Horticulture Newsletter
May 2002

A Look Inside. . .	Archived newsletters

Upcoming Activities Improper Mulching Apogee Update 2002 Cucumber Beetle Control Using Acetic Acid As A Broad-Spectrum Herbicide

Swimming Pool Pesticide Certification and Recertification Class
Tuesday, May 14, 8:00 a.m. to 5:30 p.m. at the Elk/Cameron Community Education Council in St. Marys. Pesticide credits available for licensed applicators.
For information please call 776-5331.

Landscaping

Improper Mulching Can Kill Trees

While proper choice and application of mulch can dramatically improve the overall health and vigor of landscape plants, improper mulching can needlessly stress and potentially kill landscape plants. This article was written to explain how improper mulches and mulch applications needlessly stress plants and gives solutions to common problems.

Proper and Improper Mulches and Mulching Techniques

The two basic mulch choices commonly available include inorganic and organic mulches. Inorganic mulches such as lava rock, mineral rock and gravel, pulverized rubber, geo-textile fabrics, etc. do not decompose as readily as organic mulches which is why many contractors choose to use them. Organic mulches are usually derived from plants or plant parts and may include cocoa hulls, conifer needles, leaves, grass, newspaper, straw, hardwood and softwood (conifer) bark, woodchips and other wood products. When considering mulch longevity, it should be remembered that wood decomposes more quickly than bark, conifers (softwoods) more quickly than hardwoods, fine mulch more quickly than coarse, succulent tissues more quickly than woody materials and fresh tissue more quickly than dry. Conifer bark nuggets from large mature pine, cypress or other softwood trees contain very high amounts of lignin, wax and protected cellulose that resist decay. Wood from these same species however rots quickly because the cellulose (in young trees especially) is not yet protected by the production of lignin waxes and tannins which resist decay.

Hardwood tree bark, even from large trees, contains large amounts of cellulose (like ground wood from most tree species) that is NOT protected from rotting. Wood and hardwood bark, due to their high cellulose content, have high carbon contents and low nitrogen contents, i.e., high carbon to nitrogen (c:n) ratios. Microorganisms that decompose wood utilize nitrogen in the decomposition process which is why plants sometimes exhibit temporary nitrogen deficiencies. This phenomenon is especially noted when high wood content or hardwood bark mulches with particles smaller than 3/8" to prevent this problem. Other producers additionally compost their mulch for a minimum of six weeks and add one to three pounds of actual nitrogen per cubic yard of mulch to speed composting and lower the carbon to nitrogen ratios to the ideal twenty parts carbon to one part nitrogen. This composting procedure kills plant disease pathogens, eggs of insect pests and produces a product that returns plant nutrients rather than ties them up.

Organic mulches have the distinct advantage of returning essential nutrients to the beneficial soil micro-flora and surrounding plants upon decomposition. Because of this decomposition, however, organic mulches need to periodically be replaced and that is where many problems arise.

The "monkey-see, monkey-do" plague of over-mulching landscape plants unfortunately continues because of a continued lack of education. Not only is over-mulching a waste of mulch, (and a potentially costly one at that), it is rapidly becoming the number one cause of death to azaleas, rhododendrons, dogwoods, boxwood, mountain laurel, hollies, cherry trees, ash, birch, linden, spruce and the list goes on.

There are many reasons why over-mulched woody and herbaceous ornamentals may become stressed and die. Improper mulching has the potential to stress and/or kill by:

• Promoting excessive soil moisture and subsequent root rots;
• Inner bark tissue (phloem) death of above-ground "stem flares;"
• Fungal/bacterial diseases, root rots, crown rots and butt rots;
• Excessive heat generation preventing tissue "hardening off;"
• Rodent chewing on phloem tissue and "stem girdling;"
• Anaerobic microorganisms producing toxic organic acids, alcohols and volatile gases such as ammonia;
• Promoting nutrient deficiencies and imbalances and possible allelopathic toxicities (allelopathy);
• Lowering soil temperatures during critical "root growth" periods which may suppress overall root and plant growth;
• Preventing moisture penetration due to dry fungal masses becoming hydrophobic and actually repelling water.

Lowered Soil Oxygen Levels

One of the most common causes of stress by over-mulching is oxygen starvation or suffocation of plant roots. Repeated and excessive applications of finely textured mulches on poorly drained soils can contribute to poor soil aeration by slowing soil water loss through evaporation. Roots must respire (breathe) and take in oxygen unlike leaves, which give off oxygen. When oxygen levels in the soil drop below ten percent for many species, root growth declines. When too many roots decline and die, the plant will eventually succumb.

Most finely textured organic mulches have a high water holding capacity and can literally hold up to 10-20 times their dry weight in water. When young, shallow rooted species have mounds of mulch applied over their roots systems, needed oxygen levels in the root zone can begin to decline. This phenomenon is especially prevalent during wet growing seasons and in the spring and fall, which are the most crucial times for root growth in temperate climate plants. This oxygen deprivation is especially prevalent on soils, which do not drain well, characteristic of many urban soils in the United States. For recently planted trees, shrubs and flowers, excessive mulch applications coupled with excessive rain and/or irrigation can retard or even prevent new root growth due to a waterlogged soil. These conditions also enhance attack by root rot pathogens that can kill plants. If leaves are used, they should be chopped or shredded prior to application to prevent matting and water percolation into the soil below. Likewise, if grass clippings are used alone, they should be dry and spread in a one-inch layer. Thicker layers cause similar matting and putrid decomposition. Also be sure to never use grass clippings that have herbicide residues or you can damage tender ornamental plants.

It is most important to remember that the problems caused from yearly over-mulching are not immediate, but progress slowing with time. The symptoms on long-loved woody plants may take three to five years to express themselves and sometimes longer, depending on the species and soil type. Unfortunately, by the time the plant symptoms are recognized (off-color foliage (chlorosis), abnormally small leaves, poor annual twig growth, and die-back of older branches), it is generally too late to apply corrective measures. At this point, the plant has usually gone into an irreversible decline and will most likely die.

Inner Bark (Phloem) Stress

A second major cause of plant decline and death results from piling mulch directly against the stems of trees and shrubs. Above ground stem and trunk issue is morphologically different from root tissues. Roots have evolved many mechanisms to survive in continually moist environments while the trunk of most woody species have not. The above ground stem issue of most trees, shrubs and perennials must be able to freely exchange adequate amounts of oxygen and carbon dioxide through their lenticels. Researchers have documented that when fine-textured mulch is piled onto stem tissue; gas exchange decreases with inner bark (phloem) tissue eventually becoming stressed and later dying.

Research scientists from the Bartlett Tree Expert Company have verified that the constant presence of moisture on truck tissue inhibits the movement of oxygen and carbon dioxide in and out of the phloem (inner bark).

This occurs when the above-ground trunk flare is buried with anything that will hold moisture such as fine textured mulches or soil. Phloem death readily occurs when "pop-up" irrigation heads continually saturate the mulch placed against the plant's trunk and root flare. When the inner bark dies, roots are thus malnourished and weakened to the point where they suffer reduced water and nutrient uptake, which subsequently affects the health of the whole plant. If abnormally wet conditions persist for a long enough period of time, the phloem tissue may die thereby depriving the roots with needed photosysnthate produced by the leaves via photosynthesis. This will subsequently cause root "starvation," top dieback and plant death!

Fungal Diseased and Secondary Opportunistic Borers

A third stress factor associated with the application of mulch next to stem tissue involves fungal and bacterial diseases. Most plant diseases require moisture to grow and reproduce and may gain entry into the stressed, decaying bark tissue caused by the piling of mulch next to the stem tissue. Research has shown that collar rot caused by Phytophthora spp. is more likely to occur when trunk tissues are kept moist from soil, mulch and constant irrigation. Similarly, Armillaria root rot caused by Armillaria spp. has been observed under the trunk flares of excessively mulched woody ornamentals. Research has also shown that Verticillium dahlia, a common fungus causing the wilting and death of many species of ornamental trees and shrubs can be transmitted in fresh mulch and kill susceptible plants in the garden and landscape. Similarly, Rhizoctonia salami, a fungal plant pathogen that causes damping-off of seedling plants has been shown to kill herbaceous ornamentals and is actually stimulated by fresh mulches, especially following nitrogen applications. Although the transmission of pathogens in fresh mulch is possible, it is rarely seen in the landscape when applied to established, woody ornamentals. If it were a problem, large tree care companies like the Davey and Bartlett Tree Expert Companies (which have applied fresh woodchips for decades) would have documented it in their plant diagnostic labs from field samples. It appears that for woody ornamentals to become inoculated with a fungal pathogen from fresh, infected wood chips, the mulch must be directly placed adjacent to the susceptible plant, the susceptible plant must usually be injured and environment conditions must favor disease survival and development. In essence, the disease triangle requiring a host, the pathogen and a conducive environment must be present. Potential disease transmission problems from infected mulch can easily be corrected by short-term composting of fresh mulches under high-temperature conditions (130-160 degrees F). When temperatures exceed 130 degrees F. for just a few days, pathogens, weed seeds and insect eggs are easily killed. Turning the mulch so that all parts are exposed to these high temperatures is critical in destroying all pathogens and pests. After pathogen destruction, the organic matter in mulch must be stabilized enough to prevent pathogens from utilizing it as a food source. If this is not done, plant pathogen populations can actually increase. Such is the case with Rhizoctonia, a plant pathogen that can actually grow on the fresh, non-stabilized mulch. In conclusion, even though the likelihood of transmitting diseases and pests via fresh mulch is remote, it is considered prudent to compost all fresh mulches for six weeks before use. Applying one to three pounds of nitrogen per cubic yard of fresh mulch, or adding grass clippings at 10-20% by volume, composted poultry manure at 10-20 pounds per cubic yard, or adding any other nitrogen source to reduce the carbon to nitrogen ration to 20:1 is the quickest way to make the fresh mulch usable. When composted properly, you should not be able to smell any ammonia odors and the mulch should begin to smell like soil. Avoid applying fresh residues to ornamental plants when the crop is susceptible to disease. Fall and winter applications help avoid this problem.

Insect borers and bark beetles have also been associated with stressed, over-mulched trees and shrubs. Whenever the vascular system of woody ornamentals is weak and stressed from drought, flooding or over-mulching, many species of insect borers find it relatively easy to invade and become established. Though clearwing borers can occur fairly high on the trunk or branches of their host plant, most can be found attacking at or slightly below ground level. Once established, borers and bark beetles tunnel through sapwood weakening the plant structurally and functionally. Proper mulch placement (away from the trunk) and depth (two to four inches thick) will help prevent these problems and help to invigorate the plant so they can naturally repel these pets.

Tree Fruit

Apogee Update 2002

Many fruit growers used the new plant growth regulator Apogee on a trial basis last year, its first season of being registered for use on apples in New York. Most growers had positive results and most of those who used Apogee last year report that they are planning on using it again this year. The following information is provided to help growers fine tune the use of Apogee in 2002.

Apogee and Pruning: The big incentive for using Apogee is saving on labor for summer and dormant pruning. In several research studies on commercial orchards, Apogee has reduced pruning time by 25-45%. However, Apogee has worked best when applied to well-pruned trees. It is better to prune dense trees hard during winter and use Apogee to help control the regrowth than it is to apply Apogee to overgrown trees without corrective pruning.

Apogee provides poor control of one kind of shoot: water sprouts. Apogee must be applied prior to the first flush of terminal shoot growth in order to be effective, but that timing of Apogee is too early to reduce the growth of water sprouts, which have not yet formed at the start of the season. Heading or stubbing back cuts (i.e. bench cuts) should be avoided when pruning to limit the number of water sprouts that are often stimulated by such cuts.

Apogee is best used as a tool to supplement good pruning practices. It will not erase excessive limbs that are already in place or make up for poor pruning practices. Thus the importance of good pruning prior to application of Apogee cannot be over-emphasized.

Apogee and Thinning: The effect of Apogee on fruit set and fruit size is the biggest concern voiced by growers after its first season of use. While some growers saw no such effect, others reported that Apogee increased set, and reduced fruit size due to the increased crop load. Likewise, some research studies have shown that Apogee can increase fruit set, while others have shown no effect.

Apogee rate affects the fruit thinning response. The rate of Apogee (dilute basis) at which this effect of increased set usually becomes apparent is 12 ounces per 100 gallons. Dr. Duane Greene at the University of Massachusetts showed that McIntosh fruit set increased linearly with increasing Apogee concentration. Stated another way, the lower the Apogee rate, the less effect it had on fruit set. To minimize potential adverse effects on thinning and fruit size, use the lowest rate of Apogee necessary to get good growth control. Increasing the rate from six ounces per 100 gallons to 12 ounces per 100 gallons is more likely to affect the duration of the growth control than the degree of growth control. If a grower has had problems thinning Apogee treated trees, it may be better to make three applications of six to eight ounces per 100 gallons than to apply 12 ounces per 100 gallons twice.

Be assertive when thinning. It has always made good sense to evaluate initial fruit set and it is even more important to do so on Apogee blocks. If the initial set is heavy, thin early and thin often! The growth inhibition from Apogee takes a week or longer to become apparent, which suggests that thinning early, before the Apogee kicks in, may be a good way to reduce crop load on Apogee treated trees. Petal fall thinner applications may have an important role in combating excessive set on Apogee treated trees. Starting the thinning program at petal fall also gives the grower a longer window of opportunity to adjust the crop. This strategy is a good one without Apogee. With Apogee, it is the only way to go.

Another way to remove more fruit is to increase the dosage of a chemical thinner, or use two or more chemical thinners in combination. Our initial recommendation to adjust for Apogee was to increase the strength of the thinner by 30-50%. For example, if you determined that untreated trees in a given block would require a thinning spray of 5 ppm NAA, then Apogee treated trees in that block may require 7.5 ppm NAA or 5 ppm NAA plus Sevin to get the same degree of thinning. Beware the laws of diminishing returns when considering rates of NAA in excess of 10 ppm. The increase in thinning diminishes as the rate of NAA is increased. Dosages of NAA higher than 10 ppm can stunt the growth of the fruit that remain, resulting in little improvement in fruit size.

Another way to remove more fruit from heavy fruit sets is to add a penetrant such as oil to the thinning spray. Duane Greene showed that adding a quart of spray oil per 100 gallons to a tank mix of Accel plus Sevin XLR increased the amount of thinning to an acceptable level of Apogee treated trees. If you try this method, remember that one should NOT concentrate the oil when using it as a surfactant in a concentrate spray mix. Also remember that oil and captan can't be applied together or within a few days of one another, due to the possibility of spray damage.

Leave check trees! Many factors affect fruit set, and it is nearly impossible to assess the effects of Apogee or chemical thinners without leaving some untreated trees. Some growers might find that their thinning program was inadequate even BEFORE they applied Apogee! A few check trees would answer this question.

Different varieties may respond differently to Apogee. McIntosh and Gala were the varieties that growers most often mentions as problems following Apogee applications. In a two year study on Golden Delicious in the Hudson Valley, no difference was found in fruit set due to Apogee and Apogee treated Goldens responded to NAA no differently than untreated trees.

Don't Use Apogee on Empire: Apogee can cause fruit corking and cracking when applied to Empire. The occurrence of this injury is sporadic and the circumstances that lead to expression of the injury are not known. However, it has occurred across several years in Michigan, New York, Ohio and Pennsylvania. Apogee has been applied to many commercially important varieties, but Empire is the only variety identified as being sensitive to Apogee thus far. The sporadic nature of the fruit damage to Empire suggests that one or more environmental or application factors contribute to this problem. Apogee is applied as a foliar spray, so other agricultural chemicals applied with Apogee or near the time of the Apogee application(s) may contribute to the problem. One category of chemicals that have been investigated is water conditioners.

Applicators are advised to add a water conditioner such as ammonium sulfate (AMS) when mixing spray solutions from a hard water source. Foliar applications of nitrogen fertilizer salts can cause plant damage when applied at excessive concentration, under poor drying conditions or when applied with surfactants. Butch Palmer, ACDS Research, has reported that adding AMS to the spray mixture worsened Empire cracking. Studies were conducted in 2001 to evaluate the effect of Apogee and water conditioners on fruit damage of Empire. Fruit damage was severe in these studies, despite the use of the lowest labeled rate of Apogee (3 oz per 100 gal). The damage was worsened by the addition of a water conditioner. However, AMS applied with Regulaid but without Apogee had no effect on either the severity or extent of fruit injury. It is concluded that the fruit injury is directly caused by the formulated product Apogee itself and that this product should not be used on Empire.

Vegetables

Cucumber Beetle Control

Two species of "cucumber beetles" [striped cucumber beetle and spotted cucumber beetle] feed on cucurbits. Two additional species [northern corn rootworm and western corn rootworm] invade late in the season. Within this complex, the striped cucumber beetle is present in the highest density and over the longest time span. Adult feeding during early plant growth can cause stand reduction and rind-feeding by adults or larvae later in the season renders crop unmarketable and may serve as routes of entry for pathogens. The major pathogen is Erwinia tracheiphila, the causal agent of bacterial wilt. Disease management currently relies on vector management. Even a low beetle density during colonization of young plants can result in significant plant disease and the severity of disease over time correlates to beetle density during early plant growth. Disease development is strongly influenced by inoculum dose. Cultural methods can manage the problem in machine-harvested, short-season processing pickles that have high plant populations, but the vector/disease complex presents very difficult risks for long-season, fresh-market vine crops that are grown at much lower plant populations. Bacterial wilt is most severe on melons and cucumbers. It was rare to see bacterial wilt in squash and pumpkins in the past, but recently this disease seems to be infesting some squash and pumpkin plantings.

Timing and Cultural Controls: The striped cucumber beetle overwinters as an adult both inside and outside of cucurbit fields. Adults invade fields soon after transplanting and lay eggs at the base of plants. The hatching larvae feed on the roots of vine crops, pupate and then emerge as new adults in about 25-30 days. There are at least two, and probably more, generations per year in Pennsylvania.

Early immigration can be the most serious. Up to 10% of the immigrating adults have tested positive for carrying the pathogen that causes bacterial wilt using ELISA tests. Where the immigration has traditionally occurred very soon (within three days to a week) after transplanting, it's recommended an application of Admire to the transplants. Roots of these young plants may not be sufficiently established to take up material applied through drip irrigation. After transplanting, the roots are training to the drip lines and a single application through the drip irrigation at that time (about ten days to two weeks after transplanting) should control the immigrating adults. It's anticipated that no, or only one or two foliar insecticide applications would be needed if that early immigrating population is prevented from establishing. If this approach is successful for several years, we may no longer see this dramatic invasion of fields soon after transplanting. Should that occur, scout fields and add Admire when immigration starts.

An effective cultural control method is the use of row covers. Put the covers on right at transplanting. The row covers effectively exclude the beetles for as long as you have the crop covered. If you can find a self-pollinating cultivar (there are some good ones now for cucumbers), then you can leave the floating row cover on until harvest. Otherwise you need to remove the cover to allow bees to pollinate the crop.

Admire 2F: Imidacloprid, the active ingredient of Admire, is a relatively new systemic neonicotinoid insecticide now labeled for striped cucumber beetle in vine crops. Admire is intended for soil applications and translocates to new leaf tissue when taken up by the roots. The foliar formulation - Provado - is NOT labeled for cucurbit crops. In a gallon of formulated product, Admire 2F contains two pounds active ingredient Vegetable growers must correct for application rates when using plasticulture and applying materials through drip irrigation since the effective treated area is reduced to that of a "mulched acre." Smaller growers will need low application rates, which require measuring volumes of less than one fluid ounce.

To control cucumber beetles invasion at transplanting with Admire, use a very low rate (0.02 ml/plant) to treat transplants about one day prior to planting in the field. To treat a flat of 200 transplants with Admire, dilute 4 ml (0.135 oz) of Admire in a volume of water sufficient to soak to soil mix evenly. This treatment will protect the plants for about two weeks and can be followed by field application if needed. To help make conversions: multiply 0.02 ml per plant times the number of plants in your flat. For example, use 20 ml to treat 1000 transplants. This rate is just a little higher than suggested for tomato transplants, which is - 15 ml - or 0.5 ounce - per 1000 transplants. You can convert ml to oz by dividing by 29.6 (there is 29.6 ml in a fluid ounce). Be careful of phytotoxicity (burning the plants) at higher rates. Burning of leaf margins was observed at 0.04 ml/melon plant at the two-leaf stage, although these plants did grow out of this in about two weeks.

The best way to apply Admire to the field is through an injector connected to a drip irrigation system. As with all chemical injections, the irrigation system should be primed with water prior to beginning the injection, and the material should be injected slowly to provide even distribution. Remember: the more evenly distributed the material is, the better job of protection, and the chance of phytotoxicity will be reduced (i.e. the more emitters on your drip tubing the better). It is also important that the zone of moisture created by the drip tubing the better). It is also important that the zone of moisture created by the drip tubing be within the root zone of the crop because it is the roots that will move the material into the plant. Below is a table which shows the number of ounces (oz) or milliliters (ml) of Admire that need to be applied per 100 row-feet of a cucurbit crop for application rates of 16-24 oz/A. The amount required to treat a 100 foot row is very small and not easily achieved by commonly available measuring devices. We recommend that growers with small fields use milliliters as a measurement unit and use syringes if available (without a needle to avoid plugging up the syringe and to help with safety) for extracting these amounts. A syringe without a needle will provide a very accurate and safe method of measuring these small quantities.

Table 1. Linear rates of Admire in cucurbits for 100 row feet (Remember: do not apply Admire within 21 days of harvest!).

* These rates were calculated for irrigation systems supplying a three-foot bed. The amount of material applied should be the same regardless of whether a single or double row occupies a bed of this size.

If a drip irrigation system is not available to deliver the pesticide to the root zone, it can be applied by directing a spray or drench into the soil at the base of the plant. It is important that the material be applied when the soil is moist, or with enough water to soak the soil to the depth of the root zone. If it is applied only to the surface of the soil, the roots will not have good access to it. Apply the material directly to the base of the plant when the soil is moist and with at least 10 ml of water per plant for young plants (<4 true leaves) and 50 ml of water for larger plants. This can be achieved with a backpack sprayer rather efficiently, but you need to calculate how much time it takes for your backpack sprayer to deliver 10 ml volume of water. To calculate the total volume of water to put in your backpack sprayer, multiply the number of plants you want to treat by the volume of water per plant. For example, let's say you wanted to treat a 100-foot row of cantaloupe with 16 oz/A of Admire. If your plant spacing were two feet then there would be 50 plants in that row. If the plants were young you would only need 10 ml of water per plant (minimum-please feel free to increase this amount if you wish). Multiply 50 plants by 10 ml and you will discover you need 500 ml of water to treat this row. Add 500 ml of water to the backpack sprayer and then add 3.26 ml of Admire (see Table 1), mix well and apply. The trick with this type of delivery system is to estimate how long it takes to deliver 10 ml (a very small quantity) from your backpack sprayer. Often this is just a quick shot from the trigger. Remember to keep the nozzle close to the soil and direct the spray toward the base of the stem. Please test the output of your sprayer with water prior to performing these calculations, and remember-do not apply Admire within 221 days of harvest.

Read the label. You should always read the label. Information on the label supercedes anything written here and good information about rates per 1000 linear feet if on the label.

Home Garden

Using Acetic Acid (Vinegar) as a Broad-Spectrum Herbicide

Horticulture professionals as well as home gardeners constantly ask Cooperative Extension Educators about alternatives to pesticides for turfgrass management. A specific interest is in alternatives for the broad-spectrum herbicides, such as glyphosate (sold as "RoundUp" or other trade names). Broad-spectrum herbicides affect all plants in an applied area and are used in a variety of turfgrass and landscape renovation projects, such as the removal of an existing lawn area to install new sod or seed, the removal of a lawn for other landscaping projects or general weed management in paved and graveled areas. While a new lawn or garden bed can be managed without pesticides, a broad-spectrum herbicide is generally needed to create a new bed or lawn, since the other alternatives (stripping the existing sod with a sod cutter, rototilling the existing sod into the soil, etc.) are often not practical or desirable.

Recently, a great deal of interest has been expressed in the use of acetic acid (vinegar) as a broad-spectrum herbicide. While many anecdotal reports of success with vinegar have been published in the popular press and on the internet, research to substantiate these claims is limited. This research project evaluated the broad-spectrum herbicidal activity of two new acetic acid type herbicides, two "home-made" treatments of acetic acid and a traditional herbicide.

* Vinegar off the store shelf is approximately 5% acetic acid.

The study was conducted in August 2001, on a partially irrigated home lawn in Castleton, New York. Applications for each product were made either once (at 0 days) or three times (at 0, 7 and 14 days), except for glyphosate, which was applied only once. Plant populations in the plots included quackgrass, crabgrass, ground ivy, dandelion, broadleaf plantain and Kentucky bluegrass. A 0% to 100% visual rating scale was used, with 0% appearing like the check plots (no injury) and 100% appearing as total injury.

Results and Discussion: All acetic acid treatments quickly caused a dramatic discoloration and browning of foliage on all plant species. In a few hours, the foliage became blackened and water-soaked. No twisting or yellowing was seen for any treatment or species. After 24 hours, control in all plots with an acetic acid product was 95% to 100%.

In general, three applications of the products gave better results than just one application (see tables below). Nature's Glory, BurnOut and the 20% acetic acid performed in a similar manner, giving good control (80% or over) for up to five weeks when sprayed once and for nine to 13 weeks when sprayed three times. The 5% acetic acid gave good control for less than two weeks when sprayed once. This increased to five weeks or more when it was sprayed three times, but it still did no perform as the treatments containing a higher concentration of acetic acid. Glyphosate, as expected, provided 90% or better control from two weeks to 13 weeks.

Average percent control for selected dates for plots sprayed with one application:

Average percent control for selected dates for plots sprayed with three applications:

Conclusions: All treatments of acetic acid provided excellent control of crabgrass and broadleaf plantain, two annual weeds, with virtually no re-growth of these species during the 13 weeks. If this experiment was conducted earlier in the growing season or under less droughty conditions, it is not known whether re-growth or new seedling germination would have occurred. Ground ivy appears to be very susceptible to acetic acid. Virtually all treatments provided excellent initial and long-lasting control of this often difficult-to-manage species. Although all of the acetic acid treatments did a good job of initially controlling quackgrass (which has a very aggressive root system), it re-grew by the nine week observation date for many treatments. By 13 weeks, the percentage of quackgrass for many treatments actually increased beyond what was initially seen in the plots!

This study showed that acetic acid is useful as an herbicide. Acetic acid at 5% concentration (as would be found on the supermarket shelf) provided only short-term control of most perennial weeds, but did effectively control crabgrass and plantain. Pesticide applicators and gardeners following the advice of various gardening media who suggest vinegar as an herbicide should be aware that repeated applications may be necessary. The highest concentration of acetic acid (20%) and the commercial formulations provided good control, but were not as effective long-term as glyphosate. Possible ways to improve the performance of acetic acid will be examined in future studies.

Sincerely,

Gregory K. Burns
Elk-Cameron Extension Director

Supporting Agents:

This publication is available in alternative media on request.

Where trade names are used no discrimination is intended and no endorsement by Penn State Cooperative Extension is implied.

Penn State encourages qualified persons with disabilities to participate in its programs and activities. If you anticipate needing any type of accommodation or have questions about the physical access provided, please contact Greg Burns at (814) 776-5331 (Elk) or (814) 486-3350 (Cameron) in advance of your participation or visit.

The Pennsylvania State University is committed to the policy that all persons shall have equal access to programs, facilities, admission and employment without regard to personal characteristics not related to ability, performance, or qualifications as determined by University policy or by state or federal authorities. It is the policy of the University to maintain an academic and work environment free of discrimination, including harassment. The Pennsylvania State University prohibits discrimination and harassment against any person because of age, ancestry, color, disability or handicap, national origin, race, religious creed, sex, sexual orientation, or veteran status. Discrimination or harassment against faculty, staff, or students will not be tolerated at The Pennsylvania State University. Direct all affirmative action inquiries to the Affirmative Action Office, The Pennsylvania State University, 201 Willard Building, University Park, PA 16802-2801, Tel 814-865-4700/V, 814-863-1150/TTY.