Home « Gardening News « July 15, 2007

Yard and Garden News

Woolly Alder Aphids

There have been many reports of silver maples (Acer saccharinum) infested with insects. People typically have described silvery or white silky threads coming from these affected trees. These strands often fall and cover cars, patio furniture, and anything else that is underneath. If you look closely at the twigs and leaves, you will find small fly-like insects clustered on them. Leaves are sometimes deformed, being folded lengthwise as a result of this insect feeding.

This mystery insect is the woolly alder aphid. They overwinter on silver maple, their primary host. In the spring, as they become active, they create conspicuous white waxy filaments. They can also produce large amounts of sticky honeydew, a sweet waste product, as they feed on sap. Honeydew often results in sooty mold, a black colored fungus.

Despite their appearance, woolly alder aphids are not harming either maple or alder and no treatment is necessary. Even sooty mold has no real effect on plants and can be ignored. Woolly alder aphids will soon be moving from silver maple to alder, their secondary host. If you see something that looks like a very small piece of cotton seemingly floating in the air, that is a woolly aphid flying by.

Japanese Beetles

While we don’t see Japanese beetle in many areas of Minnesota, they are gradually becoming more common here. They can be very injurious but don’t panic if you find some in your garden or yard. The presence of just a few Japanese beetles does not signal the end of your plants, since small numbers are unlikely to be that damaging. Use common sense when dealing with Japanese beetles.

Japanese beetle adults feed on over 300 different plants. They commonly eat the flowers and foliage of roses and grapes. They also feed on the leaves of many trees, such as crab apple, cherry, birch, Norway maple, mountainash, willow, and linden. The beetles skeletonize the foliage, eating the leaf tissue between the veins. Japanese beetles particularly like to feed on plants in sunny areas and typically will start eating foliage at the top of plants and work their way down.

Carefully examine your garden and yard to determine how severe Japanese beetle are and which plants they are attacking. Japanese beetles are active through August so be sure to continue monitoring throughout the summer. If there are only small or moderate numbers present, consider handpicking them. Just pick them off or knock them into a pail of soapy water. Handpicking is easier in the morning when it is cooler and the Japanese beetle are not very active. Check your plants regularly as more beetles can fly into your garden. They are strong fliers and can travel at least one to two miles.

If you are interested in using a lower impact insecticide, try products containing neem. This insecticide deters Japanese beetle from feeding but is only effective on small or moderate numbers of Japanese beetle. There are a variety of residual garden insecticides that work against these beetles, such as products containing bifenthrin, carbaryl, cyfluthrin, esfenvalerate, lambda‑cyhalothrin, or permethrin. Be sure the plants you wish to treat are on the label of the product you want to use. Repeat applications may be necessary, especially if large numbers are present. Another insecticide option is imidacloprid (one example is Bayer Advanced Tree and Shrub Insect Control), a type of systemic insecticide. This is not immediately effective as it takes at least two weeks before the insecticide is taken up by plants and kills Japanese beetles.

Pheromone traps are available in garden centers and catalogs to help people capture Japanese beetles. The lure mimics virgin female Japanese beetles and is very attractive to male beetles. While these traps do capture Japanese beetles, sometimes in large numbers, they are not very effective in reducing them in your yard and garden. Research has shown that traps actually attract more beetles into an area than they capture.

Japanese beetle grubs are also pests feeding on turf grass. However, if you have Japanese beetle adults in your garden, you may or may not have their grubs in your lawn. Treat for grubs if you find they have damaged your turf but do not rely on applying insecticides to your grass to reduce Japanese beetle adults.

See Japanese Beetle Management in Minnesota.

A Bounty of Butterflies

Does it seem like there have been more butterflies this spring and summer than normal? Not only in numbers but perhaps in different species observed. You would not be alone if that is what you were thinking as many other people have had the same experiences. Turns out it is not just our imaginations but there is some truth to it. And we can thank the weather for this fortune. Our early, warm spring with just enough rain to go with it encouraged an above average population of butterflies. How long will this last? If we hit a dry stretch of weather, like many areas have had recently, we will probably see fewer butterflies. But for as long as it lasts, enjoy the butterflies you see.

 

June-bearing strawberries provide an abundance of fruit early in the growing season. After many weeks of picking and eating strawberries, many gardeners are ready to stand up straight once again and take a break from the strawberry patch. Before getting too comfortable in the lawn chair, however, it is time to take one more, close look at the strawberry patch. This time the search is not for juicy red fruit but for fungal leaf spot diseases.

Strawberry Leaf Spot, Leaf Scorch, and Leaf Blight are common fungal leaf spot diseases in Minnesota. All three diseases can infect both wild and cultivated strawberries. Leaf spot diseases can be a minor problem if only a few spots are present or a major yield reducing force if the fungi that cause these leaf spots are allowed to spread uncontrolled.

The fungi that cause Strawberry Leaf Spot, Leaf Scorch, and Leaf Blight survive Minnesota’s winter in the mature perennial leaves of the strawberry plant or in leaf debris in the strawberry patch. Spores are spread by splashing water or by wind to start infections on new leaves early in the growing season. Although disease may not seem severe at this time, the fungi will continue to grow and spread all season long whenever the weather conditions are favorable. This can result in very severe disease by the end of the summer. The strawberry plants, weakened by the fungi, will have fewer leaves, roots, and crowns. As a result, far fewer strawberries are produced the following year.

The good news is that renovation of the strawberry patch can significantly reduce problems with fungal leaf spot diseases. (See next article for details on renovating strawberry patches) Renovation reduces leaf spot diseases in several ways. In the first step of renovation, plants are mowed and clippings removed. (In small plantings infected leaves can be selectively cut and removed.) Leaf spot diseases produce spores on infected leaves throughout the summer, increasing the severity of the disease. In addition, the fungus is able to survive the winter in existing leaf spots. By removing the infected leaves from the patch, the fungus is no longer able to spread in this season and survive to the next. The new leaves produced in August will be healthier without the presence of infected older leaves.

The fungi that cause leaf spot diseases grow well in moist conditions. During renovation rows are narrowed and weeds are removed. This allows better air circulation through the patch. As a result, leaves will dry quickly, and the fungi will have a more difficult time initiating disease.

In patches where leaf spot diseases have been severe in the past, other control strategies may need to be considered in addition to renovation. Resistant cultivars are available for Leaf Spot and Leaf Scorch. Unfortunately no cultivars are known to be resistant to Leaf Blight. Overhead watering can increase the moisture on strawberry leaves, resulting in increased problems with leaf spot diseases. If possible switch to drip irrigation or use a soaker hose. If using overhead irrigation, water early in the morning so foliage dries quickly in the sun. Also consider the location of the strawberry patch. Plants should have a minimum of 6 hours of sun each day. If possible, choose a site where strawberries receive morning sun so that dew dries quickly from the leaves.

Know your Fungi

Strawberry Leaf Spot, caused by the fungus Mycosphaerella fragariae, starts out as small purple spots. As they grow, the center of the spot turns light gray or white but maintains a purplish red border. These spots remain small and circular earning the common name “bird’s eye spot”. In severe cases, several spots can grow together and kill the leaf.

Strawberry Leaf Scorch, caused by the fungus Diplocarpon earlianum, also starts out as purplish spots, but Leaf Scorch spots never develop a white center, instead the tissue around and between the spots turns purple or red. As the infection progresses the edges of the leaves dry and turn brown, making the leaf look scorched. Depending on the susceptibility of the cultivar, a severe case of leaf scorch can reduce next years yield by 20-40%! Severely infected plants are also easily killed by heat, drought, or cold extremes.

Strawberry Leaf Blight, caused by the fungus Phomopsis obscurans, starts as a purplish red spot with a gray or tan center. These are easily confused with Leaf Spot infections. As these spots grow, they widen out into a large V-shaped lesion with a brown center of dry dead tissue edged by a purplish red V-shaped border. The leaf tissue right outside the V-shaped lesion is often yellow fading to green. The fungus that causes Leaf Blight can also cause a soft rot of strawberry fruit.

All three fungi are also able to infect leaf petioles, stolons (the stem connecting the mother plant to the daughter plant), and the fruit.

 

Junebearing strawberries need to be renovated each year after harvest. Renovation helps to thin out the stand, re-shape the rows and reduce foliar diseases. Figure 1 shows three rows of strawberries in dire need of renovation shortly after harvest. The basic steps of renovation include:

1. Mow the foliage - especially in areas between rows and if leaf diseases are present. Remove diseased foliage from the garden by using a bagging attachment on the mower or collecting loose clippings. In northern areas of the state mowing can be skipped due to the shorter growing season there.

2. Narrow the rows - Using a rototiller or a hoe, narrow the rows to a width of 9 to 12 inches (see Figure 2).

3. Remove weeds - In small strawberry patches weeds can be removed by hand. Registered herbicides can be used if needed on larger patches.

4. Fertilize with nitrogen at the rate of 0.1 lb N/100 square ft. after shaping the rows and 0.1 lb N/100 sq ft. once the vines begin to run (about 4 to 5 weeks later). Have your soil tested to determine if other nutrients are needed.

This article was adapted from an earlier version. For more detailed information on renovation see: http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-July0199.html

 

 

If you carry firewood from one location to another, you may be introducing invasive insect species and tree pests. Invasive species and tree diseases can devastate our forests!

Here’s how you can protect our forests –

• Don’t transport firewood. Rather than bringing firewood to your campground or cabin, buy locally grown firewood at the campground or in the town you’ll be visiting.
• Buy only the amount you need for your trip and burn it all. Firewood can harbor invasive insect species and tree pests. If you leave any firewood behind or take it home, you could be spreading these tree pests.

Firewood may harbor invasive insects and disease pathogens that can harm our trees and forests.These threats include:

• Emerald ash borer is a non-native insect that has killed nearly 20 million ash trees since 2002 when it was accidentally introduced into Michigan from China. This invasive insect is spreading to other states and Canadian provinces due to the movement of infested firewood and nursery stock. While emerald ash borer has not been found in Minnesota, firewood sold in Minnesota may come from other states and could be harboring emerald ash borer.
• Gypsy moth is among America’s most destructive tree pests. The moths have caused millions of dollars of damage to forests in eastern states and now threaten Minnesota forests. Gypsy moth eggs can be transported on firewood.
• Oak wilt is a disease that kills large numbers of oak trees in Minnesota every year. Oak wilt can be spread by insects that carry the fungal spores from infected firewood to healthy trees.
• Dutch elm disease has devastated our elm population, and continues to kill more elms every year. It can be spread when insects carrying the fungal spores travel from infected firewood to healthy trees.

Reprinted with permission from Minnesota Department of Agriculture
http://www.mda.state.mn.us/index.htm

For more information on:

Emerald ash borer
Gypsy moth
Oak wilt
Dutch elm disease

 

Mulch is a good thing. Just a few inches of organic mulch spread around trees and shrubs and in flower beds does wonders for holding in soil moisture, moderating soil temperature, and reducing weeds. Wood-product mulches (pine bark nuggets, shredded hardwood, chipped brush, etc.) are particularly popular for landscape use. They are readily available in bulk or bagged form and can normally be used without any problem.

But occasionally even good mulch goes bad. A case of “toxic mulch” was reported recently at the University of Minnesota Landscape Arboretum in Chaska. Wood chip mulch (from tree trimmings) was spread over a planting bed. Shortly afterwards, gardeners reported shriveled and scorched-looking foliage on cotoneaster and mock orange shrubs in the bed. Some nearby perennials also showed symptoms, according to gardener Jeffrey Johnson. IPM (Integrated Pest Management) specialist Dan Miller diagnosed the problem as toxic mulch syndrome.

Toxic mulch develops when anaerobic conditions exist within the mulch. This lack of air is most likely to occur when wet mulch is piled higher than six feet and/or when the mulch is heavily compressed. Johnson reports that the arboretum mulch pile was over 10 feet tall and tightly packed.

Decomposition under these conditions may produce toxic waste gases such as methane, hydrogen sulfide (think rotten eggs), alcohol, and ammonia. When the mulch is then spread around plants the toxic gases can damage or kill plant tissue.

Potentially toxic mulch is usually detectable by smell; it often smells sour or vinegary, or may have sulfur or ammonia smells. Good mulch should smell something like freshly cut wood or have a fairly pleasant “damp earth” smell. A pH test can also detect toxic mulch: it has an extremely low (acidic) pH of around 1.8 to 3.6.

How to avoid a toxic landscape experience:

• Give bulk loads or bags of mulch a thorough sniff test before spreading them in the landscape.
• If mulch smells like vinegar, sulfur, or ammonia, don’t use the mulch immediately.
• To redeem toxic mulch, spread it out on a hard surface like a driveway to a depth of one foot or less. Water the mulch repeatedly, or let it sit through several rainstorms. Turn the mulch and allow it to sit for a week or more before using it.
• If you have a pile of chipped wood, keep it less than 6 feet tall and turn the pile occasionally before using as mulch.

 

While most of our beloved garden and landscape plants are growing gangbusters this time of year, so are most weeds. There are varied definitions for a weed, but the best one I know is “a plant out of place”. For some plants, such as dandelions, it may be difficult for us to envision that it may ever have an appropriate place. For other plants, such as cultivated ornamentals that can reseed under favorable conditions like Monarda (bee-balm), seedlings may be considered weeds or welcomed additions depending on the location, the abundance of seedlings, and one’s perspective. Multiple factors can influence the severity and types of weeds we encounter, including what weed species are in our seed bank (dormant seeds in the soil) and their growth patterns (i.e. annual or perennial, clump forming or spreading, and abundant or light seed producers), past and current weed management practices, and the current year’s growing conditions.

Even with the best of intentions and preventative action (e.g. mulch and herbicides) some weeds can still take hold in our gardens and landscapes. Here are some mid-summer tips to help tame what may be getting out of hand.

Know the Enemy!

Identify and learn about the weeds you routinely encounter. It will go a long way in finding a reasonable and effective control strategy. For instance, weeds like ragweed and lambsquarter are annuals. They have an upright, relatively strong stem that can be easily pulled out when the soil is moist or hoed away as young seedlings. Hoeing these weeds will seldom result in the remaining root system regenerating shoots. On the other hand, more care must be given to a perennial weed like a dandelion where the main tap root has the ability to regenerate new shoots when severed. One may decide to repeatedly hoe off the newly emerging shoots until the energy reserves are depleted, carefully pull or dig out most of the taproot, or use an herbicide. Some of the more aggressive perennial weeds like Canada thistle and quackgrass produce abundant underground rhizomes that can emerge long distances from the original plant. Such plants can quickly spread throughout a garden or landscape. For such weeds a more aggressive control strategy is typically needed and often includes frequent hoeing or pulling and the use of an herbicide.

Smothering Weeds

Relatively short, lower vigor weeds can be smothered with a fresh layer of mulch. Some weeds may grow through the mulch and can be controlled in the same or another manner later. Smothering is a great option for larger expanses of space such as for walkways. Besides standard mulches, other useful things to use to smother weeds include plywood, carpet, and landscape fabric.

Pulling Out Weeds

Individually pulling weeds can be labor intensive, but in many situations is the simplest and most reasonable course of action. This is especially practical if one has a small garden to manage or if weeds are intermingled with desired plants. In fact, it can become a very satisfying chore because it brings us in close proximity to our garden beds where we can better inspect and appreciate our plants and the results are immediately evident. Weeds typically are easier to pull out when the soil is moderately moist and when we grip them close to their base where they enter the soil. Often the day after a rain or a deep watering is perfect for pulling weeds. Pulling typically is also the least disruptive control measure for nearby desired plants.

Hoeing Weeds / Cultivation

There are many different tools designed over the millennia to aid in the physical damage and demise of weeds. Some tools are designed to lightly cultivate the uppermost layer of soil, exposing and drying out the root systems of newly germinated seedlings. Other tools are designed with blades to slice off the tops of relatively young or tender weeds. Still other tools are sturdy enough to aggressively chop away at the base of older, tougher stemmed weeds. In most situations, hoeing or cultivating when the soil is moderately moist is best. When the soil is too moist it can be compacted and clods can form and when the soil is too dry it is hard for the tools to penetrate. Avoid watering after hoeing until weeds have dried to prevent weeds from rerooting. Some succulent weeds like purslane dehydrate slowly and are especially prone to reestablishing themselves if they are left on the soil surface and are irrigated.

Factors in deciding which tool to purchase and use in a particular circumstance include what types of weeds one has, how difficult they are to cut, the size of the weeds one typically allows to grow before hoeing or cultivating is done, and how much space there is between plants and the maneuverability of the tool.

Karyn Vidmar, manager of the University of Minnesota Display and Trial Garden on the St. Paul Campus, and her employees have found that a short-handled weeder and a stirrup hoe have been particularly helpful in controlling weeds. The hand weeder can be used in tight places and the elongated, pointed ends are especially helpful to grab and scrape off weeds growing close to desirable plants. The stirrup hoe works both as it is pulled and pushed across the soil surface and is especially useful in larger, open spaces to remove small to medium sized weeds.

Some of the Most Problematic Weeds

Some of the most problematic garden and landscape weeds include aggressive perennial weeds that spread underground by rhizomes. Some of these types of weeds like Canada thistle and field bindweed are also among the eleven primary noxious weeds of Minnesota and it is required that they be controlled on all public and private land. More information about the Eleven Primary Noxious Weeds of Minnesota (pdf).

Another very problematic group of weeds include woody weeds such as tree and shrub seedlings, especially if they are growing in confined places near a foundation or fence. If they are allowed to grow a season or two before they are dealt with, they can become well-rooted and very difficult to remove. One control option is to repeatedly prune off sprouts to the ground throughout the summer with the hope of eventually depleting the stored energy resources.

Use of an Herbicide

For the more problematic weeds it can be very difficult to achieve adequate control without also including an herbicide. Some of the more challenging weeds often require repeated herbicide applications and carefully timed applications. For instance, glyphosate applications in early fall are especially effective on many perennial weeds. The herbicide will more readily be transported to overwintering structures and kill the entire plant. Glyphosate can also kill the same weeds in the summer, but may take more applications. Sometimes due to the sheer number and location of weeds, the use of a contact herbicide in summer is a favorable option even for weeds that can readily be controlled with non-chemical means. For woody weeds near a foundation or fence, pruning off shoots and then painting the stumps with an appropriate herbicide can be very effective. When using any herbicide, carefully read and follow the label.

Keeping up with summertime weeding will help the chore remain manageable and it keeps us regularly inspecting and enjoying our gardens and landscape. Over time our weed seed bank will decline as we remove weeds before they reseed. In addition, weed management will become easier as we eradicate the pesky, hard to control perennial weeds from our gardens. Early and frequent intervention with weeds typically pays off by overall making the job much easier.

 

 

While parts of the state have received some moisture, many lawns as well as other landscape plants are struggling through some very warm, dry conditions. Here are some things to consider when wondering what, if any, lawn care one should be doing during our hot, dry summer conditons.

• Maintaining a typical Kentucky bluegrass lawn in an actively green and growing state during the summer months of July and August will generally require about 1 to 1 ½ inches of water per week, including any rain that falls. If some browning of the lawn can be tolerated, one can cut back to about ½ to ¾ inch of water every week to 10 days. Under this condition, those areas of the lawn that tend to dry out more quickly may indeed turn brown. However, the spreading stems and growing points of the grass will usually contain sufficient moisture to be able to resprout and begin growth again when conditions moderate later in the summer. Where lawns are allowed to go completely brown (summertime dormancy), applying about an inch of water every two to three weeks will again help keep just enough water in the plant so that it can resume growth later this summer. While these are general guidelines, they are not guarantees of lawn grass survival. Extremely hot and/or dry conditions can push even well-prepared grass plants over the edge and result in some permanent die back. During periods of very high temperatures and no significant rainfall, the above mentioned watering intervals should be reduced by 30 to 50 percent to help moderate high plant and soil temperatures. That is, apply similar quantities of water but more frequently during those very stressful periods. Remember, you are the person who knows the conditions of your lawn most thoroughly and are in the best position to judge the amount of stress that can be tolerated as well as the lawn' s water needs.
• It is usually best to avoid lots of play and traffic on lawn areas under drought stress or in a summer dormant state. Lawn grasses are much more vulnerable to serious injury and dieback from wear and tear under water stress conditions than when actively growing and green.
• Many of our common lawn weeds remain alive and growing while our lawn grasses are struggling through heat and drought stresses. While it may be tempting to go out armed with a weed killer to destroy those unwanted plants, it is best to avoid applications of herbicides to heat and/or drought stressed lawns. The reason for the caution is that under stressful conditions, lawn grasses can also be susceptible to injury from those same weed killers designed to destroy broadleaf weeds or certain annual grasses. While the weeds may be greener in color, they too are under stress and often not actively growing. Hence, control of the weeds may also not be that good. For many of our perennial lawn grass weeds such as dandelion, broadleaf plantain or clover, it is best to wait until later this summer or into early fall to apply weed killers. Assuming the lawn is actively growing and green by that time, there should be little chance for injury to our lawn grasses and control of those perennial broadleaf weeds will be much more effective.

 

For larger views of most images, just click on the image.

Back issues Yard & Garden Line News for the past nine years are online at http://www.extension.umn.edu/projects/yardandgarden/ygline-news.html.

For plant and insect questions, visit http://www.extension.umn.edu/askmg. Thousands of questions have been answered, so try the search option in the black bar at the top left of the board for the fastest answer.

Receive an e-mail reminder when the next issue of the Yard and Garden News is posted to the web:

Name

Email

unsubscribe

Happy gardening!

Nancy Rose
Editor
Regional Extension Educator - Horticulture

Home « Gardening News « July 15, 2007

©2007 Regents of the University of Minnesota. All rights reserved. Trouble seeing the text? | Contact U of M | Privacy

The University of Minnesota Extension Service is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation.