Common lawn weeds: Black medic and white Dutch clover

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By David Graper

SDSU Extension Horticulture Specialist

Black medic (Medicago lupulina) is a bothersome weed in lawns, flower beds and even in cracks in sidewalks or between pavers. It is usually a low-growing annual plant, but it can grow to about a foot wide and tall in sites where it is not mowed and has other plants to support the stems. It has small, clover-like leaves composed of three leaflets attached to very fine stems. Probably the most striking feature of this plant are its tiny, clusters of bright yellow flowers that begin to appear in early summer and continue until a hard freeze kills the plant. After the flowers fade, clusters of nearly black fruit develop in their place, each one containing a single seed. While black medic is usually an annual plant, it can sometimes be a short-lived perennial as well.

The challenging aspect of controlling black medic occurs in flower beds and borders. Young plants usually go unnoticed as they often grow up amongst the foliage and stems of other plants. This makes them difficult to see and pull out because it is hard to find the base of the plant where all the stems are attached to the main tap root. Pulling on the side branches usually means that they just break off. Each plant can produce hundreds of seeds, so it only takes a few plants to create a significant infestation of this weed. Besides pulling, hoeing out the young seedlings can be effective but often difficult in a densely planted flower bed. Applying an organic mulch 2 to 3” deep, around the flowers, to cover the soil and any weed seeds that might be there, may also be helpful.

Black medic can also be a problem weed in lawns, particularly lawns that have not been adequately fertilized or are stressed for other reasons. Properly fertilizing the lawn and increasing the mowing height will help to make the lawn grasses more competitive with the black medic. Most broadleaf lawn herbicides will also help to control the problem in lawns.

Use a broadleaf herbicide or a non-selective herbicide, like glyphosate, for effective control of black medic in sidewalks and pavers.

Dutch white clover (Trifolium repens) is an even more common plant found in lawns in our area. While some people do not mind it and even like it, since it is tough and remains green pretty much all summer, many people want to get rid of it if it shows up in their lawn.

It is difficult to control. The best way to achieve control is to use a three-way combination broadleaf herbicide that lists clover on the label. The key is to apply it in the fall, usually about the end of September is best, then repeat treatment in about 3 weeks. Treatment the following year will likely be necessary as well. Improving the growing conditions for your grass will usually reduce the problem as well. Fertilizing the lawn, especially in the fall will help to make the lawn grass more competitive. Raising the mowing height to allow for healthier grass and a deeper root system to better compete with the clover will help as well. If the lawn is irrigated, only water about once a week when it needs it, but apply enough water to saturate the root zone, down about 6″. This usually means applying about 1” of water per week. Frequent shallow irrigation is not the best approach to a healthier lawn.

Woody plant concerns for early July

By John Ball, SDSU Extension Forestry Specialist

Aphids, aphids and more aphids

Aphids seem to be the major cause for concern this past week. This is not too surprising as they are also some of the most common pests in the ornamental landscape. Fortunately, they tend to be more of an annoyance rather than a life threat to trees so rarely are treatments necessary.

Aphids are small pear-shaped insects, about 1/8-inch long. They have a pair of pipe-like cornicles protruding from the abdomen that emit an alarm odor to communicate danger to the “herd.” Some adults have clear wings. Aphids are colorful insects with species ranging from yellow, to orange, green, red, and black. Some aphids also have their bodies covered with waxy threads and these are known as “woolly aphids.”

Aphids live in colonies, so you rarely find an aphid, you find a cluster of them on the underside of a leaf or along a shoot. Almost all the aphids you see during the growing season are females. They reproduce asexually, and even give birth to live young rather than an egg. Each female can deliver three to five daughters per day and each daughter becomes an adult in about 10-days, so populations can increase very rapidly.

Aphids feed by sucking sap from tender shoots and leaves. They are generally concentrated on the newest growth – the most nutritious – so can be found on branch tips. If the colonies are large enough, they can remove enough sap so that the leaves become yellow and wilt. The infested leaves often curl, providing protection for the aphids that are inside.

Aphids also produce honeydew. This is a sugary liquid that is excreted by the aphids as they feed. It rains down from the tree canopy and leaves everything beneath the tree covered in a light sticky film – it’s hard to clean it from a car windshield. This sugary material serves as a food source for a sooty mold which leaves the honeydew film covered with a black, powdery material.

This honeydew is also attractive to ants. So, often when you find ants crawling along the shoot tips you usually can also find aphids. The ants collect the honeydew from the aphids (hence another name for aphids – ant cows) and will protect their herds from predators.

Everything likes to eat aphids, so the ants have their work cut out for them. Lady beetles, lacewings, hover flies, and other insects find aphids a tasty snack. The best treatment is often no treatment and just let the natural enemies do their job. If you want to help reduce the aphid population, a jet of water from a garden hose will usually dislodge many of them and since they do not fly, and are easily crushed, you’ll kill quite a few and have minimal impact on the beneficial insects.

Emerald ash borer update

The emerald ash borers are still flying in the Sioux Falls area. We are just beyond the peak flight time and some of the earliest eggs should have hatched by now. Beginning next week, I will be doing an update on larval development in infested trees.

The trees that have been infested for several years or more are clearly presenting symptoms of the repeated attacks. The emerald ash borer emerges as an adult in June or July and will often lay eggs on the same tree it emerged from. It may take four or five years of repeated attacks to finally kill a tree.

Tree that have been attacked for four years usually have more than half the crown dead and there are numerous sprouts coming up near the base. These are trees that are beyond the stage for treating and instead will be removed and the wood destroyed this fall after the flight period has ended and all the insects are inside the tree.

Dogwood sawfly (Macremphytus tarsatus) is feeding on redosier dogwoods. They are almost completely defoliating some shrub beds. Sawflies feed in colonies so they can usually go through a shrub in a few days or a week. Often the damage goes unnoticed until its almost too late for treatments.

The larvae are about 1/2-inch long at this stage and have a white powdery appearance. They will go through several color changes as they develop. They feed in small colonies along the margins of the underside of the leaves, feeding on everything but the major veins, a type of damage referred to as skeletonizing.

In another month or so the larvae will drop to the ground and overwinter in logs, landscape timbers or other protected locations. The following spring the adults – a small wasp-like insect -begin flying about the time the leaves have expanded.

The insects can still be treated with a foliar application of Carbaryl (Sevin) or Malathion. However, this window is rapidly closing as once they reach about 1-inch long they are finished feeding and the only value to a spray then is revenge, not treatment.

Strange bugs

Q We have a lot of these little orange and black bugs. (See photo.)

A These are the larval stage of argus tortoise beetle. We have had several samples of these, often seen on stiff sunflower and they’ve also been reported eating creeping jenny and some other weeds. They are a leaf beetle but do not seem to be a real problem for our garden plants.

A potato question from Fall River County, SD

Q My potatoes are blooming. Should I snip them off or let them go to seed?

A It is common to see potatoes blooming. Their flowers look very similar to tomato or eggplant flowers and are actually rather ornamental. However, potatoes seldom set fruit, so removing them is not really necessary because they really do not take that much energy from the plant. If you did want to remove them, it would be most beneficial to do it as soon as you see the flowers developing. As a side note, if potato fruit do develop, do not eat them, they can make you sick. Also, some people will think that their potatoes crossed with their tomatoes to cause the fruit to develop. But this is not true. Those two plants will normally not cross with each other because they are not of the same species.

A tomato question from Pennington County, SD

Q A few of my tomato plants are getting these spots on the leaves. (See photo.) What kind of disease is this and how should I treat it?

A Last spring and summer we observed numerous tomato plants that were looking sick after being transplanted. In some cases, the leaves had some purplish-bronze tinge to them, or the growing points turned brown and died. In other cases, the leaves had scattered small brown spots, particularly toward their stems. The symptoms were not quite what we had seen before, but laboratory tests confirmed that the cause was Tomato Spotted Wilt Virus (TSWV). This virus can be a problem in both field and greenhouse situations, and most frequently its effects in South Dakota are observed on tomatoes and peppers.

Leaves and Foliage: Symptoms on the leaves can be variable. On approaching the plant, one may notice that the newer foliage has a bronze or purplish tinge to it or it may even die back. Sometimes the plants are stunted or look wilted. Leaf symptoms may first appear as small, light brown flecks. Sometimes a portion of a leaf (either base or tip) will look normal, while other portions may be dying back or exhibiting one or more green spots bordered on the outside by brown tissue. Spots with normal-looking centers are almost always a sign of a virus. In contrast, fungal and bacterial diseases start from the center and spread outward, so there is no green center with those diseases.

Fruit: The fruit is usually infected also and may exhibit raised circular areas, sometimes ringed with small brown spots. As the fruit ripens, the distinctive spots can quickly become very obvious against the red background. Again, if the spot on the ripe fruit has a red center or red and yellow target pattern, one may be quite sure the cause is a virus. If the plant is infected when it is very young, it may not set fruit at all.

For more information on how to properly identify and manage issues with TSWV, see https://bit.ly/2ueUOj9.

— Dr. Rhoda Burrows, SDSU Extension Horticulture Specialist

A question from Moody County, SD: Tomato dry rot

Q A gardener has dry rot on the bottom of her tomatoes. She has a bunch of them that are green and starting to turn red, and I was wondering if there was anything she could do to prevent it or slow it down?

A The dry rot you are referring to is more commonly called blossom end rot because it occurs on the blossom end of the fruit. It is caused by a calcium deficiency in the tomato fruit, which results from the plant not being able to take up sufficient calcium from the soil. Generally, there is adequate calcium in the soil but uneven watering, excessively wet conditions or other environmental factors that limit water and calcium uptake that lead to the problem. There are also differences between tomato varieties, and determinate vs. indeterminate types of tomatoes. Often this is a more common problem for the first tomatoes that are harvested then it gets better later in the season.

The best treatments call for management of water to the plant. Do not let the plant go through water stress then followed by heavy watering, try to maintain even moisture content in the soil. It can be difficult when you are dealing with extended periods of saturated soils from rain for example but do the best you can to even out moisture availability. An organic mulch can help to shade the soil and help it to retain water during hot and dry periods. Adding lime, which is calcium carbonate, is not recommended because there is likely already enough calcium in the soil and the addition of lime will raise the pH of the soil which can cause additional problems with the availability of other nutrients. Calcium applications to the leaves are not generally effective either. I have even heard of people applying milk to their tomato plants – this would not be a good idea as the souring milk would likely attract insects, animals or various molds that could be harmful to the tomatoes and be a source of food-borne pathogens that could be dangerous to a person eating the fruit. For additional information go to https://bit.ly/2ffGz7u.

Dutch white clover in lawn. iGrow photo
Blossom end rot on tomatoes. iGrow photo
Black medic in lawn. iGrow photo
A close up of aphids taken on July 4. iGrow photo
A tomato spotted wilt virus-infected tomato leaf. iGrow photo
Argus tortoise beetle larvae. iGrow photo