Lawn weed killers can cause damage to gardens, trees

Don Kinzler
Forum News Service

There aren’t many things in the world of gardening that cause fear. I’m not concerned that a giant Venus fly trap is going to swallow my arm, and if I trip over a cucumber vine, I’ve got plenty of padding. But I honestly do fear one thing, and that’s collateral damage from weed killers applied to area lawns.

This summer I’ve received an alarming number of photos and samples of ailing vegetables, trees and shrubs whose symptoms, when analyzed, were caused by weed killers applied to nearby lawns. A weed killer is more appropriately called an herbicide, a word that literally means “plant killer,” and an herbicide doesn’t always distinguish between a dandelion plant and a tomato plant.

Much good can be said about herbicides, whether synthetic or organic, because they can prevent invasive, non-native plant species from overrunning our yards, gardens, prairies and forests. But if not used wisely, cautiously and judiciously, while carefully following all label instructions, herbicides can cause unintended consequences, damaging the plants we value.

When we apply herbicides to weeds, there are several ways the product can damage the desirable plants in our yards and gardens. Herbicide spray droplets can drift through the air and land on valuable plants adjacent to the area being sprayed. Some herbicides volatilize, becoming gas vapors that travel into unintended areas, gassing desirable plants. Some herbicides move readily downward through the soil into the rootzone of trees, where the plant-killing compounds are taken up by the tree’s root system and absorbed internally into the tree.

If plants in our yard or garden are doing poorly, how can we tell if it’s from herbicide damage or another cause, such as disease? The symptoms vary by the type of product.

Herbicides used on lawns commonly list one or more of the following active ingredients on the label: 2,4-D, dicamba, triclopyr, MCPA or MCPP. These herbicides kill broadleaf weeds like dandelions, and can likewise damage broadleaf yard and garden trees, shrubs, vegetables and flowers if the product comes in contact. Common damage symptoms include twisted leaves, leaves that cup upward or downward, narrow strap-like leaves on the newest growth, and distorted leaves and twigs. I’ve received many such samples this season of tomato plants, beans, potatoes, trees, and flowers.

Herbicides containing the active ingredient glyphosate (original Roundup) are non-selective, meaning they kill or injure nearly all susceptible plants, weeds and desirables. Symptoms of exposure to glyphosate include yellowing of leaves, general decline or death, and sometimes a twiggy “witches’ broom” growth on branch ends.

Following are ways to prevent injury to desirable plants when spraying for weeds:

  • Read the label directions completely to see how, when and where the product should be applied. The label is a legal document.
  • Fine spray droplets drift more readily than large droplets. Use a coarse spray with low pressure, instead of a fine, high-pressured mist.
  • Spray when the air is calm.
  • High temperatures above 80 to 85 degrees F. during or immediately after application may cause some herbicides to vaporize and move to other areas.
  • Spray nozzles might apply product in a wider band than is visible, causing over-spray on adjoining desirable plants.
  • Lawn herbicides containing dicamba and triclopyr are well-known to travel downward into the soil, causing damage to trees as the roots absorb the chemical. Don’t apply these products over the rootzone of trees, which can extend outward from the trunk a distance equal to twice the tree’s height or more.
  • Spot-spray weeds when possible, instead of blanket-spraying the entire lawn.
  • Consider alternatives such as digging and smothering.
  • Use protective cardboard spray shields if spraying close to tree trunks or other desirable plants.
  • Farm-type chemicals might not be registered or safe for use on turfgrass. Much damage is caused by non-label use of farm herbicides on rural lawns.

There’s another less visible, but very serious way that herbicide injury is occurring in flower and vegetable gardens. Compost, made from herbicide-treated lawngrass, may injure plants when added to the soil. Manure can be tainted when horses and cattle eat feed that was treated with herbicides that pass through their digestive system.

All about the iris

Which perennial flower has the greatest range of colors? It’s iris, appropriately named after the goddess of the rainbow, available in white, yellow, blue, purple, lavender, orange, rust, brown, black and near-red. With a flower shape so elegant, it’s no wonder it’s called the poor man’s orchid.

Iris, blooming mostly in late May through June, are among the winter-hardy perennials that form the backbone combination of a good perennial garden. Even the sword-shaped blue-green leaves give foliage contrast when flowers fade.

Iris purchased in pots can be planted throughout the season, but August is the traditional time to dig and divide established plantings. Bareroot divisions are often available from garden centers now. Late August and early September are ideal for planting so roots become established before winter.

There are different species of iris, but the most common is called bearded or German iris (from the botanical name Iris germanica.) Bearded refers to the bushy “beards” on each of three drooping petal-like sepals, called falls. The true petals are called standards and are the upright part of the iris blossom.

Iris grow from thick underground stems called rhizomes that are sometimes mistakenly called iris “bulbs.” Leaves grow in a fan shape arising from the top of the rhizome, and the actual fibrous roots grow below.

As years pass and the iris grow, the center of the clump often becomes choked with old leafless rhizomes. This causes a reduction in flowering and makes iris more susceptible to insects and disease. It’s best to divide clumps every three to five years while they’re still healthy. Dig the entire clump and reset healthy divisions in its place.

To divide and transplant iris, lift the whole clump by gently prying them out of the soil with a spading fork, which works better than a shovel because it’s less likely to injure roots and rhizomes. With a sharp knife, separate the clumps into divisions, each having a single rhizome with a fan of leaves attached. Cut leaves back to about 4 or 5 inches, leaving a neatly trimmed fan shape. Discard leafless old rhizomes from the clump’s center.

Iris bloom best in full sun, with six hours a minimum. They also don’t tolerate “wet feet.” An easy way to ensure good drainage is to rake soil into a slight mound for an iris grouping. Heavy soil can be improved by adding peat moss or compost.

Dig a shallow hole for each division, leaving a ridge in the center. Place the rhizome over this ridge with the roots spread out on either side. Add soil over the roots and gently firm. Just barely cover the rhizome with soil.

Planting depth is very important. Whether using potted iris or bareroot divisions, the rhizome should be just barely covered. Remember, the actual roots that require soil contact are below the rhizome. It’s preferred if the rhizome is slightly visible at soil level after planting, especially in heavy clay. If planted too deeply, flowering is hindered and rhizomes can rot.

Space rhizomes 12 to 15 inches apart when planting groups. Arrange several iris of the same variety in drifts with the fans pointing in the same direction, or in a triangle with the “toes” pointing inward. Water well after planting. If iris rhizomes are plump and healthy with a nice fan of leaves, they will usually bloom the first year after replanting. Small, thin rhizomes might take until the second year.

Iris should be mulched for winter protection, especially if their location doesn’t receive snow cover. Apply 6 to 12 inches of leaves or straw in early November after cutting back leaves to about 4 inches.

Besides bearded iris, several other types merit greater use. Siberian iris are extremely hardy and produce a circular clump of deep green foliage up to 3 feet tall that’s attractive even when they’re not flowering. They can remain eight to 10 years without division.

Similar to Siberian iris, and also becoming increasingly popular, are Spuria iris. Forming an attractive circle of 4-foot-tall foliage, they create a nice accent at the rear of perennial beds.

Fielding questions

Question: Our tomato fruits have yellow tops. Can you tell me what causes this condition? They are the Goliath variety. — Bill Novacek

Answer: The disorder is called green shoulder, or yellow shoulder. It’s caused by a potassium deficiency in the fruit, causing failure of red pigment to develop in the shoulders, but instead they remain green or yellow. The tomato flesh in those areas is often hard, tough and not full-flavored.

It’s possible the soil has sufficient potassium, but the plant wasn’t able to absorb it, resulting in this disorder. The situation is often weather-related, frequently triggered by hot weather, not just when the fruit is ripening, but also when the fruit is first developing.

Some tomato varieties are more susceptible than others. It’s often worse when the shoulders of the fruit are exposed to the sun, heating up that portion of the tomato, which triggers the heat/potassium problem.

There’s not much that can be done about spells of hot weather. Adding a well-balanced fertilizer such as 10-10-10 at planting time, and again a month later can help assure that potassium levels are adequate. Soil tests are a wise idea to provide a garden soil nutrient baseline. North Dakota State University and University of Minnesota both provide soil testing of samples submitted.

Q: We are trying to get rid of dandelions in our lawn. We sprayed them about a week ago with Weed B Gon and they are yellowing. Is it best to remove the dying plants or just mow over them? Would all of the weed killer already be absorbed throughout the roots? —Lana H.

A: It’s probably better to leave the dandelions in place and eventually mow over them, after they are well-injured from the product you’ve applied. Removing the plants too early could leave a taproot in place that hasn’t absorbed enough product yet, increasing the chance of regrowth.

Dandelions are quite susceptible to products containing 2,4-D and the plants disintegrate fairly rapidly, so there’s usually no need to remove the dead plant. If mowing at the recommended height of three inches, the dandelions don’t get scalped off, giving the herbicide further chance to work, and the grass quickly conceals the diminishing plant.

Q: Just a note about your stinging nettle story previously in The Forum. While it’s true that nettles can be a real pain when one accidentally brushes up against the plant unknowingly, you really don’t have to use gloves to pull them out of the garden! — Richard Pemble, professor emeritus of biological sciences at Minnesota State University Moorhead

A: Thanks, Richard, for your fascinating response to my suggestion of using gloves to pull stinging nettle. Richard continues, “As youngsters we had to pull weeds, including nettles, from our family’s large strawberry patch. I remember the little rhyme my grandmother taught us about how nettles could be pulled without gloves. It went like this ... Tender handed stroke the nettle, and it stings you for your pains; grasp it like a man of metal, soft as silk it will remain!

“The explanation is that the stinging hairs of the nettle are like tiny hypodermic needles so if you gently brush against them you get a hypodermic shot of several different itching compounds. But, if you ‘grasp it like a man of metal’ you crush the hairs so they don’t inject the compounds.

“I’ve shown this over the years to many of my students in my field botany classes and they are amazed it works. I’ve also shown them if they do get stung a most effective cure for the itch is to crush the leaves of the touch-me-not on the itchy skin and almost immediately the itch is gone! Again, my students were really surprised that this works!”

I’m a firm believer in counting the day lost you don’t learn something new, and today I learned something new. Thanks Richard!

Last week’s questions and answers were:

Q: I listen to you on WDAY’s Lawn and Garden Radio on “The Jay Thomas Show.” We have a plant that started out growing in just a small area by our deck, but now it’s expanded and is creeping around to the side of the house. Do you know what it is, and is it some kind of weed? How do we get rid of it, if so? — J. Lanford.

A: The plant in your photo with the white and green variegated leaves has several common names, including bishop’s weed, goutweed and snow-on-the-mountain, although the latter is also used for several other plant types. The botanical name is Aegopdium podagraria ‘Variegatum.’

It’s sometimes planted as a low-growing groundcover along shaded house foundations, and does very well in low-light, moist sites. However, it can spread easily by its underground structures, and whether you consider it a useful groundcover or a weed depends on your situation and outlook. Any plant growing where you don’t want it is a generally accepted definition of a weed.

If the snow-on-the-mountain isn’t desired, it can take persistence to remove it. If the area is small, you can dig the plants, being sure to remove all the roots and underground pieces, as plants can readily sprout from them. Or you can apply a weed killer such as Weed-B-Gon or glyphosate (original Roundup), carefully applying the herbicides following label directions only to the plants you want to kill, as these products will kill “good” plants as well.

Q: What is the best care to have the short Stella d’Oro daylilies rebloom more through the summer? Do you cut off the “spent” flower pod stems? — Becky Brockberg.

A: There are several ways to keep Stella d’Oro daylilies blooming as much as possible. In May, spread granular 10-10-10 fertilizer around the base of the plants following label directions, and water well. This will give them good nutrition and energy to build the most flower buds possible.

Daylilies thrive best with better-than-average moisture, so water regularly throughout the growing season. Soak the soil well when watering, rather than frequent light sprinklings, because daylilies have a hefty root system. Do this about once a week, unless an inch of rain takes care of the weekly watering for you, which normally penetrates 6 to 8 inches into the soil. Mulches like shredded bark help conserve moisture.

As daylilies bloom, remove any seedpods that form after the flower petals shrivel. It’s not enough to just remove the withered petals, but if a round seedpod begins to form, remove it along with the attached stem. Deadheading, as it’s called, prevents the daylilies from wasting energy on seed production, and can prolong the period of time that Stellas flower and repeat.

When all the buds on a stalk are gone, stalks can be cut down to foliage level, which approves the appearance of the planting. By following these guidelines, all daylilies will bloom their best.

Q: I tried seeding a late crop of radishes, but as they started to grow all the little leaves have tiny holes, and the poor seedlings don’t look healthy. Any suggestions? — Linda S.

A: Radishes are fast-growing and crops to enjoy late in summer and early fall can be planted in August. It only takes about 45 days from seeding until harvest.

In the heat of summer, insects are active, and your radishes are likely being attacked by flea beetles and other insects. For best results, as soon as seedlings emerge apply insecticides like Sevin, Eight or organic spinosad, which will protect the tender seedlings from being weakened by insects. Adequate moisture also helps radishes grow quickly and vigorously.

A tomato plant shows leaf distortion on new growth due to herbicide exposure.
Iris, named after the goddess of the rainbow, have a wider range of available colors than any other perennial.
Siberian iris requires less frequent division than some other types of iris.
This tomato is suffering from a disorder called green shoulder or yellow shoulder. It’s caused by a potassium deficiency in the fruit, causing failure of red pigment to develop in the shoulders, but instead they remain green or yellow. The tomato flesh in those areas is often hard, tough and not full-flavored.
A reader asks what this plant is that’s expanding and creeping around to the side of their house.