Plant Lovers’ Almanac: Questions about weeds

By Denise Ellsworth
Special to the Beacon Journal

Published: January 17, 2014 – 12:47 PM

 Hairy bittercress

Hairy bittercress is one of the most common cool-season weeds in our landscapes. (Denise Ellsworth/Ohio State University)

Q: In spring, my garden beds are full of a tiny weed with white flowers. The leaves are divided, and grow in a rosette. I never used to see them, but now see them everywhere. What is it?

A: Hairy bittercress is the name of this tiny member of the mustard family. Rarely seen about a decade ago, it is now one of the most common cool-season weeds in our landscapes. Plants and seeds are often introduced to the landscape as hitchhikers in container plants.

Hairy bittercress begins as a low-growing rosette, followed by a flower stalk, producing small white flowers and eventually growing to anywhere from 1 to 8 inches. Flowering begins even as snow covers the ground, and continues through spring. Seed capsules explode when ripe, dispersing seeds into surrounding soils.

Bittercress can be easily removed by pulling, just be sure to remove plants before seed capsules have matured to avoid further spread.

Q: I don’t understand the idea of winter weeds. Why do I have to worry about weeds when there’s snow on the ground?

A: Although a dusting of snow is enough to hide the evidence, winter weeds are hard at work right now in the garden. Bittercress, groundsel and annual bluegrass are three pesky winter weeds. Unaffected by the cold, they rest under snow while bitter winds blow, waiting for the slightest rise in temperature to flower and produce seed.

Take advantage of the next winter thaw to pluck these pesky weeds out of landscape beds before they can flower and set more seed. Even though they are small, winter weeds compete with desirable crops for soil nutrients.

Q: My lawn is free from dandelions, and it greens up every year, but I’m not happy with the texture of the grass. I have coarse grass blades in patches that want to lay down instead of growing upright. These patches are darker green, and seems to hold up better to summer stress. Every year I have more and more of this grass taking over the lawn. What can I do to even out my grass?

A: The weed you describe sounds like tall fescue. This weedy grass is not desirable in the lawn because of the characteristics you’ve described: it’s coarse, it grows erratically, and it is often a deeper shade of green than the rest of the turf, making it stand out. Unfortunately, there is no easy method for dealing with tall fescue in the lawn.

In the case of dandelions and other broadleaf weeds, there are many herbicides available which will selectively kill those weeds while leaving the turf grass unaffected. Since tall fescue is a grass, there is no product that can kill it while leaving the rest of the lawn intact. Some homeowners choose to selectively kill patches of tall fescue with a nonselective herbicide, then reseed the area.

If a lawn is heavily overgrown with tall fescue, the only real solution is to tear up the lawn and start over. This may be more cost and effort than you’re willing to put into the lawn; only you can evaluate how unsightly the lawn is, and whether it’s worth the investment to rid it of tall fescue.

Q: How can I control aegopodium in my flower beds? It’s taking over.

A: Also known as bishop’s weed, aegopodium is a creeping ground cover that can quickly take over entire flower beds. The variegated bishop’s weed is less aggressive than the green ground cover. Once the green bishop’s weed has crept into flower beds, it can be very difficult to eradicate. Pulling the plant isn’t enough, since new sprouts are quickly produced off the roots. Digging out the plant, root and all, or treatment with a nonselective herbicide can give good results. Both practices may need to be repeated to eliminate the planting.

In severe infestations, desirable perennials may need to be dug out, and the bishop’s weed teased out of their roots. After the entire bed is killed with a nonselective herbicide, the perennials can be replanted. Check the bed frequently, and treat any emerging sprouts of bishop’s weed with a nonselective herbicide.

Q: We have “onion grass” spreading throughout the yard. At first it was only a few tufts, but each year the amount increases. We have been told that there is nothing that can get rid of it. It is impossible to pull up as leaving only one tiny bulb underground will continue its spread. Is there any way to rid our yard of this pesky weed?

A: Wild onion and wild garlic are very tough weeds. Their texture and rapid growth rate make them unsightly in the lawn. With repeated applications, a broadleaf herbicide with 2,4-D may offer some control.

The smooth leaf texture makes it difficult for herbicides to adhere to the plant, so a spreader-sticker product may be necessary in conjunction with the herbicide. In isolated areas, a nonselective herbicide such as Roundup may be used to kill the weed. The area will then have to be replanted with grass.

Q: I’ve moved into a new house, and some of my beds are infested with Canada thistle. The previous owner said she sprayed with Roundup, but it didn’t kill the thistle. Is Canada thistle resistant to Roundup? What should I do?

A: Canada thistle is one of the worst perennial weeds to deal with in landscape beds, right up there with quack grass. What makes these two weeds so difficult to manage is the fact that they spread by underground stems called rhizomes. These rhizomes make the weed function as one huge colony, so that instead of dealing with an individual plant everywhere you see a sprout, you’re actually dealing with an underground monster.

One application of glyphosate, the active ingredient in Roundup, may have little effect on a large Canada thistle colony. The weed isn’t resistant, but the plant has mammoth reserves and can continue to sprout. With persistent application, however, glyphosate can effectively take out Canada thistle. This will likely take several applications and several months.

If the Canada thistle is growing right in among desirable plants, your job is much more challenging. Drift or overspray of the herbicide will kill any desirable plants, so great care must be taken to protect surrounding plants.

An effective method used by gardeners and professionals is to use a chemical-resistant glove, available at hardware stores, covered by a cotton garden glove. Carefully apply the herbicide to the cotton glove, then rub the herbicide on to the weeds. Wait a few weeks to see where new sprouts emerge, then repeat this procedure until all weeds have been killed.

Other options are available to control Canada thistle, but pulling is not one of them. Pulling Canada thistle is like harvesting broccoli; everywhere you cut a piece off, small new sprouts will emerge.

Large areas can be covered with plastic, thick newspaper or landscape fabric to smother the weed. This may take some time and persistence as new sprouts may emerge in different locations.

Learn what small weed seedlings look like so that you’re sure to eliminate Canada thistle seedlings from the garden as soon as they’re seen. This will hopefully prevent any future weed nightmares.

Q: Creeping Charlie has moved into my lawn. How can I get rid of it?

A: Ground ivy, also known as creeping Charlie, is a vigorous perennial weed of landscape beds and turf areas. Ground ivy’s creeping stems take root at leaf nodes, making this weed difficult to remove by hand, especially once the weed has moved into lawn areas. Crushed stems and leaves emit a disagreeable, mint-like odor.

Introduced as a gardening plant for hanging baskets, ground ivy escaped from cultivation and is now a common landscape weed. Small pieces, cut when mowing, can take root, spreading ground ivy throughout the lawn and garden.

A broadleaf herbicide, applied in fall, can help to control this pest in lawn areas, although repeated applications may be necessary. Hand pulling or spot herbicide applications work best in landscape beds.

Learn more about weeds at the Holden Arboretum from 1 to 3 p.m. Feb. 8. I’ll be teaching this class to share some of my favorite weeds. We’ll focus on identification, natural history and management strategies for some of the most common weeds in Ohio. Registration is $15. Contact the Holden Arboretum for more information or to register at www.holdenarb.org, or 440-946-4400.

Denise Ellsworth directs the honeybee and native pollinator education program for the Ohio State University. If you have questions about caring for your garden, contact her at 330-263-3700 or click on the Ask Denise link on her blog at www.osugarden.com.

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