Controlling Weeds in Perennial Beds Back »

Quackgrass along the edge of a lawn.

Perennial Beds: Weed Management Tips

Quackgrass and bromegrass are often two of the worst weeds in perennial flower gardens and in perennial vegetables like asparagus. Kentucky bluegrass and other lawn grasses can also be a problem. Perennial broadleaf weeds are also other common weeds among flowers and perennial vegetables. They are both aggressive plants that can grow among other plants so tightly that it is difficult to get them out. In addition, plants like quackgrass, bromegrass, creeping jenny and others produce creeping, underground stems called rhizomes that allow the plant to spread a foot or more in a season, producing new plants as they grow. You will be able to see those when you dig the plants up. They are usually white and about 1/8” in diameter. Note that quackgrass is entirely different from crabgrass. Crabgrass is an annual that only germinates from seed later in the spring and it does not produce rhizomes. It will creep along the ground as it becomes more mature, later in the summer, but it does so by horizontal stems on the surface of the ground. Gardeners frequently ask how to get rid of these tough weeds, so here are a few tips.

Quack grass and brome grass spread by underground stems called rhizomes.

Tip 1: Prevent weeds by planning ahead.

First, prevention is usually the best way to deal with a weed problem. This is the place to start if you are thinking of establishing a new garden somewhere. Check the site to see if quack, bromegrass, or other perennial weeds are already established there. If they are, you are going to want to try to control them with repeated cultivation or better yet, a couple applications of a non-selective, non-residual herbicide like glyphosate. Wait until the vegetation has gotten several inches tall, then treat it according to label directions. If you are doing this during cool, spring conditions, try to wait about a week before tilling up the site after treatment. You only have to wait a couple of days if you are doing this later in the season in warm weather. If you can, wait a month or so and repeat the process to get rid of as many of the perennial weeds and newly sprouted annual weeds as you can before planting.

Controlling perennial weeds in an asparagus patch can be difficult.

Tip 2: Exchange plants carefully.

The second aspect of prevention is to be careful not to introduce new weeds into your garden bed. Gardeners love to exchange plants with each other. However, often those plants have unwelcome weeds growing along with the desirable plants. So, take a little time to carefully remove any weeds that might be along for the ride into your new garden bed. Plants purchased from a greenhouse, nursery or discount outlet store might also contain unwanted weeds, so always check before planting. Also, remember the definition of a weed – a plant growing out of place. Many gardeners love to share plants with friend but often the recipients of those plants later wish they had never planted them as the plants have taken over their garden. So, take a look at the plants growing in your friend’s yard first to see if if this might be an aggressive plant that might end up becoming a weed later on for you in your garden.

Weedy globe thistle.

Tip 3: Use edging around beds.

Another way to help reduce the chances of having weeds creep into your planting beds is to use exclusion, usually in the form of some type of edging. Most people use a black plastic edging but other kinds are also available like brick pavers or even steel, but these can be rather expensive. It’s easiest to install the edging before you begin planting so you don’t have to worry about damaging newly installed plants. Most plastic edging can be installed using a flat garden spade or better yet, rent a power edger to cut the small trench along the edge of your bed to install the edging.

Crabgrass will often creep into the edge of a flower bed.

Tip 4: Consider a precision herbicide application.

If you are dealing with an established perennial bed, using an herbicide like glyphosate gets much more difficult because you have a much greater chance of misapplication, getting the spray on some of the plants that you want to keep. You might be able to do a little spot spraying but you may still get damage if you are not extremely careful. Use a coarse spray and only when there is as little wind as possible. Cover desirable plants with buckets or other containers if possible to protect them from the spray and leave the buckets in place until the spray has dried. There is another option in using some of the spray foam formulations of glyphosate. These products shoot out a narrow stream of the herbicide solution allowing you to pinpoint where you apply it, allowing you almost surgical precision during application. But, it does take some practice to hit what you are aiming for so still be very careful. Keep some water handy so you can immediately wash off any leaves of desirable plants that accidentally get hit with the foam.

Some spray foam formulations allow you to make precision applications of herbicide around desirable plants.

Tip 5: Review herbicide labels closely before using.

Gardeners may be tempted to use various broadleaf herbicides to kill broadleaf weeds like dandelions, thistles or creeping jenny. But, be sure to read the label. I would probably never suggest using any of these herbicides in a mixed perennial flower bed because so many of the flowers we grow will be damaged by phenoxy herbicides like 2,4-D, Banvel or Trimec. Even trying to spot spray would be very risky because of the likelihood of drift or volatilization of the product that would likely damage nearby plants. Some of these products are labeled for use on asparagus but the directions must be followed closely. Never use any kinds of herbicides that say they are ground-clearing products that kill everything and will prevent weeds from coming back for several months. They are generally non-selective herbicides that will seriously damage your perennials or kill them, just like they would weeds. Garden weed preventors, like Preen for example, can be used, as long as the plants you are using them around are listed on the label. However, keep in mind that these kinds of herbicides prevent seeds from germinating. They will generally not have any effect on already-growing perennial or annual weeds.

Creeping jenny infestation.

Dandelion among daylily plants.

Tip 6: Spread an organic mulch.

Using organic mulches is another great way to help reduce the likelihood of weed problems developing around your perennial plants. I prefer using an organic mulch like shredded hardwood bark. It tends to stay in place, looks attractive, may be available in a variety of colors, helps to conserve moisture and will help to prevent new weeds from getting established by covering the soil. Bark or wood chips can also be used but may be washed off beds during heavy rains more easily than shredded bark. Apply the mulch about 2-3” deep to be effective. You do not need to apply a weed barrier fabric first, and I generally advise against using black plastic that can interfere with mater and air movement into the soil. As the mulch decomposes, it will add organic matter to the soil too. Even though rock mulch is quite popular, I usually advise against using it as a mulch. It is heavy and difficult to work with. It tends to heat up during the summer time causing heat stress to most perennial plants and is very difficult to remove if you decide to change a planting bed later on. While many think it is a low maintenance alternative, weeds frequently grow in rock mulch as the weed seeds are blown in between the rocks. Black plastic is often used in conjunction with the rock, which is not good for the perennials either. if you are really interested in having healthy perennial plants, an organic mulch is a much better option.

Peony with weeds.

Tip 7: Dig the weeds out by hand.

Generally digging out weeds by hand can be very difficult and often just results in causing more plants to spring up as you cut rhizomes in the process. However, at this time of year, the newest plants, which are sprouting up from nodes along the rhizomes, are just beginning to form their roots. Therefore, they are easier to dig now, than they will be any time during the rest of the season. Use a sturdy trowel to dig a few inches back from the new leaves you see growing from the ground. Lift up the soil and plant to loosen it from the ground. Then, rather than just jerking it out, see if you can loosen up more of the connecting rhizome and additional plants. You might be surprised as to how far one individual, but connected plant, can reach. If you dig carefully, you can get fairly close to established perennials and get rid of much of the quack or brome.

Quack grass and brome grass are easier to dig early in the spring before their new root systems become established.

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