Weed Control in Ornamental Beds

Published: June 9, 2016

Someone once mused, “It takes a lot of water to grow a garden, much in the form of perspiration”. Quite likely, that remark was made after pulling or hoeing weeds on a warm, humid summer day. Along with death and taxes, weeds are an inevitability in the life of a gardener.

One of the first steps in effective weed control is to “know the enemy”. In general, weeds can be grouped into three categories: grasses, sedges and broadleaved weeds. A knowledge of the life cycle (e.g. annual versus perennial), reproductive habit, rate of spread, etc. of a weed is helpful when attempting to control it. A number of good pictorial guides are available on the internet to help with weed species identification.

The majority of weeds that plague ornamental plantings are annuals and emerge from seeds in the soil. For the average flower garden where a wide range of flowers are planted, weed control via mulching should be considered. Mulches control weeds by depriving them of light. They provide an easy, safe and “environmentally-friendly” way to accomplish weed control of an entire bed planted with a number of different ornamental species. To control weeds using mulch, it should be applied uniformly to a depth of at least one to two inches. Mulches should be from materials dense enough so they are not easily blown away. Pine bark, leaf mold or pine needles represent good choices.

In more permanent beds, landscape (weed control) fabric may be placed beneath the mulch to provide even more effective weed control. After carefully placing the fabric over the bed to be planted, cut an “H” into the fabric where plants are to be placed. After planting, fold the flaps of the H back toward the plant, taking care not to allow any disturbed soil to remain on top the fabric for fear of weed seed contamination. After plants have been installed, the fabric is ready to be covered with an organic mulch.

Perennial weeds such as quack grass, horse nettle and field bindweed not removed before planting a bed may push their way through even deep mulches. Using the afore-mentioned landscape fabric beneath mulches with help to deter them. However, some might still emerge through holes created in the fabric when it was put in place or when the mulch was applied to cover it. At such times, either hand weeding or the use of some type of herbicide may be needed to control them.

Unfortunately, there is no safe and effective herbicide that controls all types of weeds mixed among garden plants without damaging the garden plants also. The use of a nonselective herbicides (i.e. one that kills every plant it contacts) very carefully applied to the weeds often is the only alternative to hand removal. Perhaps the most widely used nonselective herbicide today is glyphosate, which is available in several different trade names (e.g. RoundUp®). For those who prefer organic weed control, horticultural vinegar (20% acetic acid) is an option worth consideration as a non-selective herbicide.

When nonselective herbicides are used as “spot treatments” among desirable plants, extreme care must be taken during application to avoid contacting desirable plants. First, keep the sprayer pressure low and use a coarse spray to make drift less likely. Apply herbicides when the air is very still. Early morning or late even are good times. Finally, a shield of some type placed between target weeds and desirable plants is a good idea. If the shield is moved during the treatment of a large bed, make certain to keep the same side toward target weeds, since herbicide accumulated on the shield can damage desirable plants.

Wick applicators are a novel and relative new way to safely apply nonselective herbicides. These devices feature materials such as a sponge that is kept continually moist with herbicide as it passes from a small bottle atop the applicator through its hollow handle. The herbicide-laden sponge is rubbed against the leaves of target weeds while desirable plants are avoided. Spray drift thus is not a problem.

Some of the most difficult and invasive weeds to control in ornamental beds are annual grasses, such as crabgrass. Several pre-emergent herbicides that may be used among garden plants to prevent the germination of annual weed seeds are available commercially. Examples of materials available for this purpose include trifluralin (Preen®), DCPA (Dacthal®), oryzalin (Surflan®), pendimethalin (Halts®) and isoxaben (Gallery®). Unfortunately, some of the previous are not readily available to home gardeners, since their primary use is by professional applicators. In all cases, careful reading of the herbicide label is important, since not all herbicides can be used among all ornamental plants and certain herbicides require special application techniques. For example, trifluralin must be soil incorporated within 24 hours after application.

Among those pre-emergent chemicals more readily available to homeowners, DCPA and trifluralin have been two of the most widely used in ornamental beds of annuals and perennials. As in the case of all pre-emergent herbicides, they must be applied to the garden after ornamental plants are established and before weed seedlings have emerged. Pre-emergent herbicides act by forming a chemical barrier that prevents weed seeds from germinating and emerging. If the  barrier is disrupted in any way, the herbicidal action in that immediate area is lost.

There are relatively few selective herbicides (i.e. ones that kill only certain types of plants) that can be used in ornamental beds to control grasses after the grasses have emerged. Classified as post-emergent herbicides, they are applied uniformly across the planting and kill only grasses, leaving broadleaved plants unharmed. Examples include, sethoxydim (Poast®), fluazifop (Fusilade II®), fenoxaprop (Acclaim®) and clethodim (Envoy®). Check the labels of each herbicide before using for labeled bedding plants, susceptible weeds and any precautions that should be observed.

Although herbicides control a wide array of weeds, none is able to control them all. Therefore, total weed elimination through the use of chemicals probably is not a realistic goal for gardeners. However, even though herbicides may not control all weeds, they do control a large number of them and can be real “labor savers” for many gardeners.