Weed control is an age-old subject, one that green industry companies have struggled with for decades. Perhaps the biggest driver of this issue is the sheer number of weed seeds in the soil. Soil and weed scientists tell us that a conservative number is 10,000 weed seeds per cubic foot of soil volume. This is hard to imagine, yet all you have to do is drive by a vacant lot, even one that has been recently abandoned, and even in dry years the space is covered with a mass of weed growth.
Weeds compete with desirable plants for water and nutrients. In most cases they compete very well. Due to their location on the soil surface, they get first crack at water and nutrients. If that weren’t enough, most of us, and more importantly your customers, consider them to be downright ugly. Unless you’re working for a fan of scruffy, gnarly water and nutrient robbers, they’ve got to go.
Turf — another weed
The definition of a weed is “a plant growing out of place” or “a plant that is located where it’s not wanted.” If turfgrass is growing up to or near the trunk, it’s considered a weed. Why? For the same reason broadleaf weeds such as plantain or ground ivy are considered to be weeds: they compete with the tree for nutrients and water. In addition, they’re not natural. When was the last time you saw turf thriving alongside riverbank trees? The two just don’t belong together.
The solution is simple: separate trees and turf. If your clients want both, that’s fine; they just can’t have them together. Suggest to them that they take a cue from Mother Nature and establish a landscape that supports both lawns and woody plants, one that uses gradation and plants under plants as landscape design techniques. Starting with a shade or framing tree, locate understory trees underneath, shrubs nearby, and sun/shade-adapted perennials or ground covers filling in the gaps. At the edge of the ornamentals, turf can thrive when designed in natural curved lines.
Good weed control methods
Fortunately, there are several good options in the endeavor of controlling weed growth under or near trees and shrubs.
- Mulch – Application of mulch is a good step in the replication of a natural landscape. Begin by applying 2 to 3 inches of wood chips or stump grindings starting 3 inches away from the tree trunk root flare. These materials facilitate the movement of water and air through the profile. From year to year, aim to keep the mulch layer in the moderate level, striving to avoid thin or thick layers and “mulch volcanoes.” Thin layers are inadequate in weed prevention and water retention, while thick layers tend to decrease air movement and act as a thatch roof, shedding water instead of allowing it to percolate downward.
Placement of mulch at or on the tree trunk is problematic in that it keeps the bark more moist than Mother Nature intended and can lead to crown rot or basal root flare decay. Bruce Friedrich with the Bartlett Tree Research Company puts it this way, “Mulch is a root treatment, not a trunk treatment.” A good rule of thumb with mulch is to extend it to the drip line.
- Ground covers and perennials – If the customer insists on something other than mulch, a good option is a ground cover/perennial mix. The basic idea is to shoehorn in shade-adapted ground covers and perennials in between the flare roots. Perennials or ground covers under trees is a better choice than annuals in that annuals are planted yearly, so the risk of damaging tree roots is increased. Shoehorning should be done without adding soil over the top of the roots for a planting medium. Adding even a few inches of soil over the tree roots can kill the tree because it puts the shallow feeder roots farther away from needed moisture and oxygen.
- Preemergence herbicides – This group of herbicides is widely used on lawns and landscape beds to prevent the growth of annual weeds such as crabgrass, pigweed and yellow nutsedge. They generally have limited root activity and are considered fairly safe around most woody plant species. Injury can occur if these herbicides are applied at exceptionally high rates or if drift from spray contacts wet foliage. Herbicides like S-metolachlor (Pennant Magnum) can be applied over the top of plants as long as it is washed off from foliage following application.
- Soil inactive herbicides – These are often the nonselective, systemic varieties. Care must be taken to avoid herbicide contact with the tree trunk or any green tissues, including suckers. Glyphosate (Roundup Pro) and glufosinate-ammonium (Finale) are systemics and are easily translocated within the plant tissue. Protecting the trunk with barriers or plastic can help minimize injury from nonselective systemic herbicides.
- Another option would be to use contact herbicides. They affect only the material they directly contact and are not translocated. Diquat (Reward) and pelargonic acid (Scythe) are a couple of examples. They are most effective on small or immature weeds. As weeds increase in size and maturity, these products lose their effectiveness.
Note: when using any type of herbicide to control weeds, granular formulations are the safest near trees because there is a lower probability of drift or injury when compared to spray formulations.
- Physical removal – Especially during conversion from turf to a healthy soil covering, physical removal of existing weeds is an important procedure. Usually reserved for new hires, this step often needs to be repeated several times in order to achieve the objective especially when “trash trees,” such as mulberry and Siberian elm, are growing near the trunk of a desirable tree.
Bad weed control methods
There is a right way and a wrong way to do anything, including weed control around trees.
- Tree surrounds – A southern gentleman once described a problem he was having with a “trace-around.” It took several attempts and back and forth clarifying comments to determine that he was talking about a tree surround, or planter box. Although well intentioned in terms of the primary purpose, which is to replace weed growth and thinning turf, these hardscape features are actually detrimental to the landscape. By their very nature, they place soil in constant contact with the tree trunk, keeping it moister than Mother Nature intended. They’re just not natural.
- Spraying with soil active herbicides – This group often includes herbicides with soil residual times in excess of one month or more. Bare ground soil sterilants like prometon (Pramitol) or imazapyr and diuron (Sahara) are extremely toxic to woody plants, and their misapplication is one of the leading reasons for tree death. Soil sterilants can have a soil residual of more than six months, are water soluble, and can move easily via leaching or runoff. They are readily absorbed by trees roots and translocated throughout the crown, so when working with bare ground areas be sure to study drainage patterns and the proximity of woody vegetation to the treatment areas.
Another widely used soil active herbicide is dicamba. This product is commonly used to control weeds not easily controlled by 2,4-D products. It has a soil residual of three months or more and is highly mobile and can also leach readily. Injury symptoms are most obvious on new leaves, resulting in cupping, curling and parallel leaf venation. Activated charcoal may minimize injury if applied immediately after an herbicide misapplication.
- Installing plastic or woven weed barriers – A well-intended concept of the ’80s, placement of black plastic under wood chip mulch causes a great reduction in oxygen exchange and water infiltration, which are essential for healthy tree growth. This misguided approach evolved into the use of landscaping fabric that allowed for limited air movement and water penetration, but was not an effective weed control material. In some cases, as tree litter was deposited under the tree weeds actually started growing in the fabric, adding to the problem. In most situations, a layer of rock is also a poor choice because it increases soil temperature and tends to hold less soil moisture near the roots.
Editor’s note: This article was originally published in October 2013 and has been updated.