The challenge of eliminating unwanted grass around trees without harming the woody plants is common, but selective control is achievable due to fundamental physiological differences between grass and trees. Understanding these biological distinctions allows for the precise use of chemical and physical methods that target the grass while ensuring the health and long-term survival of established trees.
The Biological Basis of Selectivity
The ability to kill grass while sparing a tree stems from the division of flowering plants into two major groups: monocotyledons (monocots) and dicotyledons (dicots). Grasses are monocots, characterized by a single seed leaf, parallel leaf veins, and a fibrous root system. Trees, conversely, are dicots, typically featuring two seed leaves and a taproot system.
These structural differences extend to the cellular level, particularly in how they synthesize lipids. The enzyme acetyl-CoA carboxylase (ACCase) is responsible for the first step in the creation of fatty acids, which are essential for building cell membranes. Grasses possess a specific type of ACCase that is highly sensitive to certain herbicides. Dicot plants, including trees, have a different, less-sensitive form of the enzyme, providing them with a natural tolerance.
Another protective feature of established trees is the bark, which prevents foliar-applied herbicides from being absorbed into the tree’s vascular system. Grasses, with their exposed, thin foliage, readily absorb the chemicals. This combination of enzyme tolerance and physical protection forms the foundation for selective grass control.
Chemical Solutions Selective Herbicides
The most effective chemical method for selective grass removal involves specialized herbicides known as ACCase inhibitors. Active ingredients like Fluazifop-P-butyl and Sethoxydim target the unique ACCase enzyme found in grasses. When absorbed by the grass leaves, these chemicals halt the production of lipids, causing cell membrane destruction and eventual death of the grass.
These grass-selective herbicides are generally safe to spray near trees because the woody plants either possess the resistant form of the ACCase enzyme or rapidly metabolize the chemical into an inactive form. For preventing new grass growth, pre-emergent herbicides are another option. Products containing active ingredients like Prodiamine or Dithiopyr create a barrier in the soil that inhibits seed germination but are not significantly absorbed by the established, deep root systems of trees.
It is important to avoid broad-spectrum herbicides and those known for soil persistence, as they can severely damage or kill trees. Herbicides containing ingredients like Imazapyr, Prometon, or high concentrations of Dicamba should be avoided in the tree’s root zone. These water-soluble chemicals can be absorbed by the tree’s roots, leading to crown dieback, abnormal foliage growth, or death, especially in younger or stressed trees.
Non-Chemical and Physical Methods
For those preferring to avoid herbicides, deep mulching is an excellent, long-term solution for grass control around tree bases. Applying a layer of organic mulch, such as wood chips, four to six inches deep effectively smothers existing grass and prevents new seeds from germinating. It is important to utilize the “donut” method, keeping the mulch several inches away from the tree trunk to prevent moisture buildup and disease.
Manual removal, while labor-intensive, ensures absolute selectivity and zero chemical exposure. Hand-pulling or using a hoe near the tree trunk is the safest approach, as it avoids damaging shallow surface roots that may be present. Physical barriers, such as metal or plastic edging installed at the soil line, can also prevent grass runners from creeping back into the cleared area.
Soil Solarization
On a larger scale, soil solarization can be used to clear an area before planting or mulching near established trees. This process involves covering the area with clear plastic sheeting during the hottest months of the year, trapping solar heat to kill weeds and grass seeds. This method requires four to six weeks of high temperatures and must be done carefully to avoid excessively heating the tree’s immediate root zone.
Safe Application Procedures Near Trees
Careful application technique is necessary to prevent accidental tree damage regardless of the method chosen. When using selective herbicides, the greatest risk is spray drift, where fine mist is carried by the wind onto the tree’s foliage. To mitigate this, applicators should use low-pressure sprayers and only apply chemicals on calm days with minimal wind.
Understanding the tree’s root zone is paramount, as roots often extend far beyond the canopy’s edge. Non-selective or soil-active herbicides should never be applied within this extended root area. Protecting the trunk is also important, especially on young trees with thin or green bark, as this tissue can absorb chemicals directly into the vascular system.
For any application, always consult the product label to confirm it is safe for use around woody ornamentals and follow all dilution and application rate instructions precisely. Misapplication, such as oversaturating the soil or applying too high a concentration, can allow selective chemicals to be taken up by shallow tree roots, resulting in stress or injury.