Grass and other vegetation competes for water and nutrients with newly established trees. Control of this vegetation is recommended and is usually best achieved through mulch or herbicide application. Credit: Priya Jaishanker
Weeds can compete with trees planted in urban areas, robbing them of vital water and nutrients. If competition from weeds is strong enough, trees can experience lengthened establishment periods, which means that they will require more maintenance over a longer time period than if the weeds were not present. Many people choose to eliminate weeds using mechanical means, such as mowers and string trimmers, but these methods can lead to severe damage of young trees. For example, if the blade of a mower or the string of a trimmer gets too close to the base of a tree, it can damage or remove the bark. Vital nutrient and water carrying components of the tree lie just below the bark. As a result, bark injury caused by mowers and string trimmers can result in compromised structural integrity and tree health. As well, bark injuries provide an entry point for diseases and fungi, leading to decay and health problems. These injury areas, though repaired by the tree over time, will always be a weak point on the tree and may cause future property damage during a storm.
Mulching and applying herbicides are safer options for controlling weeds while maintaining the health of the tree. Mulch prevents weeds from absorbing sunlight and suppresses their growth. It also returns nutrients and organic material to the soil. Herbicides, such as glyphosate (RoundUp), can be very effective in killing weeds and have low toxicity to humans, but they require additional planning and precautions before use. Personal protective equipment, including long sleeves and pants, gloves, and protective eyewear, should always be worn when using herbicides to prevent contact with the skin. Glyphosate is foliar active, meaning that it can damage any plant that has green foliage and undergoes photosynthesis; because of this, it is important to not let the herbicide splash or spray on any off-target vegetation. Spraying on low-wind days prevents the glyphosate from being picked up by the wind and drifting on to nearby plants. Adjusting the applicator to only spray larger droplets and holding the nozzle close to the ground helps to reduce drift as well. Applying glyphosate directly to the bark of the tree should also be avoided, as this can allow the herbicide to enter the tree through wounds in the bark and cause damage. Placing a sheet of cardboard against the base of tree and over any exposed roots can prevent the herbicide from coming into contact with the bark. Herbicides should be applied evenly in a circle extending four to five feet away from the base of the tree.
Watch the video Controlling Weeds around Urban Landscapes to learn more. This video is part of a series, Trees for Energy Conservation, developed by the Southern Regional Extension Forestry and Virginia Tech Cooperative Extension with funding provided by a National Urban and Community Forestry Advisory Council grant.
Written by: Connor McDonald, Southern Regional Extension Forestry