Lawn grasses are very competitive with trees and shrubs, especially young stock, for nutrients and water. Grass roots will slow down and restrict the growth of tree and shrub roots that are planted in a lawn. Some grasses produce substances that are antagonistic (alleopathic compounds) to other plants that are planted in the grasses’ sphere of influence. Older, established trees and shrubs can survive and grow, although at a reduced rate, in a lawn environment as they are able to withstand the competition of the lawn grasses.
When planting trees and shrubs in a lawn area, a grass-free zone should be maintained for about one foot outside the original rootball for at least a year while the new plant becomes established in its new surroundings. If you want your tree or shrub to thrive, the grass-free zone should be maintained out to the drip line of the plant as it grows.