For trees and other plants, sometimes it’s just too hard. The soil, that is. Compacted soil, packed down so tight that roots can hardly move or get air or water, is one of the most common causes of plant problems.
“If the soil particles are pressed together, all the air is squeezed out,” says Sharon Yiesla, Plant Clinic assistant at The Morton Arboretum. “The roots don’t have enough oxygen to function. And the spaces are so small that it’s too hard for roots to penetrate so they can grow.”
In healthy soil, the particles are in clumps, with gaps between the clumps for air and water to flow, Yiesla says. When those clumps get broken down, the gaps fill in. Air and water can’t get through the soil to reach the roots.
Plants in compacted soil may have their growth stunted. Lawns won’t thrive. Trees with compacted soil around their roots may grow slowly or start to die back at the top of the canopy.
The condition is most obvious in lawns, but plants will struggle in any place that has compacted soil, Yiesla says. Soil is often packed down around the base of trees by foot traffic or playing.
In lawns, core-aerating once a year in spring or fall will help, and early autumn is a good time to do it. In general, though, compacted soil is hard to repair, so it’s better to avoid packing the soil down in the first place. Here are some tips for avoiding soil compaction:
Stay off wet soil: When soil is wet, it's easily compacted. After any rainfall, let the soil dry out thoroughly before playing or working on the lawn or garden, Yiesla says.
Use stepping stones: Flat stones in garden beds will give you places to set your feet and distribute your weight more widely so you compact soil less.
Make paths: Grass can’t grow in hard-packed soil, so if there is a route in your yard where people constantly walk, make it a path.
Plan for construction: Plan a remodeling or construction project so that heavy objects are stored on paved surfaces, not on soil or in the root zones of trees. If necessary, lay thick sheets of plywood to distribute the weight of foot traffic or heavy equipment.
Mulch around trees: A wide ring of wood-chip mulch, 3 or 4 inches deep, around the trunk of a tree will discourage people from walking or playing in its root zone and packing down the soil.