FAQ: How do I add compost to mature trees and shrubs?
Q: My established plantings need some compost, but won’t all that digging hurt the root systems?
A: Think “massage” and not “excavation” to add compost to mature trees and shrubs. It’s hard to overdo it when using compost and as little as 1/8 of an inch can net visible results.
While there are many ways to apply this soil amendment, these are among the easiest:
For mulched trees
Scrape away mulch and, using the spreading method of choice, apply up to 2 inches of compost out to the drip line (the widest point of the tree canopy).
Rake lightly to even out the surface. But no need to dig in. This application method is called “top dressing” for a reason. Happily, over the coming weeks and months, Mother Nature will take care of soil incorporation for you. Simply reapply the mulch once the area has been covered with compost.
If some of the area under the tree is grassed
Gently work up to 1/2 inch of compost into the turf with a rake or broom. You are giving the earth a gentle back scratch, not plowing. But it’s okay to scratch a little harder where you have bare spots or fairy rings, because compost has been known to help solve some of these types of yard maintenance issues.
A light sprinkle with the hose or irrigation system can also help move compost from the grassy surface to the soil.
But keep the water use to a minimum. If the application area gets too wet, you’ll just lose all that compost to runoff, wasting both the water and the compost while becoming a contamination source for receiving waters or the stormwater system.
For shrubs and planting beds
Remove mulch, lay down up to 2 inches of compost, and remulch. As an alternative, if you want to apply 3 or 4 inches of compost, it can serve as mulch.
Just be careful to avoid piling compost up around woody stems and tree trunks. This practice invites insects and could, eventually, kill the plant.