I have some large and old spruce trees in my backyard. A lot of shade is produced by these behemoths and so nothing can survive below them expect shade loving plants like hostas. As growing grass is mission impossible, I was thinking of laying a flagstone patio with lots of drainage between them with pebbles. My question is whether this will be bad for my spruce trees. I don’t want to do anything that endangers these trees as I like them and they would for sure cost me a fortune to remove if they die.
You are correct to be worried about damaging your spruce trees by putting a flagstone patio below them as this could potentially harm your trees. The weight and compaction of the soil caused by the flagstones could restrict the flow of air, water and nutrients to the roots of the trees. As well, as spruce trees have a shallow wide spreading root system which extend beyond the tree’s canopy; the necessary disturbance caused by excavation and compaction to install the patio may damage this root system causing harm to your trees. However there are some techniques which could mitigate the potential harm, for example using permeable pavers; widely spacing the impermeable flagstones and separating them, as you have suggested, by pea gravel or other small stones; or using only gravel, as organic mulch or other permeable option for the surface of the patio instead of flagstone or other pavers, or creating a raised patio surface such as a slatted wooden deck, in lieu of something directly on the surface of the ground. In any case you should not bring the patio surface to close (at least five feet away) to the trunks of your spruce trees.
Instead of a patio you might also consider a ground cover which can thrive in deep shade. Examples of native ground covers which grow in those conditions include:
- wild ginger (Asrum canadense)
- Oak sedge (Carex pensylvanica)
- running strawberry (Euonymus obovatus, and
- Canada mayflower (maianthemum canadense)
Here is a link to a Toronto Master Gardener Guide on Native Plants for Shade which you might find useful if you decide to grow something rather than lay flagstones: https://www.torontomastergardeners.ca/gardeningguides/native-perennials-for-shade/
Before installing a patio, consider consulting an arborist for advice on how best to do this in a way which will minimize the negative effect on your trees.