A tree's root system provides vital functions for whole tree health, including nutrient and water uptake, energy storage and anchoring the tree. Most of a tree's root system is located within the top few feet of soil. The best way to ensure that a tree will remain healthy is to locate the patio or walkway outside the tree's protected root zone, the area of the ground within the tree's drip line. However, for many projects, this spacing is not feasible. In this case, the University of Minnesota extension recommends disturbing no more than 25 percent of the roots located within a tree's drip line. The characteristics of individual tree species should also influence construction considerations. Some trees can withstand more root damage than others. Additionally, tree roots can grow near or above the soil surface and lift the flagstone.
Laying the Flagstone
Certain flagstone-laying methods or options will help minimize the effect of the patio on the roots and can also help prevent large surface roots from pushing up the flagstone. Curtis W. Smith of the New Mexico State University Cooperative Extension Service recommends using chimney flue tiles with one side removed to create a sort of bridge to make a tunnel that allows for root enlargement and air exchange. Avoid digging or making cuts into the ground during hot, dry weather, do all digging by hand, water the tree regularly and apply mulch to the ground around the project area. Do not use mortar or concrete underneath or between the flagstones. Instead, use a base and infill of crushed rock, pea gravel or sand that will allow moisture and air to reach roots.
Tree Care After Patio Construction
Certain steps can be taken after the root-damaging activity in order to reduce damage. The destruction of roots and compaction decrease the tree's ability to take up water and nutrients, but corrective actions can help. Thin out branches selectively to reduce the tree's water requirements, irrigate the tree regularly so it has adequate moisture and consider applying fertilizer that will stimulate root growth. Avoid using excessive nitrogen.
Recognizing Damage to Trees
Careful monitoring of the tree after placement of the flagstone patio will allow for the prompt recognition of construction-related damage and may indicate a need for corrective action. A tree suffering the effects of root damage will often suffer from wilt, leaf distortion and early leaf drop. Adjust the care regimen and mitigate any construction damage if possible. A significantly affected or particularly sensitive tree can also decline enough that it eventually dies and fails to leaf out during the following growing season. In this case, the tree should be removed and the patio setup should be adjusted or a more suitable tree specimen should be planted.