Leyland Cypress (X Cuprocyparis leylandii), hardy from U.S. Department of Agriculture plant hardiness zones 6 through 10, may seem like a good candidate to set in pairs around doorways or screen a street side window. Before you plant one, however, heed the advice of the University of Georgia Center for Urban Agriculture to consider “size, sunlight, soil type, irrigation and care required” when planting these giants next to your house and its foundation.
Leyland cypress is a big tree with big roots. It may grow 70 feet or taller and up to 20 feet in diameter, big enough to swallow up a small ranch home. Set it too close -- closer than 15 to 20 feet -- to the house and its crown suffers from poor air circulation at maturity. Cypress roots grow past the tree's driplines and against the foundation. There are only two options for roots in this predicament -- stop growing or burrow into the foundation. The former produces a top-heavy tree with a weak root system that could easily tip over in wet weather. The second option leads to foundation damage.
A big Leyland cypress needs full sun all around because grows fast -- from 18 inches to 3 feet per year. When you plant the tree close to a house foundation, the building shades at least part of the tree. If you plant the tree on the south side of the building, the back of the tree sits in shade. Trees drop needles and lower branches thin out completely when planted in part shade. Roof overhangs increase shade, stressing the tree further.
Like many evergreens, Leyland cypress needs a wide area of fertile, well-drained soil in which to spread its shallow, brittle roots. Soil near foundations can be alkaline, with a pH above 7.0 -- at the extreme end of the Leyland cypress’ tolerance. The tree’s ideal soil pH is 6.0 to 6.8 and alkaline soil or soil heavy in clay will encourage weak growth.
These big trees require steady moisture, particularly when they’re young. Planted next to a foundation, the tree has fewer roots to gather moisture and might be sheltered by an overhang or gutters. Too much water or dry weather can encourage canker diseases, so trees need careful, deep watering during drought or when sheltered by the building. Trees with need enough water to soak the ground 12 to 18 inches deep every seven to 14 days. A cypress under the stress of an inadequate root system and roof overhang will need careful, continual irrigation.
You might have other concerns in addition to irrigation and soil supplements for your Leyland cypress when you plant it near a foundation. Stresses on its root system intensify its vulnerability to Botryosphaeria, or bot, canker, that affects individual branches and Seiridium canker, that kills entire limbs. Constant pruning to remove diseased wood can also spread these fungi.