Though it is an attractive tree, Leyland cypress has become increasingly prone to a number of serious disease and insect problems that are difficult or impossible to treat, and some authorities now recommend against its use. For best results, plant the trees when they are dormant in the fall, about six weeks before the first frost. Official sources do not list Leyland cypress as a toxic plant, but minor skin irritation is known to occur when the plant is handled. It’s best to wear gloves when pruning or otherwise pruning or handling the plant. There are also concerns about grazing animals becoming ill from browsing on Leyland cypress. You may not want to plant this specimen around horses, goats, or if you have pets that are prone to chewing on branches. Be sure to prune them early and often; otherwise, due to their fast growth rate, Leyland cypress tends to get too tall too quickly and can overwhelm a landscape. Be on the lookout for canker and root rot diseases, which are often fatal.
Light
While Leyland cypress is forgiving of most light conditions, it does not tolerate dense shade. It grows best in open, sunny conditions or partially shaded areas.
Soil
Although it prefers moist, fertile, well-drained soil, this tree tolerates a wide range of soil types—clay, loam, or sand, and acidic or alkaline. However, dense, wet soils are a prescription for root rot, which is usually fatal.
Water
To help its roots get established after spring or summer planting, water your Leyland cypress regularly. A Leyland cypress will take a few months to get established and needs frequent deep watering during this time, but after that, a good rule of thumb is to give the tree about 1 gallon of water per week for each foot of height—a 10 feet tree will need roughly 10 gallons of water weekly. This can be divided into two equal waterings each week during the active growing season. In winter, reduce watering to just once or twice a month. Use soaker hoses, not sprinklers that will wet the foliage. The tree tolerates occasional drought or brief waterlogging, but take care not to allow it to soak in wet soil for long periods, as this can encourage root rot.
Temperature and Humidity
Leyland cypress trees are best grown in USDA hardiness zones 6 to 10, with temperatures no lower than minus 8 degrees Fahrenheit. However, zone 5 gardeners have been successfully growing them by providing mulch and an A-frame shelter in the winter months to protect them from snow and ice damage. Such sheltering is feasible only while the plants are young—unless you keep them short by pruning. Luckily, once the plants have matured, sheltering becomes unnecessary, as they are often sufficiently cold-hardy in zone 5.
Fertilizer
Fertilize Leyland cypress in early spring, before new growth begins. Spread a general-purpose, 10-10-10 fertilizer around the tree’s drip line.
Types of Leyland Cypress
The official classification of Leyland cypress has changed several times as horticulturalists have debated which genus it belongs to and its parentage. Thus, you may see it listed as Cuprocyparis leylandii, Callitropsis × leylandii, ×Cupressocyparis leylandii, or Cuprocyparis x leylandii. As of 2019, the official name in the ITIS (Integrated Taxonomic Information System) is Cuprocyparis leylandii. However, many authoritative sources continue to use other scientific names. There are many cultivars of Leyland cypress, usually named for the color of the foliage.
‘Leighton Green’ has dark, forest-green foliage, making it ideal for holiday decorating or for use as a Christmas tree. Heavy and stout, this cultivar has a coarser appearance than other varieties.‘Silver Dust’ has a leaf structure similar to ‘Leighton Green’ but with white variegated splotches on the foliage.‘Naylor’s Blue’ is known for its attractive blue-gray foliage. It grows to up to 60 feet tall and 10 to 15 feet wide. The scaled needles vary in color depending on the season. In winter it takes on a ghostly grey hue.‘Castlewellan’ tends to grow in a conical shape. Its delicate, lacy foliage is a favorite feature. In the winter, the tree’s exterior turns a gold hue while the interior remains green.‘Robinson’s Gold’ has yellow-golden foliage and is considered one of the best of the golden varieties.Emerald Isle® has bright green foliage in flat sprays. It grows to 25 feet, making it a more manageable tree.
Pruning
This tree tends to grow best with a central trunk, so it’s wise to trim away competing leaders immediately after planting it. (Leyland cypress can, however, be grown as a multi-stemmed shrub, a favored method if using them in a hedge.) The height of a Leyland cypress can be controlled but only through persistent pruning that starts when the plants are young. Trim the sides of Leyland cypress trees every year in July. After the leader has reached the height you want the tree to retain, make a pruning cut a few inches below that (which will leave room for the vertical growth of minor branches) to preclude any further significant upward growth, as you would do when pollarding a tree. When individual branches turn brown and die back, they should be pruned out immediately, as this can be a sign of canker disease beginning. If caught early enough, you can sometimes save the tree from immediately perishing by pruning away affected limbs before the fungus spreads throughout the tree. But this usually just slows the progression of the disease rather than stopping it. If growing these plants as hedge shrubs, they will bear pruning as much as three times during each growing season.
Propagating Leyland Cypress
Since the tree is a hybrid, the seeds produced by Leyland cypress are often sterile, and if they are fertile, planting them usually results in a tree that looks quite different than the parent plant. Thus, this tree is best propagated from cuttings. According to legendary woody plant authority Michael Dirr, a good method is as follows:
Potting and Repotting Leyland Cypress
Leyland cypress is such a fast-growing tree that container culture is not very common, though it is sometimes used as a living Christmas tree. If you decide to try growing it in a pot, use a large, wide, container with good drainage holes (heavy-duty plastic is a good choice), and fill it with a standard well-draining potting mix. It will likely need repotting every year as the roots fill the pot, but rather than potting up to larger containers, it’s best to trim back the roots, then replant in the same container with fresh soil around the reduced root ball. Handled this way, you may be able to slow the otherwise rapid growth rate of this plant. Once it is 5 to 7 feet tall, the tree is ideal as a living Christmas tree, but when it grows larger than this, it is best to either discard it or attempt to transplant it into the landscape. Most growers grow weary of root-pruning this sizable tree every year to keep it small enough for a container.
Overwintering
These trees can be prone to branch damage in regions with heavy snowfall, so vigorous pruning up through mid-summer may prevent this from occurring. In colder regions, shielding the shrub with a burlap tent or wrap can prevent it from winter burn, especially while the tree is young. An evergreen covered with burlap does not make for a very attractive winter specimen, however, so gardeners in zones 5 and 6 may want to grow a different species. Watering should be reduced to once a month in winter, as these trees are especially prone to root to in cold, wet soil.
Common Pests & Plant Diseases
The frequency with which Leyland cypress succumbs to insect damage and disease is one reason why some landscape experts now shy away from recommending this plant. A very serious insect problem for Leyland cypress is bagworms, which can strip a tree of foliage within a few weeks unless you diligently pick off the silky nest bags as soon as they appear. You may also experience infestations of spider mites on this tree. A good solution for this problem is to spray with neem oil. Leyland cypress is susceptible to root rot caused by Amillaria and Phytophthora fungi, and cankers caused by Seiridium and Botryoshpaeria fungi. Cankers are most likely to occur during periods of extended drought. Both root rot and canker disease are incurable, and they are prevalent enough to make planting this tree a questionable choice, especially in areas where the diseases are commonly seen.
Common Problems With Leyland Cypress
Aside from common problems with canker fungus and bagworms, the most common complaint with Leyland cypress is that it grows faster than expected—and to a size that can overwhelm landscapes. And this fast growth, combined with a shallow root system, means that this tree can topple over easily in strong wind, especially when soil becomes saturated. This is a tree that requires vigorous pruning yearly if you want to keep it at a manageable size.
Browning or Yellowing Branches
It is normal for Leyland cypress to experience some dieback of individual limbs as it grows large. These can simply be pruned off. This is a very dense tree, and the center portions can die back due to lack of sunlight, so selectively pruning out limbs to open up the center to light and air is a good practice. If the browning is found on one side of the tree, you are likely witnessing winter scald—caused by a combination of cold temperatures and harsh winds. This is most common with younger trees, and it can be prevented by using burlap to wrap or tent the plant for the winter. Finally, dieback on the inside or outside of the tree can be a sign that a serious canker disease is beginning. Affected limbs should be pruned back to well below whatever canker or oozing wounds you find. If cankers continue to form (likely) the tree will need to be removed. Similarly, root rot can cause this kind of dieback, especially in wet conditions or if the tree is planted in wet soil where it doesn’t get enough sun. Withhold all watering during wet weather, and if the tree’s branches continue to die back, the tree will need to be removed. Bring the tree indoors for no more than seven to 10 days at holiday time, and keep it moist but not soggy. Remember that this is a fast-growing tree, so it can be hard to use it for the same purpose a second time the following year. Once it serves its holiday function, the tree is often transplanted into the landscape or discarded.
