Finer points of planting trees in containers
A single tree growing in a large, handsome container adds a significant grace note to any garden. Carefully placed, it can become a sculptural focal point. It can define an entry or an outdoor living area--or screen one. Place it on a sunny deck or patio and it will obligingly cast shade for you or a cluster of smaller potted plants.
The keys to success are selecting a tree that grows slowly to a manageable size (see "Eight Trees Made for Containers," below), potting it in an ample container, and providing proper long-term care.
A tree in a container is like a bird in a cage: To keep it happy and healthy, you have to provide the right environment. Choose a generous pot that will allow the roots to develop; a 4-foot-tall tree will need a pot at least 20 to 24 inches in diameter and 16 to 24 inches in depth. Plastic and glazed ceramic containers won't crack in freezing weather, and they retain soil moisture much better than unglazed terra-cotta and wood containers.
Fill the container with a high-quality potting mix. Most trees (except pines) benefit from controlled-release fertilizer mixed into the soil at planting time. Use about 1/4 cup of fertilizer for every 5 gallons of potting mix.
A big container filled with soil and a tree is heavy; it will be much easier to move around a deck or patio if you set the pot on a mobile platform with wheels or casters attached to the undercarriage.
Remember that trees in containers need water more frequently than those in the ground. Every three years or so, take the tree out of its container and prune off the large old roots that have coiled around the outside of the rootball. Repot the tree immediately in fresh mix, moving it to a larger container if necessary.
Eight trees made for containers
Bristlecone pine (Pinus aristata); Sunset climate zones 1-6. Dense, rich green needles on a tree that grows slowly, rarely to more than 20 feet. Nursery-grown stock has a pleasing conical form. Cold-hardy and drought-tolerant. One mail-order source is Forestfarm Nursery, 990 Tetherow Rd., Williams, OR 97544; (541) 846-7269 or www.forestfarm.com. Catalog $4; pines from $12.
Dwarf Alberta spruce (Picea glauca 'Conica'); zones 1-6 and 14-17. This conifer with a pyramidal form and bright green, thickly set needles will reach 7 feet in 35 years. Protect it from hot, drying winds and intense reflected light.
Holly (Ilex); zones vary by species. Glossy evergreen leaves, often with ruffled edges and prickly tips, sparkle in sunlight or rain. A number of small-scale varieties such as I. 'September Gem', I. fargesii, and I. altaclarensis 'Wilsonii' will live for years in containers. One good source for hollies is Heronswood Nursery, 7530 N.E. 288th St., Kingston, WA 98346; (360) 297-4172 or www.heronswood.com. Catalog $5; hollies from $6.
Japanese maple (Acer palmatum); zones 1-10, 12, 14-24. This delicately scaled deciduous tree comes in many shapes and leaf and bark colors, but most grow slowly and seldom exceed 20 feet in height. For lacy foliage and drooping branches, look for laceleaf Japanese maple ( A.p. 'Dissectum'); for deep red foliage and upright form, consider A.p. 'Burgundy Lace'; and for an upright plant with a rounded crown, try A.p. 'Globe'. Give any of them regular water and shelter from intense sunlight.
Southern magnolia (M. grandiflora 'Little Gem'); zones 4-12, 14-24. This small evergreen magnolia grows slowly to 15 to 20 feet. Leaves are glossy, dark green on the top, covered with bright, rust-colored fuzz underneath. Give it full sun and protection from cold winter winds.
Staghorn sumac (Rhus typhina); zones 1-10, 14-17. This deciduous tree with deeply cut, fernlike leaves grows 15 to 20 feet tall in time. Crimson fruit clusters last all winter atop fuzzy branches that resemble deer antlers. Takes full sun; very drought-tolerant.
Vine maple (Acer circinatum); zones 1-6, 14-17. This multitrunked Northwest native adapts well to container culture. Leaves are bright green. Locate it where the sun won't beat down on the trunk.
Windmill palm (Trachycarpus fortunei); zones 4-24. Hardy to 10°, this palm grows slowly in a container (in the ground it eventually reaches 30 feet). Fan-shaped leaves 3 feet across are borne on toothed stalks; the trunk is covered with dark brown, hairy-looking fiber.