By Adrian Higgins The Washington Post
In this rush to lay bare the winter garden, we may be missing an opportunity. Any garden, big or small, can be transformed by a hardy tree grown in a planter outdoors year-round.
The obvious is you get a plant of architectural stature where you don’t have soil. A tree in a planter can act as a focal point to the patio or any garden space. More than one can announce your entrance, separate the driveway or provide instant screening on an exposed terrace. In greater numbers, they can turn a hot and unwelcoming area into a leafy grove.
It probably needs to be a small tree, or a sculptural shrub or conifer. The container should be large and frost-tolerant. It is useful to think of a tree in a container as a yin-yang pairing: The plant and its home should be matched in size, allowing room for a few inches of root spread.
One of the most convincing pairings I have seen is at Eastwoods Nurseries in Virginia, where owner Henry Eastwood plants Japanese maples into handmade planters of white oak. They are square and shallow and angled outward to produce a bonsai-like display. The effect of putting the trees in such a planter bestows desirable bonsai-like qualities on them: They are stunted and take on an aged character early.
If you want more heft in your planter tree, consider the work of professional gardener Nick McCullough, of Ohio, who installed two full-size upright Japanese maples for a client 11 years ago, and placed them in large, extravagant oaken containers. The planters are 42 inches across and stand four feet tall. They are used to mark key points of entry to the terraces around the house.
Proving that even full-size shade trees will take this treatment, Longwood’s Gentry grows the red maple variety Franksred in large decorative teak containers — Versailles planters — that are 48 inches square and 36 inches high. The trees grow in stainless steel sleeves within the boxes.
The planters are just one of several vital elements to consider when cultivating trees this way, along with the soil mix, the selection of varieties, and watering and feeding.
Woody plants for planters
Japanese maples (Acer palmatum and A. japonicum) make good candidates for containers because they are relatively small, grow slowly and have fibrous roots.
Developed over centuries in Japan, these maples come in many growing habits, leaf shapes and colors. All types work in the planters, Eastwood said. Whatever variety you pick, deft formative pruning would enhance the effect. Other deciduous shrubs and trees are excellent, especially smaller or more compact cultivars.
Trees in planters have roots exposed to air temperatures and behave as if they are in colder climates, so you will need to make sure your chosen variety is tough enough. You should shift to at least one plant-hardiness zone colder.
What to avoid? Be careful with boxwood; you may be bringing in boxwood blight that will spread to any existing boxwood in the yard. If you have a boxwood in the garden that needs moving, it would transfer happily to a planter. Generally, however, broadleaf evergreens such as hollies, camellias and rhododendrons don’t make ideal planter specimens because of winter wilting and leaf burn.
They need to be large enough to allow root development relative to the top growth and to anchor the tree. Even the smallest trees should have pots at least 20 inches across. You can undersize the pot if you compensate with diligent watering and feeding. Size will minimize the stress of fluctuating soil temperatures and moisture.
Planters need to be frost-proof and resistant to UV damage. Wooden, fiberglass, cast concrete and high-grade plastic are desirable. Metal may get too hot and cold. Most terra cotta pots are prone to cracking.
Planters must drain freely. Most trees die in waterlogged conditions. One drainage hole may not be enough. Planters on hard paving should be elevated for effective drainage.
Tall containers drain better. The planters should be lined with porous cloth to prevent soil from washing out and roots from growing through and beyond the drainage holes. If the top of the planter is narrower than the sides, root growth will make it difficult to repot or root-prune the tree.
Do not use topsoil or anything not designed for container use. Even potting mixes should be amended with sharp sand or, better yet, chicken grit because peat moss, wood and bark mulch, and other organic ingredients eventually break down to a heavy soil consistency, impeding root aeration. The soil can be freshened if you decide to root-prune, which can be done by slicing roots where they abut the planter sides.
Woody plants in containers will need watering more often than those in the ground, as much as twice a week or more in high summer. Water until the planter drains. Don’t rely on rain. You need to water from October to April to prevent the roots from desiccating. Water every two or three weeks in the offseason but only when temperatures are above freezing.
Trees in planters need to be fed regularly, but not excessively. Incorporate a slow-release fertilizer at planting time. Organic options are available, including dried kelp meal. Stop feeding for the year in August to allow the tree to prepare for dormancy.
A simple mulch of pine fines, river stones or pea gravel will help to retain soil moisture and thwart weeds. Some gardeners plant flowering annuals or perennials beneath the tree, but that may be gilding the lily. Demure, fine-textured ground covers or even moss would be a more elegant approach.