When people think about container gardens, they often imagine dainty hanging baskets or herbs in window boxes. Let's think big! Many trees grow well in containers, and if you have a small garden (a deck? a terrace?) a containerized tree is a terrific option.
There are a few special considerations when planting a container tree. It's not harder than planting a tree in the ground…just a little different.
Trees in containers are more vulnerable to cold than their in-ground counterparts; there's not nearly as much soil insulating tree roots from winter temperatures. To be safe, subtract one or two USDA hardiness zone for planting in a container and one more for every ten floors above street level. For example, if you live in zone 7 and your terrace is on the tenth floor, subtract one zone for growing in a container and another for being 10 floors above ground. That means you should choose a tree hardy to zone 5.
When choosing your container, first measure the diameter of your root ball and then add 12 to 16 inches to that measurement to get the size of your new container. An extra six to eight inches of soil on all sides of the root ball will help insulate the roots. Also, providing this extra space at planting time means your tree won't outgrow the container right away.
If you live in an area where winter temperatures get below freezing, choose a frost resistant container. Porous containers absorb water, and repeated freezing and thawing throughout the winter may cause them to crack. Wood, fiberglass, and metal are durable choices.
Check the bottom of your container for drainage holes; you may need to add more. You'll want one inch holes at six inch intervals across the bottom of the pot. In addition to your container, you'll need something to elevate your pot off the deck or ground. You can use special pot feet or some short pieces of pressure treated wood. Raising the container an inch off the ground will improve drainage.
To prevent the soil in the container from dribbling out the drainage holes in the bottom, cut a piece of landscape cloth or flexible screening to cover the holes. Add a few inches of potting mix to hold the cloth in place, then turn your attention to your tree.
If yours is a balled and burlapped (B&B) tree, now is the time to cut away at least the top third of the wire basket and any twine from around the trunk. Also trim away the burlap from at least the top third of the rootball; if burlap is left above ground, it will wick moisture away from the roots. If the ball is wrapped in natural burlap you may leave the rest of the burlap in place because it will biodegrade. However, if the rootball is wrapped in synthetic burlap or woven plastic fabric instead of natural burlap, you must remove the entire covering as it won't break down and will hinder root growth. To make it easier to get the tree into the container, you can leave the bottom portions of the wire basket and rootball covering in place as you set the tree into it; then reach into the container and clip the remaining wire and covering away before backfilling the container with soil.
If you're planting a containerized tree, slide the tree out of its pot. If the pot won't slip off the root ball, roll the container along the ground while applying pressure, to loosen the roots. If this doesn't work, you can cut away the container.
Once the roots are exposed, make sure they aren't circling around the rootball. If they are, pull firmly at them to open up the root ball and spread the roots into an outward growing position. This is where people get timid, but being tentative slows you down, and the longer a tree's roots are exposed to the air, the more likely they are to dry out. If the roots are too tightly wound to separate by hand, use a pruning saw or serrated knife to vertically slice the sides of the root ball at 3" intervals, going about a half inch deep. Cut an X across the bottom of the root ball and use your fingers to pull the root ball apart from inside the X.
Whenever you plant a tree, whether it's in a container or in the ground, it's important to make sure it's planted at the correct depth. To do this you need to look at the tree trunk to find the root flare (also called the trunk flare). This is where the straight sides of the trunk flare out at the base to meet the topmost roots in the rootball. This junction of trunk and roots is where the soil line should fall when the tree is planted. Burying the root flare can lead to rot and planting too high can expose roots that should be protected by soil.
However, you can't always depend on the root flare being at the correct soil level in either a B&B or container rootball. Often when B&B trees are dug, soil is thrown up around the trunk, burying the roots too deep. Container grown trees may have been planted too deep. You may need to remove soil from the top of the rootball in order to uncover the root flare.
Once you've located the root flare, place the root ball in the new pot to see how much soil you need to add to raise the tree to the proper height. Remove the tree, add the soil, place your tree back in the pot, and double check the planting depth. Fill in around the tree with potting mix, firming in the tree as you go. Add water until it runs out the bottom of the container.
Several species that thrive in containers include Juneberry or serviceberry (Amelanchier species), crabapple (Malus species), Japanese maple Acer palmatum), purple leaf plum (Prunus cerasifera), and crape myrtle (Lagerstromia indica) . Container trees will be smaller than their in-ground counterparts because limiting root growth limits top growth. But that doesn't mean you won't have a happy, healthy tree…it'll just be a little smaller. Perfect for a small garden.
Ellen Zachos is the owner of Acme Plant Stuff (www.acmeplant.com), a garden design, installation, and maintenance company in NYC specializing in rooftop gardens and indoor plants. She is the author of numerous magazine articles and six books and also blogs at www.downanddirtygardening.com. Ellen is a Harvard graduate and an instructor at the New York Botanical Garden. She lectures at garden shows and events across the country.