How large can potted patio trees grow in the US northeast?

Question

I have a large sunny patio that I would like to shade with the largest potted trees that I can grow practically in the northeast of the U.S. (New England, 6a). What trees are suitable and how large can I expect to grow (or purchase) them? Ideally I would like a mixture of deciduous and evergreen trees. Trees that need to be taken inside are not practical (and would be too small anyway if they could fit inside).

Answer 1

You really need to find a specialist nursery that supplies large plants in large pots - I'm in the UK and we do have those here.

You haven't said what USDA zone you're in, but I can see that New England doesn't get any higher than zone 7a,and that is an important factor. Many trees which are hardy in the ground are not so hardy when in pots, Laurus nobilis (bay laurel) being a good example; should the soil in the containers freeze in a particularly cold spell for longer than a fortnight, even trees with more resilient roots will be killed. In large enough containers, you could get a silver birch to grow to 12 feet easily, but once it reaches that size, that's it, it can't get any bigger, and over time will suffer from being contained. Acer negundo can be grown to a similar height, it's all down to how much root room the plant has, that's what limits growth. Usually, trees or large shrubs chosen for container growing are those which are naturally a bit smaller, so those I've mentioned already, plus plants like Amelanchier lamarckii, which is not enormous in the ground, and which should perhaps make half its mature size in a large container. There will certainly be coniferous plants that will grow in containers and get fairly large, but these don't usually cast much shade and may be blown over in high winds in winter. I'd suggest the advice of a specialist supplier in your area is essential.

UPDATE: in response to your comment - there are complicated things such as root pruning, but that's a highly skilled job, not to mention extremely difficult with a tree that large. Getting an arboriculturalist to prune it back periodically might help to keep the roots in check a bit. Size of container for Birch should be a minimum of 36 inches wide by 48 inches deep, but with extreme winter temperatures of -17.8 to -15 deg C, I would be seriously concerned about root freeze. Topsoil in the ground will freeze at -12 deg C. The larger the container, the less likely to freeze, but even so, arranging pots in clusters will give added protection to roots, otherwise its chancing bubble wrap or several layers of horticultural fleece round the pot and over the surface at onset of winter, and I doubt any of that will be enough. Amelanchier should reach 10 feet in a container 36 inch wide by 16-24 inches deep too, but needs a sheltered spot.

I would certainly check what's available in mature plants in containers in your area, there may be others that are better suited to your micro climate.

Answer 2

The best pot to get are the sand-cast concrete. Be sure to fill the entire pot with good potting soil. Look for soil that has mychorrizae and bacteria added. Mix into the soil Osmocote extended release fertilizer. No rock or gravel in the bottom as that actually causes poor drainage. You'll need 'pot feet' to get the bottom of the pot off the patio and increase the drainage.

The most vulnerable part of a plant are the roots and once a plant is in a pot, it loses the protection of a large body of soil. I've done lots of trees in pots for long-term use outside. To ensure the trees make it through the winter you can wrap the pots in burlap and christmas tree lights. This can be done to look great.

Bamboo is correct about the evergreens. Deciduous trees are often hardier because they are deciduous, the lose their leaves, the next most vulnerable part of a plant. Wind can blow through them without pulling the whole plant and pot over. Evergreens will get blown over, hold snow and break branches. If you need screening, use architectural screens.

Birch, however, you don't want on a patio! They get aphids, no-seeums, they drip honeydew quite badly. Lovely as a grove out away from the home and cars.

Amelanchier 'alnifolia' or 'arborea', Serviceberry would be a great choice. I prefer the multi-stemmed varieties, there's more structure and you'd get a beautiful vase-shaped canopy for shade. I would get at least 3 but 5 would be better. Stay with just one type of tree and the effect will be powerful. This is my favorite tree of all time. Few, if any, insect or disease problems. 4 season beauty; first of the trees to bloom in the spring. Beautiful clouds of white flowers that are easy to blow off the patio surface. Leaves are oval, small and a bluish dark green. Lots of blue-black berries that make great jam if you can keep the birds away. I thought the berries would be messy but it is the rare berry that makes it to the ground. Lovely branching and smooth, gray bark are sculptural in a delicate yet powerful combination. Fall color is neon rust reds, purples and flame orange. One of the most unforgettable plants for fall color and easy to clean-up leaves.

A few other trees I'd recommend are: Cercis canadensis 'Forest Pansy', a purple-red leaved Red Bud; Gleditsia triacanthos inermis moraine 'Ruby Lace' or 'Shademaster', a hardy tree with delicate foliage, tiny leaflets and light shade; Acer palmatum, upright Japanese Maple in either green or red foliage...variegated forms including Coral Bark Japanese Maples are less hardy.

This will be an investment, so I would put these trees on an automatic irrigation for pots with a timer. Water deeply and allow time to dry out. You should see water coming out of the planter when it's being watered. Adjust the timer for the seasons. Less water for the winter. Adjust heating, wraps so that the soil isn't freezing solid. They might even have pots, now, with heating coils available. Still, you want the thick, heavy concrete pots if you want 12-15' trees.

Mulch the top of the soil with 2-4" of decomposed organic mulch to feed the soil organisms and protect the roots. You won't need to re-pot if you keep mulching the top of the soil. Make sure that no mulch touches the bark of the tree and that only the roots are under the soil. Planting a tree too deeply where the bark of the stem is covered with soil or mulch will kill it.

If you buy a balled and burlapped tree, remove all burlap, wires, twine and tags before planting it. Don't stake these trees if at all possible. Purchase your trees at the same time getting the same size, same variety. Prune out any broken branches, any branches growing toward the center of the tree, any crossing branches. Cut to a main branch or trunk leaving no stumps. Purchase in the fall and you'll get a better price. The trees won't have foliage, you don't have to worry about blowing over and they can spend the winter growing roots. Don't forget to always clean your pruners (by-pass, not anvil) with alcohol before, between different trees and after.

I would consider making a patio out of gravel 3/8" plus or granulithics, 4" deep, compacted. Landscape fabric should be laid on the subsoil beneath the gravel. This way you will never lose your gravel. The fine texture and light gray color would highlight your 'grove'. A firepit and low garden walls made from CMU (concrete modular units) would be Sunset Magazine beautiful. You might even consider building a few 'planters' for your trees using these walls capped with 2" thick concrete pre-cast light gray caps. I'd also make my firepit using this material as well. Roman cobble pavers 2X7X9 concrete pavers are my favorite paver if you would like a more formal patio. Look for CMU's that come in an 'Ashlar' pattern. Try to keep your hardscape all the same color. Light gray with bits of dark gray is the best color for your garden. It looks natural, clean and shows off your plants. Brown, red, greens...look like you need to get out a pressure washer and clean off moss, dirt and mold. Just a few suggestions...I really can't think of a better tree, this is my favorite. Use the same thought process on other candidates and let us know if you have any other questions!

UPDATE: Find your trees first. You'll be able to get pots to match. If the trees are small to begin with, you don't want to swamp them in huge pots. You can keep them in smaller, black plastic pots set into mulch or dried moss or straw sitting on turned over black plastic pots for height and upgrade as the trees grow. Otherwise, figure a pot to begin with about 6"-1' wider than the root ball. So, if you've purchased a tree with a 2" caliper and the root ball is 1 1/2' wide, you'll use a pot that's 2 1/2' wide. Keep the width and height close to equal. Definitely don't get pots that are taller than the width as they will blow over easily. The final pot size should be 4' X 4', minimum. These will be heavy, but they aren't meant to be moved. At the beginning, with a young tree, pots this size can be moved by tipping them onto an edge and 'rolling' the planter where you need it to go. You can also use a tree dolly, a dolly with a rounded back and heavy duty wheels and meant for moving trees and pots. Years down the road you can use this dolly to root prune and refresh the potting soil. Simple plywood boxes could be constructed around each pot outfitted with light bulbs for heat and stored in the garage during winter for even better winter protection...

Answer 3

Here are some trees that will grow well in large planters given the right care:

Canadian Hemlock, Tsuga canadensis

Positives:

• One of the only shady evergreens that will be winter hardy in a pot in New england
• Is hardy to USDA Zone 3a: to -39.9 °C (-40 °F)
• Year-round visual interest
• Will take a heavy snow load
• Grows fast
• low maintenance

Negatives:

• Drops old needles yearly
• May bleed from wounds
• Susceptible to certain diseases in some areas
• Requires special soil

Corkscrew Hazel, Corylus avellana 'Contorta'

Positives:

• Visually interesting year-round
• Available in a purple-leaved form
• Is hardy to USDA Zone 4a: to -34.4 °C (-30 °F)
• Low maintenance

Negatives:

• Not the fastest grower

Pussy willow, Salix discolor

Positives:

• Fast growing
• Visually interesting in late winter/early spring
• Hardy to USDA Zone 4a: to -34.4 °C (-30 °F)
• Leaves out early

Negatives:

• Drops branches
• Not very long-lived
• Like lots of fertilizer

Russian Olive, Elaeagnus angustifolia

Positives:

• Likes root confinement
• Grows fast
• Winter hardy to USDA Zone 3a: to -39.9 °C (-40 °F)
• Visually interesting (foliage, bark)
• Disease resistant
• Attracts birds

Negatives:

• May be considered invasive in your area
• Drops fruit

Japanese ZelKova, Zelkova serrata

Positives:

• Interesting branching habit
• Exceptionally shady
• Fairly fast growing
• Hardy to USDA Zone 5a: to -28.8 °C (-20 °F)
• Fall color

Negatives:

• Likes lots of fertilizer

Eastern Redbud, Cercis canadensis

Positives:

• Very attractive, especially cultivars like 'Lavender Twist'
• Grows fast
• Flowering tree
• Nice habit
• Hardy to USDA Zone 4a: to -34.4 °C (-30 °F)

Negatives:

• Drops twigs
• Uses a lot of fertilizer

Corkskrew Willow, Salix matsudana

Positives:

• Visual interest in winter
• Fast growing
• Leaves out early
• Hardy to USDA Zone 4b: to -31.6 °C (-25 °F)

Negatives:

• Needs lots of fertilizer
• Drops branches in winter
• Not very symmetrical
• Not a long lived tree

Paper Birch, Betula papyrifera

Positives:

• Visually interesting in winter
• Nice habit
• Fast growing
• Hardy to USDA Zone 2b: to -42.7 °C (-45 °F)
• Shady

Negatives:

• Will drop honeydew, same as silver birch. See stormy's answer
• Susceptible to several diseases

American Yellowwood, Cladrastis lutea

Positives:

• Flowering tree
• Nice habit
• Shady
• Hardy to USDA Zone 4a: to -34.4 °C (-30 °F)
• Disease resistant

Negatives:

• May drop twigs in winter
• Drops flowers in spring/summer

Weeping Cherry, Prunus subhirtella 'Pendula'

Positives:

• Flowering tree
• Attractive year round
• Shady
• Hardy to USDA Zone 4b: to -31.6 °C (-25 °F)
• Grows fast
• Exceptional for a planter

Negatives:

• A messy tree
• Needs careful pruning every year

Flowering Dogwood, Cornus canadensis

Positives:

• Flowering tree
• Nice habit
• Hardy to USDA Zone 5a: to -28.8 °C (-20 °F)
• Interesting branch structure (winter)
• Fall color
• Spreading

Negatives:

• May drop fruit
• May drop some twigs in winter

I only added the more major plusses and minuses. These trees are going to add a lot to your property, and you will want to keep them in top condition. You will probably want them to get a regular checkup (once a year or so) by a certified plant health expert, so you don't miss anything that could be fatal.

Most of these trees are fairly common, and you should easily be able to find at least 6' specimens, most likely balled and burlapped. Make sure you have the planters and back-fill soil ready before you buy your trees. These trees will need large planters that will hold at least two yards of soil. The wider the planter the better. Also, you will get better drainage if you put an inch or two of pea gravel on the bottom of your planter, before adding any soil.

Stormy's idea of using Christmas lights as winter protection is excellent. I usually install heating cables on a thermostat, but that is much more expensive.