shade tree in container/pot

Hello Tom,

Shade trees typically reach multiple stories tall, and are categories as such because they need to be fairly tall (and thus also wide) in order to cast measurable amounts of shade. On a smaller scale for a deck, awnings tend to be more effective, but a tree that matures much smaller than a shade tree might help. The challenge with maintaining healthy trees in container is multi-pronged. Temperature and moisture levels fluctuate more drastically and rapidly in containers than they do for in-ground soil. This can contribute to root stress or, in cases of extremes, root death. Stressed plants can then be more prone to insect or disease issues. Container soil also has limited nutrients which will need to be supplemented over time; ideally, all of the old soil is replaced periodically both to remove an excess buildup of certain minerals while preventing the inevitable compaction of the soil over time. Compacted soils drain more poorly and can risk over-saturating the roots when wet. Lastly, wind can easily topple containers with large trees, even if they are well weighed-down. This would not only be tedious for clean-up, but could damage limbs and bark when it falls. Injuries to trunks can be serious with regards to a plant's health. Concrete planters would be the most sturdy in this regard, but come with the trade-off of lots of added weight and difficulty in moving them into place. Many high-end plastic containers, though very lightweight, stand up to years of UV exposure well without degrading and cracking. Ceramic and terra cotta containers should be avoided as they can crack easily in freezing conditions, particularly if the material is moisture-absorbent. Metal should be fine, but would heat up to root-damaging temperatures in summer unless shielded. Wood should last awhile if it can be treated to be rot-resistant.

With all of that said, success can be had with long-term container plants. The goal is to match the plant well to its growing conditions and use as large of a container as possible when using long-lived plants. Larger volumes of soil do not vary in conditions as quickly, are more insulating, and allow for longer-term root growth. When roots run out of space, however, this can stunt the top growth, keeping a plant much smaller than it would normally grow. (An extreme, though not perfect example of this, is bonsai.) When roots fill the pot, less soil is present, and the pot will dry out and leach nutrients faster. Predicting when a tree will reach its full potential in any given container is a bit of guesswork, so in this regard, recommendations are not predetermined.

Given the greater exposure to winter cold and the reflected heat from the deck surface in the summer, trees with a good range of cold and heat hardiness are best. Depending on the mature size you had in mind, even a large-statured shrub trained in a tree-like shape may suffice. In general, good choices might include those trees recommended for tolerating urban conditions (such as street trees) because they face many of the same challenges. Unfortunately, street trees tend not to be small growers. Candidates include:

• Hackberry (Celtis)
• Littleleaf Linden (Tilia cordata)
• Trident and Shantung Maples (Acer buergerianum and A. truncatum)
• European Hornbeam (Carpinus betulus)
• Chinese Fringetree (Chionanthus retusus)
• Corneliancherry Dogwood (Cornus mas)
• Japanese Elm (Zelkova serrata)
• Crepemyrtle (Lagerstroemia) - winter hardiness is riskier in a container
• Hawthorn (Crataegus) - canopy may not be as dense, and leaves are smaller

This is not an exhaustive list, but may be a good place to start. Many more species could work successfully if they are well cared for and conditions are not too stressful. A simpler alternative would be to use non-hardy plants as seasonal shade, and replace them each year. Large, leafy houseplants such as Bird-of-Paradise, various Palms, Weeping Fig, and Bananas can provide shade and will thrive in hot, sunny locations. While they could be overwintered inside, their size and need for bright light and high humidity would probably not make that practical. There is a hardy Banana (Musa basjoo) that dies back down to the soil in freezing weather but returns each year (if the roots are well-insulated); if the pots are sheltered, this could be an interesting alternative. It has been known to grow successfully in the ground in southern PA and even CT if adequately mulched. In your case, this would mean probably moving the containers to up against the house or into a cool garage for the winter.

Miri