I live downtown Toronto in a condo and have a 750 sq ft terrace. I would like to know what is the best tree to grow in a 30 x 30″ planter. Partial sun/shade.
When choosing the right tree for your terrace, think about what attributes you are looking for and what growing conditions you have on your terrace. Are you looking for privacy or to create a sculptural backdrop to your indoor and outdoor living rooms? Would you like to grow edibles or are you hoping to add some colourful foliage and seasonal interest?
Do you have drainage on the terrace? If not, then self-watering pots are a good alternative to prevent water seepage onto the terrace, or possibly the apartments below. Are you able to position your pot away from strong winds or intense summer heat? You mentioned that your terrace is part sun/part shade. Is it morning or afternoon sun?
Choose plants that grow in 1 or 2 hardiness zones colder than the zone where you currently live- to ensure a reliably hardy plant choice. Downtown Toronto is in zone 6a, on the Canadian Plant Hardiness Zone Map (see attached reference)
Look for dwarf species and small varieties of the trees that you are attracted to and that are cold hardy to at least zone 5a on the Canadian Plant Hardiness Scale. Note that the USDA Plant Hardiness Scale (from USA) is different to the Canadian scale and many plants are sold in Ontario with USDA zone planting recommendations- which could mean your plant selection is less cold hardy, so check which scale is used on the labels.
Evergreens are a popular choice for creating privacy screens and there are many cold hardy options. eg. Alberta Spruce or Picea glauca ‘Conica’, for its slow growing cone shaped habit and bluish coloured foliage. Degroots Spire Cedar or Thuja occidentalis ‘Degroots Spire’ for its upright narrow form and ruffled branch pattern. Uncle Fogy Jack Pine, Pinus Banksiana ‘Uncle Fogy’ makes a focal point with its unusual weeping, contorted branching patterns.
Most conifers need protection from cold winter winds, so wrapping the tree in the fall with burlap will protect the foliage overwinter. Evergreens love full sun, but some can tolerate light shade if they receive a minimum of 4 hours of full sun in the middle of the day and into the afternoon.
For deciduous trees that will give you summer privacy, fall colour and winter sunlight on your terrace: take a look at Adrians Dwarf Japanese Maple, Acer palmatum ‘Adrians Compact’ or the native Dwarf Serviceberry, Amelanchier spicata. Both would prefer a thick layer of natural cedar mulch (3-4 inches) to protect their superficial roots. Take care to scrape the mulch away from the trunk. For a dense wall of chartreuse green ferny foliage from spring to fall look at the multi-stemmed Tiger Eyes Cutleaf Sumac, Rhus Typhina ‘Tigers Eyes’.
The base of the pot can be planted with colourful perennials to cascade over the edge of the pot to add colour, texture and movement. Spring bulbs could be added to extend the seasonal interest. eg. crocus, agapanthus, hyacinths.
If your pot is overwintering outside, it needs to be insulated. You can buy insulated pots or make your own- by lining your container with a minimum of 1 inch thick rigid Styrofoam. If your large pot is curved, it would be easier to double pot your tree. Insert the pot the tree is planted in, inside your larger pot that has been lined with Styrafoam packing peanuts, or wrapped in bubble wrap 1- 2 inches thick. This creates a buffer of air around the inside pot, which is an effective insulator. Take care not to block the drainage holes of the inner pot. Check that the tree has room to grow- you may need to repot it into a larger pot before double potting it.
Use a good quality soiless potting mix, which is light weight and will allow for great drainage and aeration. After several years, root pruning may be required to maintain your tree at an ideal size. Remove handfuls of soil around the planters edge and discard. With a sharp clean blade, slice through the roots along the planter sides, discard and add fresh soiless potting mix. This will help prevent root girdling.
Check the moisture levels in your pot often. Your tree may require water every few days during the hot summer months, but only require water every 1-2 weeks in the spring and fall. Make sure it has adequate water before the winter freeze- particularly evergreens to prevent browning of the foliage.
Overwintering your pot is important, even with an insulated pot. Wrap your pot with bubble wrap several inches thick, if you are unsure of your pots insulation and burlap your tree if it is in an exposed location.