I’m interested to plant a few multistem birches in my property. I like them for their interesting exfoliating bark and light canopy. Since I want to plant a few of them I can’t plant large varieties. I would like to go native but apparently native varieties need a lot of space, usually they spread between 10-12 m. My question for you is if you could recommend a good multi-stem variety that includes my criteria. Myself I’m interested in Betula Utilis var Jacquemontii but I’m afraid the hardiezone is a little high for Toronto (Zone 5).
You obviously have done your research well. I am quite familiar with your issue as I too have a small backyard and wanted a birch tree. The most important consideration for the survival of birch trees in the city is to give them their ideal environment and follow good cultural practices so that, by avoiding stress, they can combat illnesses and insect attacks.
First, try planting species of birch trees that may be less susceptible to infestations. White or paper birch (Betula papyrifera), Grey birch (B. populifolia), and the European white birch (B. pendula) seem to be most susceptible to Bronze Birch Borer and Birch Leafminer attacks so avoid planting them.
Second, check your garden conditions: Is there enough sun and air circulation?. What is the soil type and conditions (clay/ sand; dry/moist, compacted). Most birches are shallow-rooted so keep that in mind when choosing the ideal location and mulch around the root area to avoid lawnmower damage. Is your garden completely exposed to winter winds or somehow protected (other houses, buildings, fences, conifers, etc). All these considerations will increase the possibility of your tree doing well.
Although birches may get quite large, I may suggest to plant one focal tree instead of several, particularly as you mentioned that you have a small garden. All birches need space to do well. Also keep in mind that one danger of monoculture (or planting many of the same species) is that in case of an illness, all your trees may be affected. If you think theire is enough space for more than one, consider planting two different species. I have seen very small yards in Toronto with two species of birch (River and Himalayan) creating beautiful four season interest.
In terms of native birches, the River birch (Betula nigra) is a beautiful one. Its attractive peeling bark adds to its interest in all seasons. It also meets your multi-stem criteria and it is quite resistant to pest. However, as its common name indicates, it requires moist soil conditions and full sun.
The Himalayan Birch (Betula Utilis var Jacquemontii ) is a wonderful tree and can be purchased single or multi-stem. Its bark resembles that of our native white birch. It requires full sun and rich soil. I planted this specimen in 2009 and has done beautifully even though it only gets partial sun.
Other disease resistant birches are: White Spire birch (Betula platyphylla var. japonica), Asian white birch (B. maximowicziana), black birch (B. lenta) and yellow birch (B. alleghaniensis), however, all these birches are quite large.