The only true Tree hydrangeas are in the heteromalla species. They originate from Nepal to Northeastern China and the Himalayas where, in their natural habitat, they can grow to 7m in height.
They can be grown as a multi-stemmed shrub, in which case they only reach a height of 3-4m; or, they can be grown with a single stem, or trunk, and achieve a height of 10m in southern British Columbia (zone 8).
The flowers are flat umbels with inner fertile flowers and outer ray florets. When pollination has been accomplished the sepals will invert.
Hydrangea heteromalla has ovate leaves, the upper surface may have a few hairs or be glabrous, the underside with varying amounts of hairs from sparse to pubescent.
Full sun is required to stimulate flowering. Hydrangea heteromalla is hardy in USDA zones 4-7.
There are numerous cultivars available including: ‘Arhetuse’, ‘Bellevue’, Bretschneideri Group, ‘Dumicola’, ‘Edouard Avdeew’, ‘Fan Si Pan’, ‘Gidie’, ‘Jermyn’s Lace’, ‘June Pink’, ‘Kalmthout’, ‘Krista’, ‘Long White’, ‘Morrey’s Form’, ‘Moss’, ‘Nepal Beauty’, ‘Snowcap’, ‘Willy’, ‘Xanthoneura’ and ‘Yalang Ridge’.
Unfortunately, because it adds confusion, the expression ‘tree hydrangea’ has become widely used to describe hydrangeas that have been trained as ‘standards’.
Many shrubs are trained as standards, the most popular probably being roses, fuchsias, azaleas, buddleias, lilacs and viburnums, but there are many others.
The only hydrangeas that can be pruned and trained as standards are of the Hydrangea paniculata species. There are over 30 cultivars in this species, all of which could be grown as standards.
Hydrangea paniculata can be grown as standards by selecting the dominant branch (the leader) and attaching it to a strong support. All other side shoots are allowed to grow a little but are pruned off before they can achieve a significant size. The leader is encouraged to grow until it reaches the desired maximum height, perhaps 1.5 m (5ft.).
How to train a standard hydrangea
It will take 3 years to achieve a full standard by following these five steps:
- When the cutting is about 15cm high, pinch out any sideshoots as they appear in the leaf axils.
- Continue to do this to produce a long, straight stem. Tie the plant to the cane.
- When the standard reaches the required height, leave it to produce 3 more sets of leaves and then pinch out the growing tip.
- Pinch out the tips of the sideshoots that form at the top of the stem so that they will branch further.
- Once the head has filled out, the leaves on the stem will usually drop off naturally, or they may be carefully removed.