Trees once considered rather tender, such as Albizia julibrissin, Acacia, palms and olives are becoming more widely planted in the UK (read Owen Johnson's article Trees for an uncertain future)
Trees of borderline hardiness tend to be from warmer, drier climates than that of Britain, so careful choice of site, correct planting and aftercare are vital in helping them establish and adjust to our increasingly mild, but still-changeable climate.
The best time to plant less-hardy trees is spring. Select good quality, healthy specimens. Smaller trees establish more quickly, but tend to be prone to cold damage, so it may be better to choose larger individuals. Choose a larger plant in colder areas of the country. Sheltered, sunny, well-drained sites help prevent root damage from waterlogging. On moister soils, plant onto a low mound of earth to improve drainage.
Planting and aftercare
When planting, the root collar (the point where the roots flare out from the trunk base) should be level with the surrounding soil. On container-grown stock you may need to scrape away the compost from the top of the rootball to reveal this point. Planting too deeply prevents good root development and makes the lower trunk vulnerable to disease.
On fertile soils it is not necessary to add extra organic matter, as this can discourage the roots from growing out into the surrounding soil, but a light application may be beneficial on poorer soils. Stake the trees to prevent rocking by wind. Installing temporary windbreaks with posts and netting for the winter months helps reduce wind scorch, and protect younger and smaller trees with fleece if hard frost is forecast.
Mulching with dry mulch such as bark chippings helps to protect roots from frost. Wrap pots of container-grown plants with bubble wrap, move them to a more sheltered site, or ideally into an unheated greenhouse, in the coldest periods. Overfeeding, especially late in the season with a high nitrogen feed, can lead to soft growth prone to cold damage.
Even drought-resistant species need regular watering until established. Avoid watering little and often; this leads to shallow rooting, making the tree more prone to drought damage.