While at work in the garden this week, a question from a recent episode of University Challenge popped unavoidably into my mind. The teams had been asked the meaning of the epithet sempervirens, which occurs in the names of many plants. It did not unduly tax the little grey cells and the correct answer was swiftly given: evergreen.
It came back to me a few days later, as I was raking beneath one of my finest trees – which was planted by me 30 years ago and now overtops everything else. It is a specimen of Pinus wallichiana, the long-needled Bhutan pine, a classic evergreen if ever there was one. Why, then, was I ankle-deep in shed needles?
The reason would not fox any serious University Challenge competitor because while the plant is always green, it does not retain the same leaves or needles permanently. It sheds a proportion of them piecemeal throughout the year – although some evergreen species, like my pine tree, do so over a short period, emulating deciduous trees, which drop everything during a few autumn weeks.
Evergreen trees are especially valuable in winter because they offer contrasting shapes to the skeletal outlines of leafless deciduous species, but the reason why there are relatively few kinds (other than conifers) available for British gardens is an interesting one.
It is believed that historically all plants were evergreen, and the deciduous habit evolved as an adaptation to the marked seasonal changes in temperate climates. Retaining leaves in winter (too dark for photosynthesis, and too cold for growth) was inefficient. That is why almost all our native trees are deciduous. Generally, evergreen foliage trees are plants of warm climates and not hardy enough for our winters.
But the exceptions are worth seeking out. Unquestionably at the top of my list is Magnolia grandiflora, which drops a carpet of some of its leaves in early summer. It originates from the south-eastern United States, but in this country is usually grown against a wall, where it can be pruned to size annually, although a large specimen will also thrive well as a free-standing tree.
Its advantages are not simply limited to its magnificent, glossy green leaves – but also its flowering habit. Unlike the many deciduous magnolias that are covered in flowers during one spectacular annual spring display, Magnolia grandiflora produces them a few at a time throughout the summer. And what flowers they are – up to the size of dinner plates in some kinds. Be sure to buy a named variety if you want it to flower in your lifetime – the best are “Exmouth”, “Goliath” and “Victoria”.
There are only a handful of other worthwhile broad-leaved evergreen trees. Hollies are fully hardy and there are some attractively foliaged kinds. In milder western or sheltered gardens, the strawberry tree, Arbutus unedo, has much to commend it with its dark green shiny leaves, white flowers and strawberry-like fruits.
The Portugal laurel Prunus lusitanica and the similar bay Laurus nobilis have nothing much to show in the way of flowers, but will ultimately become attractive medium-sized trees. But avoid the Holm oak Quercus ilex: it’s too large and too dismal (it was a favourite for planting in Victorian cemeteries).
An evergreen tree of some sort will enhance most gardens – but if you think it means an end to leaf collection, think again.
Next week: Nina Caplan on drink