Thinking about putting down some roots? Sally Petit, head of horticulture at Cambridge University Botanic Garden, explains why autumn is the perfect time to plant a tree – and how the cooler winter temperatures can help your new tree thrive.
It’s hard to imagine a world without trees. Many of us will have a favourite tree, whether it’s a flowering cherry we pass daily on the street, or a tree that brings back happy memories of a relative or a favourite holiday.
Here at the Cambridge University Botanic Garden, we have approximately 2,000 trees ranging from saplings of only two or three years of age to mighty giants of more than 170 years, such as our giant redwoods (Sequoiadendron giganteum).
The value of trees reaches far beyond their aesthetic or sentimental value. Trees also have economic value, with oaks being highly valued for their use in construction, while others produce edible fruits, such as the native hazel or cobnut (Corylus avellana) and exotic species like the fig (Ficus carica) and orange (Citrus x sinensis).
Beyond this, trees add so much more value to our lives and the environment. They absorb carbon dioxide, which helps slow global warming; they stabilise soils; they improve water quality and reduce the risk of flooding; they provide shade and act as coolants; they reduce noise; and they support biodiversity. It’s estimated that a single oak tree can support 2,300 species of insects, birds, mammals, fungi, lichens, liverworts and mosses.
There really are many reasons for planting a tree, and as the old adage goes, ‘the best time to plant a tree was 20 years ago and the second best time is now’.
Choosing the right tree to plant
What to choose? There is a wide variety of trees for different situations. For containers, varieties of Japanese maples (Acer palmatum) can be grown in acidic compost and will reward with intensely coloured foliage, while the cherry Prunus ‘Kojo-no-mai’ has zig- zagging stems and delicate blossom.
If you have a little more space and are looking for a tree under 5m tall, a snowy mespilus (Amelanchier lamarckii) makes a great specimen, as does the paperbark maple (Acer griseum) with its cinnamon- coloured, peeling bark. With more space you might try a crab apple such as Malus ‘Evereste’, whose striking blossom is followed by yellow-orange fruits, or perhaps the spring-flowering Cornus kousa var. chinensis, both of which grow to 6-8m.
If space is no problem and you can accommodate a specimen that will grow to 12m or more in height, you might choose an English oak (Quercus robur) to encourage biodiversity, or perhaps the Indian bean tree (Catalpa bignonioides) whose showy flowers cover the broad canopy in summer.
Why autumn is the perfect time to plant a tree
The autumn months provide the perfect window for planting trees, while the soil is still warm and autumn and winter rainfall will encourage root establishment, but avoid planting into waterlogged or frozen ground. Pot-grown trees can be planted at any time of the year but will require more frequent watering immediately after.
To encourage tree establishment, take care at the planting stage. Water the tree well before planting; dig a hole no deeper than the root ball, but twice as wide; place your tree in the hole so that the upper roots are level with the surrounding ground; backfill the hole; firm the soil to remove air pockets around the roots; then water again to help firm the tree in place and ensure the roots are in contact with the soil.
Once planted, check soil moisture levels frequently and water once the soil has dried out. If there are dry summers in the first three years, you may need to apply four to six watering cans of water weekly.
To grow a potted specimen, choose a container with plenty of drainage holes that’s approximately twice the width of the root ball of your chosen tree, add compost to the pot and place your tree on top of this, making sure that the upper roots are level with the ultimate compost level, allowing space to apply water – and then follow the steps above.
Remember, though: pot-grown plants dry out more quickly than those in the ground and will benefit from the replacement of the top layer of compost each spring, or the addition of a liquid fertiliser at regular intervals, to ensure the tree has access to nutrients for healthy growth.
When choosing your tree, be sure to match it to the conditions in your garden. There’s no point in planting a moisture- loving weeping willow (Salix babylonica) in a well-drained, gritty site, or in squeezing a cedar of Lebanon (Cedrus libani) into a 4m² front garden – as neither will survive to maturity.
But, if you choose carefully and if you nurture well, you will be able to enjoy these efforts for many years to come, while also knowing that planting your tree has wider- reaching benefits.
Sally Petitt is head of horticulture at Cambridge University Botanic Garden. Find out more at botanic.cam.ac.uk.