Aspen (Populus tremula) is a deciduous, broadleaf tree known for its habit of ‘quaking’ or ‘trembling’ in the slightest breeze. Indeed, its botanical name ‘tremula’ was given due to its trembling habit, and it’s also known as ‘quaking aspen’. Aspen tree leaves have flattened, flexible leaf stalks, which is how they are able to flutter so easily.
Native to the UK, it’s most common in north-west Scotland, where it’s found in ancient woodland, heaths and riversides. It thrives in full sun and moist soil. Its leaves emerge a coppery brown in spring and then mature to bright green. In autumn they fade to brilliant yellow. Aspens are dioecious, which means their male and female flowers (catkins) grow on different trees.
Aspen makes a fine garden tree although grows to a height of 25m so is best suited to larger gardens. Bear in mind, also, that it has a tendency to send up suckers, which are difficult to remove.
From a distance aspen can be identified by its shimmering foliage in even the slightest breeze. In spring look for coppery foliage and in autumn look for bright yellow (or occasionally red) foliage. On closer inspection, older aspen trees may be covered with lichen, which gives the trunk a black appearance, although the bark is grey and is often marked with diamond-shaped pores, called lenticels. Twigs are slender, dark brown and shiny, and the round leaves have large, irregular, blunt teeth. Look for flattened leaf stalks.
Male catkins are around 12cm long and are brown, turning yellow when ripe with pollen. Female catkins are green. Once pollinated, female catkins develop into fluffy fruits, which release white fluffy seeds in summer.
Aspen trees may be confused with birch trees. Aspen has heart-shaped leaves while birch leaves are oval. Aspen may also be confused with closely related white poplar and black poplar trees. While similar, these trees have different leaf shapes – aspen trees have more rounded leaves – and bark colourings.
Value to wildlife
Being a native tree, aspen supports a wide range of invertebrates, including a few specialists such as the aspen leaf gall midge and the aspen hoverfly, which feeds on microorganisms in dead aspen wood. Other wildlife include birds, which not only feed on the insects among the leaves and the seeds in autumn, but may also nest in holes in the trunk and branches. Aspen is a favourite food for beavers, which have recently been introduced to parts of the UK.
How to grow aspen
Grow aspen in an open spot with plenty of light, in moist but well-drained soil.
Where to grow aspen
Grow aspen in a woodland garden border where there is plenty of light, or in an open spot such as as a focal point in a large lawned area. Aspen isn’t fussy about soil type but seems to do best in rich, fertile, moist but well-drained soil.
How to plant aspen
Aspen is available as a bare-root tree in autumn and winter, or as a pot-grown tree for the rest of the year. Dig a square hole and ensure the rootball sits at the same level it was in its pot or the ground (bare-root trees have a soil ‘tide mark’ to identify its previous planting depth), and then back-fill with soil. Firm gently and water well. Stake the tree using a stake and tree tie, with the stake fixed at a 45º angle.
Caring for aspen
Aspen trees require very little care once established. Water well for the first two years after planting and mulch annually with well rotted manure or compost, which will not only suppress weeds and conserve moisture but will also gradually return nutrients to the soil. As your aspen tree grows, remove suckers that sprout up from the roots.
How to prune aspen
Aspen trees require very little pruning but if you do want to remove a dead or damaged branch, or control its shape in any way, the best time to do this is in the dormant season, from November to March.
How to propagate aspen
Aspen propagates itself from seed and by suckers. This means you can propagate new aspen trees from seed or by taking root cuttings. Taking aspen root cuttings is the easier of the two options, and takes around six months to grow a young tree from a cutting taken in spring.
How to take aspen root cuttings
- Aspen roots are shallow so take care not to damage them. In spring, using a stiff brush or fork, gently expose the roots and then use secateurs or loppers to remove lengths of root with a diameter of 1-3cm. Avoid taking too many cuttings from the same tree. Trim your cuttings to 30cm in length and plant them up as soon as you can, or place them in a plastic bag to retain their moisture until you are able to plant them up.
- Prepare a seed tray with gritty, peat-free compost, and water it thoroughly, allowing the water to drain. Lay the cuttings horizontally over the surface, spacing them a few centimetres apart, and then cover with a thin layer of compost. Keep the cuttings in a frost-free environment such as a greenhouse or windowsill. Keep the compost moist but not wet. After a few weeks, suckers will start to grow from your cuttings.
- When these suckers are large enough to handle, gently cut them away from the original root and plant them individually into pots of gritty compost (you may want to dip them into rooting hormone beforehand). Keep your potted cuttings in a heated propagator and keep the compost moist. They should develop roots within a few weeks.
- In autumn, the roots of your aspen cuttings should have started to grow out of the bottom of their pots. Pot them on into larger pots and keep them in a frost-free spot to overwinter. You can then plant them outside into their final growing positions the following spring.
How to propagate aspen from seed
Aspen trees are dioecious, meaning male and female catkins are produced on separate trees, and the female catkins produce fluffy white seeds in April and May.
- From a female tree, harvest a few catkins from a low-growing branch just as the seeds are starting to ripen (as the catkins are starting to become fluffy) and then leave them to ripen in a warm place, such as a windowsill or greenhouse bench, for a few days.
- Separate the seeds from the down and then sow them thinly in prepared seed trays or small pots of moist, peat-free compost. Do not cover. Keep in a propagator with a lid on to maximise humidity, in a partially shaded spot. Germination should occur within a few days.
- Remove the lid of the propagator to prevent the onset of fungal diseases. Leave the seedlings in their seed tray for the first growing season, watering when the compost starts to dry out, and then pot them on into individual pots the following spring. Harden off thoroughly before moving them outside.
Pests and diseases
Aspen trees are not known to be affected by pests and diseases but bear in mind they are popular with wildlife so you may notice insects on the leaves. This is nothing to worry about – in a garden with a good ecosystem there will be natural predators such as birds to keep insect numbers under control.
Advice on buying aspen trees
- Aspen trees are available as pot-grown trees or as bare-root trees in the dormant season. You may have a greater choice of tree sizes if you opt for a bare-root tree
- Ensure you have the right growing conditions for aspen before buying
- Always check plants for signs of damage or disease before planting