Is yours a late leafer?
19th May 2021
"When the oak is before the ash, then you will only get a splash; when the ash is before the oak, then you may expect a soak." So goes a saying that links leafing out to the coming summer's weather. Whether it holds true or not is uncertain, but why do some trees come into leaf so much later than others? It's often explained by their sensitivity to spring frosts.
Oak leaves can appear as early as March, or as late and May, while leaves usually arrive on ash branches in April or May. It may seem curious that two of our native trees, Quercus robur and Fraxinus excelsior, to give them their scientific names, are so vulnerable to spring frosts that they hold off releasing their shiny new foliage until relatively late in spring.
Above: Ash, oak and sweet chestnut
It's down to their physiology. Late leafers, which also include sweet chestnut (Castanea sativa) and elms (Ulmus), are usually 'ring porous'. That is, the tubes that carry water in the stem are relatively wide and arranged in a circular band around the tree stem. They are able to carry a lot of water, but are vulnerable to freezing temperatures, in a similar way to pipes in the home that can burst as water turns to ice.
The other characteristic of ring-porous trees is that they tend to form leaves at the tips of branches, doing this once or twice in the growing season. It's difficult for them to recover if the spring leaves are damaged as it takes a lot of energy to produce them.
Other species are 'diffuse porous', meaning their water carrying vessels are scattered in the growth ring, are narrower and numerous, and not as susceptible to rupturing at low temperature. So they can start to conduct water earlier in the year. Diffuse porous species sometimes keep producing leaves along their branches right through to the end of the warm seasons, so they can more quickly recover from any frost damage.
Frost damage on tender new growth usually appears as brown patches on leaves, especially on the exposed and topmost parts. Sometimes the growth turns black. Even evergreens can be affected, especially by prolonged cold snaps, and leaves can turn spotty (common on Photinia). Frost may cause newly planted shrubs to lift slightly out of the ground, so do check if they need to be firmed in again.
The good news is that trees and shrubs will usually recover, even if they lose their leaves. Occasionally you may have to wait till mid-summer to see new leaves, so don't be hasty in removing affected plants. If in doubt, the scratch test will tell you if there is life in a stem: use your thumb nail to scrape a little away from the surface - if it's green underneath, it's alive.
Above: this ceanothus has suffered frost damage (brown leaves) but is coming back very well
Species and leafing out
There's a useful acronym to remember the common ring-porous species: ELM-OACs, which stands for laburnum, mulberry, oak, ash and (sweet) chestnut. Oak here refers to English and sessile oak (Quercus robur and Q. petraea). Ring porous wood has well defined growth rings (of course you will only see these if you cut it down!).
Diffuse porous species include apples, birches, hawthorn, rowans, maples and willows, which tend to be early into leaf. Erman's birch (Betula ermanii) can be one of the earliest trees into leaf. However, not all diffuse porous species are early into leaf. For example, beech, sweet gum and sycamore are late leafers, probably due to having evolved in warm climates.
Indeed there are a host of other factors involved in the timing of greening up. A good, cold winter is often necessary before spring, for a tree to get the message to come into leaf, for example. And microclimate can be a factor, leading trees of the same species to develop leaves later in cooler sites, such as on a cold, north-facing slope, than in a sheltered sunny spot.
If you're looking at a tree and wondering if it should have leaves on it yet, you can find out more in our guide: Why hasn't my tree come into leaf yet?