Why do leaves fall? And other autumn gardening questions

As we’ve had the autumn equinox this last week, which is when the day and night are of equal length, it stands to reason that in the northern hemisphere post equinox, we have longer nights than days. So the season of autumn is definitely here. Why do leaves fall? And other Autumn Gardening Questions is here to answer some of those tricky queries that children can throw at you…

Why do leaves fall?

Leaves fall in the autumn from deciduous trees. Evergreen trees and conifers lose their leaves throughout the year.

So why do leaves fall? Leaf fall is triggered by two main factors – a decrease in ambient temperatures and a decrease in the average daylight hours. A reduction in moisture at root level is also a factor for many trees, although this can vary on location.

The reason the trees drop their leaves is to reduce stress on the tree over the winter months –
Trees with large surface areas are more likely to be damaged in winter storms, bare branches over less resistance than branches covered with leaves.
Where there is snow and frost, there will be a reduction in the amount of available water in the ground that the tree can use, so by losing its leaves, the need for water is greatly reduced.
Left around the base of the trunk, the leaves form a mulch protecting the root zone from severe frost. Come the spring, the leaves will decompose, returning nutrients to the tree.
Fruit trees, nut trees, trees that carry berries will have been working hard to produce all this abundant crop. By going in to semi dormancy over the winter, they are renewed and able to start the process all over again in spring.

The beautiful colour of many autumn trees and shrubs is part of the process of leaf fall. The production of the chlorophyll that makes leaves green stops at this point in the year; however, many pigments are left in the leaves; in particular beta-carotene and xanthophyll, leading to spectacular colourful foliage displays.

In some trees, glucose becomes trapped in the leaves after the process of photosynthesis stops. Combined with the temperatures changes caused by sunny autumn days and cool nights, a glorious red colour is produced in the leaves. Acers are a fine example of this happening.

Different tree species react with different chemical changes which is why we get yellow, red, bronze and purple leaves in autumn. Some trees and shrubs have brown leaves, due to waste matter remaining in the leaf.

The technical term for leaf fall is ‘abscission’. A layer of cells forms where the leaf meets the stem, and this slowly pushes the leaf away from the stem until a breeze will blow it off. These cells also protect the tree from having an open wound where the leaves have fallen.

What can I do with all the fallen leaves in my garden?

The first task is to check if the leaves are virus free. If they are not healthy then they need to be burnt in the garden, or sent to a green waste facility where high temperatures are part of the composting process.

If they are healthy leaves, and you have some room to store them, then you could make leaf compost to use in your garden.

Are Rowan berries poisonous for my toddler?

Rowan – Mountain Ash, Sorbus – is a beautiful tree, good in smaller gardens. The berries of Sorbus acuparia, the native mountain ash found in Scotland and across Europe, are a bright red colour, attractive to birds and toddlers alike. Other Sorbus trees have yellow, orange or white berries. And no, they shouldnt be harmful if your toddler ate a couple.

The Rowan berries contain parasorbic acid, which can cause kidney and stomach problems; although they are somewhat tart to eat raw, so not many people are likely to swallow them from choice. The raw flavour improve as the berries mature over the winter and the frosts work their own chemical changes on the fruit, so they are then theoretically, less toxic in their raw state.

However, once cooked, the toxin changes to sorbic acid which is benign, allowing a range of uses for the berries.
Rowan berries are most often used to make jelly (clear jam) and wine. Mixed with other hedgerow fruits such as blackberries, hawthorn and sloe, I have made jam from rowan or stewed the fruits together to use in pies.

Why do we plant spring flowering bulbs in the autumn?

We plant spring flowering bulbs in the autumn as that’s how long they take to develop. These bulbs are dormant, flowers waiting to happen if you like. And in order for them to wake up they need particular things to happen. One of these is a chilling period, and the cold winter weather provides the low temperatures they need.

Then the bulbs will start putting out roots from the basal plate at the flatter end. Once the roots have reached a particular stage, the leaf shoots begin to grow from the tip, followed lastly by the flower stem which has been nestled in the central part of the bulb.

One of the reasons bulbs come up ‘blind’, without a flower, can be because they have been planted to deeply. The general rule of thumb when planting bulbs is to bury them with a depth of soil above the tip of the bulb which is the same measurement as the height of the bulb. But you don’t need to get out a ruler, approximate is fine.

Can you eat daffodil bulbs?

Interestingly for such a popular garden flower, all parts of the Daffodil – Narcissus – plant are toxic to some degree.
However it is the bulb in particular that will cause problems; it contains two alkaloids, narcissine (also known as lycorine) and galantamine, plus a glycoside (scillitoxin).

If you eat a daffodil bulb having mistaken it for an onion, then you will at the very least have severe stomach cramps. The worst case scenario is death by poisoning.
The degree of toxicity does depend on the variety of Narcissus; Narcissus poeticus is the one of the more poisonous.
Some anecdotal tales of daffodil poisoning can be found here.

There is a positive side to the drug, as Narcissus alkaloids have been used to treat tumours and cancer.
If you have an autumn gardening question, do ask it in the ‘comments’ box or drop us an email. And let us know if you make rowan jelly!