This week in the garden there will be many of the of the usual autumn jobs including pot washing, end of season greenhouse cleaning, compost turning as well as other tasks that need to be done at this time of the year. We’ll be taking cuttings from our currant bushes, apples trees (scion wood for grafting in the spring) and Kiwi once the leaves have fallen off them indicating that they have gone dormant for the winter. These cuttings will be labelled, and placed in the sand boxes where by spring time they should have produced roots and will be ready to be lifted and potted on.
Moving trees is also best done in the autumn. With deciduous trees you know the time is right once they have dropped their leaves. We will be preparing the ground for transplanting a small apple tree which has been growing in a very shady part of the garden close to a hawthorn and pyracantha. Once its leaves have fallen we will be moving it so a lighter part of the woodland where it should have sufficient light to grow well.
Autumn colours and leaf fall, as seen from a biochemical perspective!
These past weeks we have been seeing some beautiful autumn colours on trees and shrubs, as well as a good deal of falling leaves. You might be curious to know about the biochemical changes that are happening inside plants that make for these attractive autumn displays. To understand these changes horticulturalists learn about five important plant hormones and how they effect the plant.
There are the growth hormones, the auxins which cause cell division and enlargement, the gibberellins which control stem extension growth and dormancy, and the cytokinins with stimulate cell division and are responsible for the bushy side growth plants put on once the growing tip of the main stem has been removed, for example when you pinch out the tip of your young sweet pea plants because you want to encourage the plants to become more bushy. These three hormone groups are responsible for regulating the growth of a plant.
The other two hormones are abscisic acid and ethylene, and these are not growth promoters. Abscisic acid is often referred to as the stress hormone. Plants produce this when they are stressed, and it causes the stomata, (small openings in leaves) to close during droughty conditions to enable plants to preserve as much water as possible. Abscisic acid is produced by wilting plants, and by plants to induce dormancy.
Ethylene is a gas which promotes ripening of fruit and causes leaves to fall. Plants produce ethylene when their fruits are ripening.
During the growing season most leaves are predominantly green and this is because of the chlorophyll in them which absorbs a wide range of coloured light but reflects green light, giving rise to the green colour of leaves.
However in autumn when the chlorophyll, (site of photosynthesis) ceases, because of the action of ethylene and abscisic acid this masking of the other pigments in the leaves stops, allowing the other colours to show through. This gives rise to autumn colours.
It is ethylene and abscisic acid that are primarily responsible for the onset of dormancy in plants causing the sap to sink and the plants to gather all its reserves from the leaves and move them to winter storage organs such as roots. This is a sign that deciduous plants are preparing for dormancy as a winter survival strategy, and it gives rise to some amazing autumn colours!