It is December 21st, the winter solstice and the shortest day of the year. Everything in the garden is sleeping – or is it?
The gardening books will tell you that plants will not grow when day length is less than ten hours. They also recommend that you should not attempt to grow vegetables if you can not achieve more than six hours of sunlight each day. At this time of year in London, our day length is about eight hours and the altitude of the sun is considerably lower than during the summer. Most of my roof top veg plot is in shadow for most of the day. We won’t get ten hours of daylight until March. So how come we can plant seeds of some species before that date?
Here are some answers;
Choosing the correct species
Only temperate plants have evolved to respond to the long darkness of winter by hibernating. Just as squirrels snuggle down, but dogs stay active, vegetables have different habits. Even some sub-sets of the same cultivars react differently to day-length.
For example, there are winter varieties of plants like onions and radish, which will not grow so well in longer days. And lettuces are notorious for bolting when the nights start to shorten around mid summer. That is why lettuces are planted in partial shade in late spring. Put them into too much sunshine and they flower profusely and delightfully but, for the veg-plotter, annoyingly!
Plants like tomatoes and cucurbits, which are tropical and sub tropical species, don’t get so fussed about day length. In the tropics the sun rises at a regular six am and sets around six pm. Tomatoes will flower early in the year as well as late into the autumn, if they are kept warm. My lime tree, which is a sub-tropical cultivar, is happily flowering away in my studio, as I write this blog on the shortest day of the year.
Maximising light for photosynthesis:
All green plants absorb carbon-dioxide and energy from sunlight through special cells within the leaves. With the aid of water, drawn up from the roots, it manufactures sugars, which forms the building blocks of all green plants. The waste product is oxygen – the stuff of our life.
In the winter the gardener can generally provide enough carbon dioxide and water, but sunlight hours are shorter and insolation (the energy from the sun) is less. In order to promote strong development of these plant sugars, which can not be substituted with fertilizers, we must make the most of the light, and especially the sunlight that is available.
Put seedlings in the lightest and brightest place you can find:
Take off plastic germination covers once the seedlings have come through. A plastic cover can reduce light levels inside the dome by 20% or more.
Turn seedlings each day, so that they are not drawn to one side by uneven daylight. Clean your windows and pray for winter sunshine, which increases the strength of the suns energy.
Drawer curtains early in the morning – do not miss out on those beneficial early morning hours of sunlight.
Balancing environmental factors:
It is not only day length that will affect seedlings. All plants need warmth, water, carbon-dioxide, and a whole host of trace elements in order to grow. So when we have lower light levels, we will need to pay more attention to the plant’s other needs.
Most plants don’t like frosts, so being able to protect your crop with forcing fabric, using cold frames, greenhouses or the sunny windowsill will all have a good effect. However heat without water is useless. Even in cold weather seedlings can suffer badly from drought.
Some people advise using a hot-bed, that is growing winter veg in a layer of compost laid on top of a foot or so of muck. But I’ve always found that difficult to achieve in an urban situation.
Once indoors, or under cover, you will also need to ensure that your plants get enough carbon-dioxide across their leaves. I know many people swear by talking to their plants. Breathing carbon dioxide onto them, as you whisper sweet nothings, may well be a very valuable exercise, especially for indoor plants which are not subject to natural airflows. In fact, still air can also encourage mould growth.
Young plants in winter may also benefit from a nutritious mulch, or foliar feeds. But, bear in mind that winter growth, however successful, will always be slower than summer growth, so keep feeding strengths dilute. Over feeding may burn the plant.
It is the careful balance between all these factors which is important. Too much of anything will kill them; not enough will see them ailing too.
One advantage of bringing seed trays indoors is that you might spend more time watching them, and can modify cultivation quickly if they show any signs of stress.
Warmth, water and oxygen are the essentials for germination. More seeds fail to germinate through lack of water, rather than lack of light. Many seedlings will germinate quite happily in a dark, moist atmosphere. Imagine what conditions might be like an inch or two underground, on a warm evening after light rain; that is the ideal situation for seeds.
Once sown and watered, a seed will swell and begin to sprout. At this stage it does not generally need sunlight because it uses oxygen to metabolise the starches, proteins and oils stored within the seed. A shoot, the hypocotyl, will sprout and start to grow towards the soil surface. At the same time an embryonic root, or radicle will appear, which will eventually nourish the new plant. Most vegetable seeds have one or two embryonic leaves which remain tightly curled around the stem, until they reach the light. These first seed leaves, or cotyledons, may be very different from the adult plant leaves. If sufficient light is present, these leaves will turn green and start the process of photosynthesis. If the light levels are too low, the first leaves may look yellow. The shoot may continue to grow upwards, despite being spindly, in order to reach more light. This is called etiolation, and is a risk with all winter and indoor seedlings. The solution is light, light and more light.
Once the seedling starts to photosynthesise it is no longer dependant upon the store of energy within the plant.
We generally wait until true leaves show before transplanting seedlings. The first true leaves are a good indicator that the root system has developed strongly enough to withstand the shock of a move.
If the seeds are not growing well, if they start to look spindly and pale, then there are a number of actions that can be taken.
Move the seeds into a lighter position, or move them around the room or greenhouse in order to catch morning and afternoon sunlight. If all else fails, you can light the plants. A hundred watt Anglepoise won’t do much good. Specialist plant lights can be purchased from garden stores. They have to be suspended close to the growing plants and they do add to cultivation costs. But, with the aid of electric light, plants will grow without any sunshine at all.
Do not despair if your seedlings start to look a bit spindly. From now on the length of day is steadily increasing. By this time next month, the levels of insolation, the suns energy, will be roughly double what they are today.
Plants will recover as the days lengthen and sun angles increase.
If you feel that the crop is really too far gone, etiolated salad and herb seedlings
can be harvested as micro-veg when they only have their cotyledon leaves. This provides a most nutritious, but slender, meal.
If the true leaves look a little spindly, they can be pruned, (which can give you another catch crop of salad leaves.) This can encourage a spurt of new growth from above the first leaf sprout. But do not expect a beheaded shoot to re-sprout. The germ or node of the plant, which provides all the genetic information, lies between the first branching leaves. Cut this away and the plant will die.
Alternatively, when you transplant veg seedlings, you can sink them deeper into the soil. Many plants, especially sub-tropicals like tomatoes, will develop strong roots from an etiolated stem, which is put into contact with the soil.
You can also consider grafting elongated shoots. This again works well with tomatoes and peppers and provides the added benefit of being able to combine the characteristics of a sturdy rootstock, with the more delicate characteristics of an heirloom.
Whatever happens, learn from your experiences. Plant a little later in the year next year, add more protection, or consider a different variety.