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Seed Sowing Times - Thompson & Morgan

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Specialised Treatment - Seed Sowing Guide

Coping with small seeds

Tiny seeds, almost as fine as dust, such as begonia and calceolaria, can be difficult to handle. Sometimes, in fact, the foil packet seems to contain nothing except a trace of dust-sized particles. Here's how to make a success of a tricky task.

  1. Use a small pan or pot for sowing, about 4 or 5in (10 or 12.5cm) is adequate.
  2. Fill the pan or pot to overflowing with the seed compost, then firm it first with your fingers, then with a wooden presser.
  3. Pour a heaped teaspoon of silver sand into the seed packet and shake to mix sand and seed.
  4. Sow the seed direct from the packet, tapping it slowly to release the sand-seed mixture evenly over the compost.
  5. Do not cover the seed with compost, simply press them into the surface with the wooden presser.
  6. Water the compost from underneath by standing the tray or pot in a bowl of tepid water.
  7. Cover with a piece of glass, cling film or seal inside a polythene bag to keep the compost moist and the atmosphere slightly humid.
  8. Remember that very fine seeds have a lower germination rate than normal-sized ones and the correct temperature for germination is very important

Vermiculite (T&M Gardening Supplies)

Some seeds benefit from pre-treatment before sowing or from being sown in a particular way. Here are brief explanations of the techniques mentioned in the following list. In almost all cases it is not critical that you carry out this pre-treatment but if it is not done the seeds will usually take longer to germinate.

Chipping

Some seeds, e.g. Sweet peas, Ipomoea etc. have hard seed coats which prevent moisture being absorbed by the seed. All that is needed is for the outer surface to be scratched or abraded to allow water to pass through. This can be achieved by chipping the seed with a sharp knife at a part furthest away from the 'eye', by rubbing lightly with emery paper or, with very small seed, pricking carefully once with a needle etc.

Soaking is beneficial in two ways; it can soften a hard seed coat and also leach out any chemical inhibitors in the seed which may prevent germination. Anything from 1-3 hours in water which starts off hand hot is usually sufficient. If soaking for longer the water should be changed daily. Seeds of some species swell up when they are soaked. If some seeds of a batch do swell within 24 hours they should be planted immediately and the remainder pricked gently with a pin and returned to soak. As each seed swells it should be removed and sown before it has time to dry out.

Damping Off disease

Damping off is one of the most common and troublesome types of garden diseases. It can affect all types of seedlings, but is most problematic on fast growing flower seedlings such as antirrhinums, lobelias, nemesias, petunias, salvias and stocks or vegetable seedlings like cabbages, cress, lettuces, tomatoes, peas and beans. Some slow growing trees and shrubs are also very prone to grey mould forms of damping off.

Symptoms are varied but all result in seedling death. Often young seedlings rapidly collapse in small roughly circular patches, or the seedlings may become weak looking and have shrivelled stems, sometimes the root system simply rot away. Some larger seedlings or young plants also sometimes show leaf spotting or other discoloration, some even show grey mould on the stems or leaves.

Organisms that cause damping off symptoms are varied. The most common ones that cause dying out in patches are the fungi Pythium and Rhizoctonia solani, surviving as spores in the soil. Stem lesions are often caused by soil-borne species of Alternaria and leaf spots are generally associated with soil-borne Phyllosticta and Pseudomonas fungi. The grey mould that often accompanies damping off is caused by Botrytis cinerea.

Treatment is difficult in the garden environment. Some chemical controls have been employed in commercial practices, but are not yet available for use on a small scale. For the amateur or small-scale grower hygiene at all stages of propagation is essential. Only use cleaned and disinfected pots and seed trays and make sure that greenhouse benches are sterile. Mains water and a proprietary sterilised seed compost, which is moist but not over wet, should also be used. Don't assume all bought compost is sterile, as most is not.

Small quantities of compost for seed sowing can be sterilised by 'cooking' in an oven at 150C for an hour or so. Care should be taken when using water other than tap water. All storage tanks should be regularly cleaned and disinfected regularly, preference should always be given to tap water on susceptible plants.

Avoid stressing plants and seedlings by preventing waterlogging and high humidity, as this will make them more vulnerable and prone to attack. Sow seed thinly and prick out as soon as possible, handling the seedlings by their leaves, not stems. Do not re-use compost that has been affected and if only part of the seed tray has shown symptoms remove all the affected seedlings, a few extra seedlings and the affected compost. Water with Cheshunt Compound or similar fungicide to help prevent any spread. Cheshunt Compound can also be used as a soil drench prior to seed sowing as a preventative measure, but this will not completely eradicate all problems. It should be used as a preventative aid.

Double dormancy

Some seeds have a combination of dormancies and each one has to be broken in turn and in the right sequence before germination can take place. For example some Lilies, Tree paeonies, Daphne etc. need a warm period during which the root develops followed by a cold period to break dormancy of the shoots, before the seedling actually emerges. Some seeds need a cold period followed by a warm period and then another cold period before they will germinate. In all cases the times and temperatures have been provided in the sowing instructions.

Outdoor sowing

Chipping, soaking, pre-chilling and double dormancy are methods that accelerate the germination process and help to prevent seeds being lost due to external hazards (mice, disease etc.) but outdoor sowing is just as effective albeit longer. The seeds are best sown in containers of free draining compost and placed in a cold frame or plunged up to their rim outdoors in a shaded part of the garden, preferably on the north side of the house avoiding cold drying winds and strong sun.

Recent tests show that much of the beneficial effects of pre-chilling are lost if the seed is not exposed to light immediately afterwards. We therefore recommend sowing the seeds very close to the surface of the soil and covering the container with a sheet of glass. An alternative method especially with larger seeds, is to sow the seed in well prepared ground, cover with a jam jar and press this down well into the soil so that the seeds are enclosed and safe from predators, drying out etc.

Pre-chilling

In some instructions you will find a reference to 'pre-chilling'. This is a pre-treatment of the seed which often helps to speed up the germination of otherwise slow to germinate seeds. However, even after pre-chilling some seeds can stubbornly refuse to germinate until a year or more has passed, so never be too hasty in discarding a seed container.

Pre-chilling was traditionally done by standing the pots outside in a cold frame during the winter. It is often quicker to adopt the following technique using a domestic refrigerator and this is of particular value if you obtain your seed outside the winter months.

To pre-chill, first sow the seed on moistened seed compost, seal the seed container inside a polythene bag and leave at 60-65F (15-18C) for 3 days then place in a refrigerator for the recommended period. For convenience large seeds can be mixed with 2-3 times their volume of damp seed compost, placed direct into a polythene bag which is sealed and placed in the refrigerator. However, there must always be sufficient air inside the bag and the compost should NEVER become either too dry or over wet. After pre-chilling these seeds can then be spread with the compost on top of a seed container and firmed down.

The seeds must be moist whilst being pre-chilled, but it will harm them if they are actually in water. During the period in the refrigerator, examine the seeds once a week and remove all the seeds into the specified warm conditions if any of them start to germinate.

Light also seems to be beneficial after pre-chilling, so pre-chilled seeds should have only the lightest covering of compost, if any is required, and the seed trays or pots, should be in the light and not covered in paper.

Soaking

Soaking is beneficial in two ways; it can soften a hard seed coat and also leach out any chemical inhibitors in the seed which may prevent germination. Anything from 1-3 hours in water which starts off hand hot is usually sufficient. If soaking for longer the water should be changed daily. Seeds of some species swell up when they are soaked. If some seeds of a batch do swell within 24 hours they should be planted immediately and the remainder pricked gently with a pin and returned to soak. As each seed swells it should be removed and sown before it has time to dry out.

Sowing In Situ

Where outdoor sowing is recommended, planting in a moist soil which is weed free and has been raked down to a fine tilth is essential. For hardy annuals and perennials sowing can be carried out from late winter onwards as soon as the ground is workable and has warmed up; half hardy annuals after all danger of frost is passed and when the soil has warmed up. Full details can be found on the separate page Sowing Outdoors.