Seeds and seedlings Q and A

Can I keep left-over seed for sowing next year?

It depends on the seed and how it is kept. Seeds deteriorate rapidly in moist and warm conditions (for instance, in garden sheds and hot kitchens). Always keep seed somewhere cool and dry, preferably in an airtight tin or box, with a bag or a dish of cobalt chloride-treated silica gel in it to absorb any moisture. When this gel turns pink, put it in an oven, dry it until it turns blue, and put it back in the box. Do not keep parsnip and scorzonera seed; and keep peas and beans only for two years. Marrows, cucumbers, most brassicas, tomatoes, beetroot, spinach, and lettuce generally remain viable for three or four years if stored carefully. Seed stored in air-sealed foil packs or glass jars keeps fresh longer than seed in ordinary packets.

How should I prepare a seedbed?

A seedbed is any ground where seeds are being sown. The rough surface must be raked to a fine tilth after it has been forked over in spring, to make sowing easy. Rake the soil several times in each direction, drawing small stones and clods to one side. Heavy or wet soil may need to be broken down in stages. A multi-pronged cultivator is useful for breaking down clods initially; then allow the soil to dry out a few days before raking, leave it again, rake it again, and so on. Small clods can sometimes be crumbled up by hand, or broken by walking lightly over the ground before raking. Dry soil may need watering before it can be raked to a tilth. After light soils are dug over in spring, cover them with a light mulch of straw. This keeps the surface in a beautifully moist condition until it is time for sowing your seeds.

How deep should seeds be sown?

Roughly speaking, the larger the seed, the deeper it should be sown. Small seeds such as carrots, leeks, lettuce, and onions can be about 13-19 mm (½ -3A in) deep; slightly larger seeds such as cabbages and marrows 19-25 mm ¾-l in) deep; sweet corn and peas 25-38 mm (1-1 ¥2 in) deep; beans 38-50 mm (lVfe-2 in) deep. Tiny seeds, and seeds such as celery which require light to germinate, should be sown on the surface. They are best sown in seed boxes, so they can be watched over and kept moist (but not wet) until germination occurs.

I get poor germination from outdoor sowings in hot weather. Can you tell me why?

Results are often poor in dry conditions because seeds dry out before they start to germinate. Here is a useful tip. Make your drill in the usual way, then, using a can with a fine spout, water just the drill fairly heavily. Let the water sink in, sow the seeds in the moist drill, press them in gently, then cover them with dry soil, firming it afterwards. The dry soil acts as a mulch, preventing the moisture from evaporating. You will then find germination will be very rapid. Seeds of butterhead varieties of lettuce will not germinate if the soil temperature is above 25°C (77°F). The problem can be overcome in hot weather if you sow between 2 and 4 o’clock in the afternoon, so that the critical germination stage occurs in the cool of the evening. Water the drill to lower the soil temperature, and shade the soil after sowing.

When does one broadcast seed, and how is it done?

Broadcasting is useful, especially in small areas, for growing seedling crops such as mustard, rape, cress, sugar-loaf chicories, lettuce, and even spinach. The less-well-known Mediterranean rocket (Eruca satiua), a tasty, spicy, easily grown salad plant, is also ideal for broadcasting. These can be grown closely and cut when 50 mm (2 in) high for salads: they will re-sprout to give several subsequent cuttings. Rake the soil smooth, and sprinkle the seed evenly but thinly on the surface. Then rake the soil again, first in one direction then at right angles to the first direction, and cover the seedbed with clear polythene until the seeds germinate. Try to avoid broadcasting on ground where annual weeds have set seed, as weeding will be difficult.

I never seem to have time to thin seedlings properly. Can thinning be simplified—or eliminated?

Under good conditions seedlings grow fast and soon become checked, over-crowded, and susceptible to disease if they are not thinned. It is a very important, and often neglected, garden task. The secret is to sow very thinly. Sow radishes, for example, 25 mm (1 in) apart. Most other vegetables can be ‘station sown’—that is, in clusters of 3 or 4 seeds together, with a gap between clusters. If the plants will eventually be, say, 150 mm (6 in) apart, make the stations 75 mm (3 in) apart; this will make thinning much easier. Start thinning as soon as the seedlings are large enough to handle; thin so that each seedling stands just clear of its neighbour. Do not pull up surplus seedlings; nip them off at the base just above ground level. This causes much less disturbance.

In many cases thinning can be avoided by raising plants in seed trays, pots, or peat blocks and planting them out.

Why is one always told to remove thinnings? I should have thought they could be left on the ground to rot.

The point is that some pests are attracted to their ‘host’ plant by its scent. Crushed or bruised foliage smells much stronger (even though we might not be aware of it) and can prove a great giveaway. This is particularly true of the carrot fly, a serious garden pest; it is attracted like a magnet to a row of carrots by the scent of thinnings, and then lays its eggs on the top of the young carrot roots. So bury the thinnings as deep as possible in the compost heap.

Why must some vegetables be sown indoors?

Sowing ‘indoors’ means sowing under cover, whether in heated or unheated greenhouse, in a heated propagator, under cloches or frames, or even on a window sill indoors. This gives plants an earlier start, and therefore a longer growing season. It is useful for several types of crops: first, for tender vegetables such as tomatoes, peppers, cucumbers, sweet corn, and French beans, which cannot be planted outside until all risk of frost is past; second, for plants such as celery, celeriac, and maincrop onions, which need a long growing season to reach their full potential, and so will do best if they are planted early; third, to get exceptionally early crops of ordinary vegetables, such as lettuces, cabbages, and cauliflowers.

Should vegetables and flowers be treated differently when sown in pots or boxes?

Most common vegetable seed is robust, relatively large, and germinates easily, so it can be sown directly into potting compost, rather than being started in a fine sowing compost (exceptions are delicate seeds such as celery and celeriac). Because relatively few vegetable plants are required at any one time, they can be sown thinly and it may be unnecessary to prick them out into pots. Cauliflowers or lettuce, for example, can be sown in a tray of potting compost, with the seeds 25-38 mm (1-1 ½ in) apart, and then planted out directly when they are large enough. Large seeds such as cucumbers, marrow, sweet corn, and French beans can be sown in small individual pots or seed trays divided into sections, and so planted out with minimum root disturbance.

What are soil blocks, and why are they recommended for raising vegetable seedlings?

Blocks are made by compressing potting compost with small hand tools, which make a hole in the top of the block where the seed is sown. The seed germinates, and having no competition, grows into an exceptionally strong plant with a fine root system. There is minimal root disturbance when the block and its seedlings are planted out as one. Vegetables which normally do not transplant well, such as Chinese cabbages, can be successfully transplanted when grown in blocks. If soil conditions are unsuitable for planting, the plants can remain in soil blocks, without harm, for longer than would be possible in a seed tray.

I’ve started using soil blocks for vegetables, but find it hard to sow the smaller seeds individually. Have you any suggestions?

Try putting the seeds on a piece of paper and gently pushing one at a time into the holes. Alternatively, get a small piece of broken glass, put the seeds in a saucer, and moisten the tip of the glass. You will find you can pick up a single seed on the tip of the glass (rather like spillikins!). The seed will drop off when you touch the glass against the hole. You can always sow several seeds in each block, nipping off at just above ground level extra seedlings once they have germinated to leave one plant per block. (Do not pull the seedlings out, otherwise the plant blocks may disintegrate).

Can you explain the term ‘hardening-off?

Plants which have been raised in protected conditions indoors need to be ‘toughened up’ over a period of 10 to 14 days before planting out or they will suffer a severe check. Start this hardening-off by gradually increasing the ventilation in the greenhouse, frames, under cloches, or wherever your plants are. Next, move indoor plants outside during the day, or into cold frames or cloches where available, initially bringing them indoors at night, and later closing up the cloches or frames. Expose the plants to increasingly longer periods outdoors, until they have no protection day or night. They are then ready for planting. Hardening-off is particularly important for plants raised in peat-based composts, which encourage softer, lusher growth than soil-based composts.

What is the best size at which to plant out vegetables?

As a general rule, the smaller they are the better, with the proviso that if they are too small they will be especially vulnerable to pests. Plant lettuce seedlings when they have about four true leaves (do not count the first tiny ‘seed’ leaves), fennel when it has two true leaves, brassicas when they are 100-125 mm (4-5 in) high, tomatoes and peppers when the first flowering truss is visible. Root vegetables such as beetroot, parsnip, and parsley can be transplanted only when they are very small, before the roots start to swell.

Have you any general hints on planting vegetables?

Everything should be done to minimise the shock of transplanting and to avoid damaging the delicate root hairs. Whether the plants are in a seedbed, seed tray, or soil blocks, water them thoroughly a couple of hours before moving them. Then, using a trowel, make a hole in the prepared ground large enough to accommodate the roots (water the ground first if it is very dry). Dig up the plant carefully, holding it by the leaves, not by the roots. With one hand hold the plant in the hole, with the other tuck the soil around the roots, firming it gently with your fingers; the lowest leaves should be just above soil level. Finally, give a leaf a gentle tug: if the plant wobbles, make it firmer. Then water it gently. In very hot weather young plants such as brassicas will wilt. Shade them with conical ‘sun hats’ made of newspaper for a couple of days until they perk up.

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