Why you should always plant more seeds than you need, according to a garden expert

As spring arrives, so the time for sowing and planting – or “establishing” – begins. Good establishment is when new plants are sturdy and growing – and that success depends on sufficient warmth and light.

Light levels are rising and, by April, when plants that have been established begin to grow, light is plentiful. But if weeds creep in or if sowing or planting has been too thick, with insufficient space between plants, competition for light leads to weak, elongated or “drawn” plants that grow slowly.

On the other hand, having too few plants leave gaps that weeds can exploit. Spacing guidance is given on seed packets. For plants, perennials or shrubs, for example, a rough guide is to plant no closer than half the plant’s eventual height shown on the label. If in doubt, plant slightly more widely.

Not every seed will produce a plant. It is best to sow extra seeds to make up for expected losses. If more plants than expected establish, they can be removed later by “thinning”. Typically, sow enough seeds for three times the required number of plants in March, declining to 10 per cent extra by late May, when weather is more favourable.

Alternatively, sow extra seeds at the end of rows for seedlings that you can lift and plant in gaps. Carrots and parsnips seldom grow well if transplanted, but most other garden plants will succeed – lettuce and beetroot, for example.

With the soil moist after winter rains, watering is rarely needed unless cold, east winds dry it out. Then shallow-sown seeds and newly planted evergreens will benefit from watering. If the soil is too wet to sow or plant, hold off until drier weather arrives – the resulting seedlings will catch up. Clay soils are notoriously slow to dry – they may not be ready until mid-April. For clay soils, raising transplants in cell trays or pots is often more successful than sowing direct.

Seeds of hardy plants germinate when temperatures reach 6-7°C. It can be hard to judge when this critical point is reached. A rough guide is that when weed seedlings appear, hardy seeds can be sown successfully.

All gardeners like to start sowing and planting early and accept the risks of this failing, for good reasons. Early flowers and vegetables can be enjoyed sooner, plants will be robust by autumn and ready to face winter, while a long growing season boosts the yield of carrots, parsnips and potatoes.

If early sowings fail, simply resow – open-pollinated or non-hybrid seeds are relatively inexpensive and appeal to less experienced growers. Resowing with cheaper seeds is less painful than with expensive hybrid varieties.

Planting sets, tubers or rootstocks is an effective and less demanding alternative to seeds. Once the soil is dry and warm enough – March for onion and shallot sets, April for less hardy tubers such as dahlias and potatoes – these can be planted for reliable results.

Raising seedlings indoors, whether on window sills or in greenhouses, gives gardeners much more control, is essential for small-seeded tender plants such as tomatoes or pelargoniums, and makes the best use of expensive seeds. It takes about six weeks to raise a transplant from most seeds until it is ready to plant out. Large-seeded plants, such as courgettes or sunflowers, are quicker; dust-like begonia and nicotiana seeds take longer.

Finding enough space to house transplants for six weeks, especially frost-free, can be challenging. But gardeners are ingenious in finding space, making low fleece tunnels outdoors or perhaps investing in or improvising cloches and cold frames. These can all be invaluable in growing new plants.