Does seed size matter?

Some veg crops have much bigger seeds than others; this tells the gardener a lot about the best time to sow, says allotment holder and RHS Chief Horticulturist Guy Barter

Young tomato plants It might seem surprising, but when it comes to tender crops such as tomatoes and pumpkins, the size of a vegetable's seed has big implications for the best sowing time.

Small-seeded tender crops such as aubergines, chilli peppers, sweet peppers and tomatoes produce little seedlings that take longer to grow than crops with larger seeds and therefore bigger seedlings.

Cucumbers and melons are intermediate in this respect. Courgettes, French and runner beans, pumpkins, squashes and sweetcorn are all good examples of crops with an inbuilt size advantage.  The extra nutrition in their larger seeds gives their seedlings much greater vigour, enabling them to grow away a lot more quickly than their small-seeded cousins.

Timing is everything

Sowing tomato seedSown in February, aubergines, chilli peppers, sweet peppers and tomatoes take until April to reach planting out size (in greenhouses only of course) – at least eight weeks of warm bright conditions at a time of year when days are short, nights bitter and light levels low.

Pricking out tomatoesSown in late March these crops also require six weeks of growing indoors, before you can plant them outdoors or, even better, under cloches or fleece.

It is feasible, with care, to raise fair tomato plants on windowsills and unheated greenhouses from March sowings. Aubergines and peppers grow more slowly and buying in plants is usually very worthwhile, particularly if the subsequent summer is dull and cool.

By April, light levels soar and the days lengthen while cold nights lose some of their bite. Larger-seeded crops, sweetcorn for example, can easily produce large plants from April sowings for setting out in late May. Indeed in the south they can even be sown in late May direct  where they are to grow.

Energy implications

Heating a greenhouse for February sowings is costly.  While insulation with bubblewrap saves energy it reduces light levels, slowing growth. Windowsills are even less well illuminated and plants grow slowly and can be weakened and ‘drawn’ by lack of light.

To buy or not to buy?

As bought-in plants are fairly inexpensive and often excellent quality there is a good case for buying plants of aubergines, chilli peppers, sweet peppers and tomatoes. On the other hand, a much wider choice of cultivars is offered as seed than as plants.

For large-seeded crops there is less financial justification for buying in plants, although it is certainly convenient and perhaps less uncertain than growing from seed, especially for inexperienced growers.

For cucumbers and melons, plant quality is critical and plants offered for sale are often rather poor, particularly around the roots. Raising these from April sowings in biodegradable pots will give good quality plants that can be planted out with no ‘check’ in growth.

More advice on growing your own

Get expert impartial advice on growing your own fruit and veg.

Gardeners' calendar

Find out what to do this month with our gardeners' calendar

Advice from the RHS

Get involved

The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.