At a glance
March is the month when seed sowing genuinely opens up in the UK. January and February required heat, supplemental light and patience – March brings improving temperatures, significantly better natural light and, in the southern half of the country by mid-month, soil that is beginning to warm enough for direct outdoor sowing of hardy crops. The range of what can be started this month is substantially broader than the two preceding months and includes the first proper outdoor sowings of the season. For many UK gardeners, March is when the excitement of the growing year begins in earnest.
The practical challenge in March is managing multiple simultaneous sowings across different temperature requirements and locations. Tender crops like tomatoes and courgettes still need indoor warmth. Hardy crops like spinach, peas and broad beans can go under cover or outside depending on the weather. Brassicas – cabbages, broccoli, Brussels sprouts, kale – need to be started in March for transplanting in May and June. Getting the timing and location right for each type avoids the twin failures of tender crops killed by cold and hardy ones that bolt because they were raised in too much warmth. The sowing guide below works through each category clearly.
Sowing indoors
March is the primary month for starting brassicas from seed in the UK. Cabbages, broccoli, purple sprouting broccoli, kale and Brussels sprouts all benefit from an indoor module sowing in March – they are transplanted to their final positions in May and June once they have developed into robust seedlings. Sow one or two seeds per module cell at 15-18°C and thin to the strongest. Brassicas do not need the high temperatures required by tomatoes and can be germinated on a cool windowsill or in an unheated propagator – indeed, excessively warm germination conditions can cause them to grow too quickly and produce weak, drawn seedlings.
Courgettes and squash are best started at the very end of March rather than the beginning – they germinate within days and grow so rapidly that a March sowing is planting-ready by late May, which is exactly the right time. Start them any earlier and you will have large, pot-bound plants sitting waiting for the weather to improve. Sow one seed per 9cm pot on its side – this prevents water sitting on the flat seed surface and causing rot – and keep at 20°C until germinated.
Direct outdoor sowing
Best varieties for March sowing
March sowing tips and checklist
Hardy annual flowers scattered in March establish better than later sowings because they have time to develop strong root systems before summer heat arrives. Cornflowers, nigella, calendula, larkspur and ammi majus are all straightforward from a direct March sowing – rake the soil surface to a fine tilth, scatter the seed thinly, rake in very lightly and water. Do not cover with fleece as most hardy annuals germinate best with exposure to light. Thin seedlings to 20-30cm apart once established. The only mistake to avoid is sowing into wet, cold soil that has not been allowed to drain – seeds sown into waterlogged soil rot rather than germinate, and March can still produce days where the soil is saturated after winter rain. Wait for a dry spell and the surface to begin to lose the grey, wet sheen before sowing.
Peas direct sown in March perform particularly well if the soil has been prepared in advance. Peas fix atmospheric nitrogen through root nodule bacteria and do not need a rich soil – they actually perform better in a moderately fertile soil than a heavily manured one, which produces excessive leafy growth at the expense of pods. What they do need is good drainage and a soil pH of 6.0-7.0. Sow in a flat-bottomed drill 5cm deep and 5cm wide, placing seeds in a double row 7cm apart. Support with twigs, netting or a proper pea frame before the plants need it rather than after – trying to thread established pea growth through netting is an exercise in frustration.
March is the one month you can sow too early outdoors as well as too late. The temptation in a warm spell is to get seeds straight into the ground, but a cold snap after sowing – common in March – can either kill seedlings outright or trigger bolting in crops like spinach and some lettuces that interpret cold as a signal to set seed. Use fleece over outdoor sowings throughout March and be patient with tender crops. The soil will be warm enough by late April for direct sowing without protection.
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