March can be a tricky month, with temperatures rising in the greenhouse and polytunnel on sunny days, but still the threat of frost at nights. You will need to be vigilant, particularly in caring for young seedlings. The reward, if the weather is mild, is the first tender harvest of early-sown salads

• Sow in pots or modules for planting out undercover: Tomatoes*, peppers*, chillies*, aubergines*, courgettes*, French beans*, basil*, cucumbers*, half-hardy companion plants (such as French marigolds, cosmos)*
• Sow in pots or modules for planting on veg plot: tomatoes*, summer cabbage and cauliflowers, parsley, leeks, broad beans
• Sow direct undercover: Carrots, beetroot, second batch of cutting salads (leaf lettuce, rocket, cress, oriental greens); green manures
• Plant undercover: Calabrese, cabbage, lettuce, kohl rabi, sugar peas, spring onions (all sown in pots or modules last month), leftover onion sets
(* crops which need extra heat) 
Early-mid March is the last chance to sow tomatoes, aubergines and peppers if they are to give you a worthwhile crop. Otherwise you will need to buy plants. To be sure of a choice of varieties, order now from mail-order catalogues for April or May delivery rather than rely on the garden centre. 
When planning a vegetable garden, remember that you want to plant all warm season vegetables after the last frost. Pay attention to these frosts, and plan accordingly in order to choose the best, safest growing option. Planting zones and recommended times are usually listed on the back of seed packets.
In mild areas, sow courgettes, French beans, and outdoor varieties of cucumber in heat for planting out in polytunnel beds. They only take three to four weeks to be ready for planting, so only sow if you will be able to keep the plants frost free by then – otherwise wait until early next month. Greenhouse cucumber plants need to be maintained at higher temperatures (at least 15C/60F), so early sowings need heated greenhouses.
While the soil we supply is nutrient enriched you will still need to feed your vegetables. Proper fertilizer is essential for your new vegetable garden. Water soluble or granular fertilizer is the best option. 15-15 or 20-20 fertilizer is a good choice, as it affords your plants the correct balance and mix of essential nutrients and ingredients they need. You should fertilize the soil before planting the seeds by at least a week or so before. The fertilizer should be worked into the top soil six inches deep. After your plants begin to grow, you can always add more fertilizer to your vegetable garden as the need arises.