What to Sow Indoors This March: A UK Growing Guide

The March Window

March marks a turning point in the UK growing calendar. While the ground outside may still be cold and frosty, your windowsills and greenhouse can become a hive of activity. Starting seeds indoors now gives your plants a crucial head start, so they are ready to plant out once the last frosts pass in late April or May.

Here is what you can sow indoors right now — and how to give each crop the best possible start.

Tomatoes

March is the ideal month to sow tomatoes indoors. Fill small pots or seed trays with moist compost, sow seeds about 1cm deep, and place them on a warm, sunny windowsill. Tomatoes need warmth to germinate — around 18-21°C is perfect.

Once seedlings develop their first true leaves, pot them on into individual 9cm pots. They will be ready to move outside or into a greenhouse from late May.

Top tip: Do not overwater seedlings. Let the compost dry slightly between waterings to encourage strong root growth.

Courgettes and Cucumbers

Courgettes and cucumbers are fast growers that respond brilliantly to an indoor start. Sow seeds on their side in 9cm pots — this prevents water sitting on the flat seed surface and causing rot.

Keep them in a warm spot (around 20°C) and they should germinate within a week. These are hungry plants, so use a good quality multipurpose compost and start feeding with liquid fertiliser once the first true leaves appear.

Peppers and Aubergines

Peppers need a long growing season, which is why March is the latest you should be sowing them. They are slow to germinate (sometimes taking 2-3 weeks), so patience is key.

Sow seeds in a heated propagator or on a consistently warm windowsill. Both peppers and aubergines love heat — they will struggle if temperatures drop below 15°C.

Leafy Greens: Lettuce and Spinach

For quicker results, sow lettuce and spinach indoors now. These crops germinate rapidly and can be harvested within 6-8 weeks.

Succession sowing is the secret here — sow a small batch every two weeks and you will have a continuous supply of fresh salad leaves right through summer. A shallow tray on a cool windowsill is all you need.

Root Vegetables: Carrots and Beetroot

While carrots and beetroot are often sown directly outdoors, early varieties can be started indoors in deep pots or modules. This is especially useful if your soil is still waterlogged or frozen.

Use a fine, stone-free compost for carrots to encourage straight root growth. Beetroot seeds are actually clusters of multiple seeds, so thin to the strongest seedling once they are a few centimetres tall.

Getting the Basics Right

No matter what you are sowing, a few principles apply:

Light: Seedlings need as much light as possible. A south-facing windowsill is ideal. If stems are stretching and leggy, they need more light.

Warmth: Most seeds germinate best at 18-21°C. A heated propagator helps enormously.

Water: Keep compost moist but never waterlogged. Water from below where possible.

Airflow: Good ventilation prevents damping off — the fungal disease that kills seedlings at soil level.

Share Your Surplus

One of the joys of starting seeds indoors is that you almost always end up with more seedlings than you need. Rather than composting the extras, why not share them with your neighbours?

That is exactly what Locavori is built for — connecting you with people nearby who would love your spare tomato plants or courgette seedlings. Growing is better when it is shared.

What is Next?

As March turns to April, you can start hardening off your indoor seedlings by placing them outside during the day and bringing them in at night. By May, most will be ready for their final positions in the garden, raised bed, or container.

Check out our growing guides for detailed instructions on each crop, including spacing, feeding, and harvesting tips tailored to UK climate zones.

Happy growing!