Spring Planting Guide: What to Sow in March

Seeds to Start Now

March is the month the garden wakes up, and the gardener must be ready to meet it. After the long pause of winter, the soil temperature is beginning to creep upward, daylight hours are noticeably lengthening, and the seed packets that have been sitting in the tin since January are finally calling. Get your timing right this month and you'll be harvesting abundantly come summer and autumn.

March marks the true beginning of the gardening year. With daylight hours increasing, it's the perfect time to sow a wide variety of crops both outdoors and under cover.

Outdoors, hardy vegetables can be sown directly into prepared soil once it is workable and not overly wet. Run the soil through your fingers — if it crumbles cleanly, it's ready. If it smears, give it another week. Good candidates for direct sowing this month include:

• Carrots
• Parsnips
• Peas
• Broad beans (if not sown in autumn)
• Salad leaves (in a sheltered spot or cold frame)
• Radishes - among the fastest to germinate and great for filling gaps between slower crops

Indoors or in a greenhouse, start your tender crops now. These plants need warmth to germinate and cannot go outside until all risk of frost has passed — typically late May in most parts of Ireland and the UK. Use seed trays or small modules filled with fine seed compost, and keep them in a consistently warm, well-lit space. Good options to start under cover in March include:

• Tomatoes
• Peppers and chillies
• Aubergines
• Celery
• Celeriac
• Leeks

Consistent moisture is key for all of these, but avoid over-watering. Soggy compost is one of the leading causes of damping-off disease, a fungal condition that can wipe out an entire tray of seedlings overnight.

EXPERT TIP
Always label your seed trays the moment you sow them, not after. Seedlings look remarkably alike in their early weeks, and even experienced gardeners have lost track of what is what. Use permanent marker on plastic labels, and note both the variety name and the sowing date.

Bulbs, Sets, and Tubers

Not everything in March starts from seed. This month is also the traditional time to get onion sets, shallot sets, and early potato tubers into the ground.

Onion sets are small, partially-grown bulbs that establish more reliably than onion seed in unpredictable spring weather. Plant them roughly 10cm apart in rows, with just the tip showing above the soil surface. Birds have a habit of pulling them up out of curiosity — if this happens in your garden, push them back in and consider covering the row with fleece for the first few weeks.

Early potatoes (known as "first earlies") can go in from mid-March onward, provided the soil is not frozen and there is no hard frost forecast. Plant them around 30cm apart and 10–12cm deep. Chitting your potatoes — standing them upright in egg boxes in a cool, bright room for several weeks before planting — gives them a head start and can bring forward your first harvest by two to three weeks.

EXPERT TIP
Stagger your sowings across the month rather than doing everything at once. Sowing carrots every two weeks from mid-March through May, for example, will give you a continuous harvest rather than a single glut followed by a long gap.

Hardening Off: The Step Most Gardeners Rush

Any seedlings started indoors will eventually need to move outside — but the transition cannot be abrupt. Plants grown in warm, sheltered conditions are not accustomed to wind, fluctuating temperatures, or direct sunlight, and moving them straight outdoors can cause serious stress or even kill them outright.

Hardening off is the process of gradually acclimatising your seedlings to outdoor conditions over a period of one to two weeks. The method is simple:

• Begin by placing seedlings outdoors in a sheltered spot for an hour or two on mild days, then bringing them back inside
• Over the following days, gradually increase the time they spend outside
• Introduce them to more exposed conditions — light wind, cooler temperatures — as the week progresses
• After seven to fourteen days, they should be ready to stay outside overnight, provided no frost is forecast

A cold frame is invaluable for this process, acting as a halfway house between the warmth of the greenhouse and the full exposure of the open garden.

EXPERT TIP
Pay attention to wind as much as temperature during hardening off. Cold, drying wind is often more damaging to young seedlings than mild frost. Start them off in a spot sheltered from prevailing winds, even if the temperature seems fine.

Soil Preparation: The Foundation of a Good Season

Before anything goes into the ground, it is worth spending time on your soil. March is an excellent month to dig in well-rotted compost or manure if you haven't already done so over winter. This improves drainage in heavy soils, increases water retention in sandy soils, and feeds the soil life that will in turn feed your plants all season long.

Avoid walking on wet soil or working it when it is waterlogged. Compacted soil drives out the air pockets that roots depend on, and it can take months to recover. Use planks or boards to spread your weight if you need to access beds that are still damp.

Rake the surface to a fine tilth — a crumbly, even texture — before sowing. For small seeds like carrots and parsnips especially, large clods and stones will obstruct germination and lead to misshapen roots.

A Note on the Unexpected Frost

March is famously unpredictable. A warm, settled spell can give way to a sharp overnight frost with very little warning, and unprotected seedlings — whether in the ground or on a windowsill by a single-glazed window — can suffer real damage.

Keep a roll of horticultural fleece to hand throughout the month. It is lightweight, inexpensive, and can be thrown over a row of seedlings or a tray of pots in minutes. Even one layer provides several degrees of frost protection and can mean the difference between losing a sowing and saving it.