March brings longer days and a burst of growth to the polytunnel, even when the weather outside still feels wintry. Keep an eye on overnight frost, give emerging seedlings plenty of light and make time to ventilate on warm days.
MARCH IN THE POLYTUNNEL: AT A GLANCE
Hardy salads, roots, brassicas and herbs. Start tender crops with suitable warmth.
Broad beans, peas and early potatoes, alongside crops from earlier sowings.
Winter salads, roots, greens and the first crops from overwintered plants.
Good to know: A bright March day can make the tunnel warm quickly, while nights can still be cold. Check conditions inside the tunnel rather than relying on the outdoor forecast alone.
WHAT TO SOW IN MARCH
March is a good time to make small, regular sowings as light levels improve. Wait if soil is cold or waterlogged, and use modules for seedlings that need a protected start.
Hardy crops under cover
Depending on local conditions, sow cabbage, carrots, cauliflower, celeriac, chard, coriander, fennel, kohlrabi, lettuce, spring onions, radish, rocket and spinach. Keep young plants watered without leaving the soil saturated, and protect them if frost is forecast.
Tender crops to start with warmth
Aubergines, peppers and tomatoes need warmth and good light as young plants. Start them indoors or in a heated propagator, then grow them on somewhere bright and frost-free. If you missed an earlier pepper sowing, March is still an opportunity, although the growing season may be shorter. Strawberry plants and garlic or elephant garlic can also be established where conditions suit them.
WHAT IS GROWING IN MARCH?
Broad beans and peas sown in January may start to flower. Plants established in autumn can be further ahead, with small pods developing as the spring hungry gap approaches. Early potatoes and the crops listed above will also be growing as temperatures rise.
Watch overwintered lettuce and pak choi for signs of bolting. Pick usable leaves while they are at their best, and replace tired plants with planned spring crops. Celery and chard can still provide welcome harvests, although older plants may soon turn to flower.
Elephant garlic planted in autumn should now be putting on strong growth. Tender seedlings started under lights earlier in the year will need enough space and continued protection from cold nights; do not move them into an unheated tunnel too early.
WHAT TO HARVEST IN MARCH
Depending on what you planted in autumn and how it has survived the winter, you may be harvesting beetroot, sprouting broccoli, cabbage, carrots, cauliflower, celeriac, celery, chard, coriander, kohlrabi, lettuce and other salad leaves, spring onions, pak choi, radish, rocket, spinach and turnips.
Pick a little at a time from leafy crops. Remove yellow or damaged leaves, and check plants that are starting to bolt so you can harvest them while their quality is still good.
ESSENTIAL POLYTUNNEL JOBS FOR MARCH
1. Make room for seedlings
March is often when the tunnel becomes a better place for young plants than a crowded windowsill. Put staging somewhere bright without shading the beds too heavily. Removable staging hung from crop bars can free up floor space; make sure it is securely supported and does not overload the frame.
2. Refresh beds as space opens
Clear spent winter crops and add well-rotted compost where the next crop needs it. Keep a note of what grew in each bed to help plan crop rotation. Avoid enriching a bed heavily immediately before sowing crops that prefer less rich soil, such as carrots.
3. Adjust watering and ventilation
Plants use more water as growth resumes, but a cold spell can slow them again. Check the soil before watering. Open doors or vents during suitable mild weather to reduce humidity, then close them before temperatures fall.
YOUR MARCH CHECKLIST
- ☐ Make small sowings of hardy crops when conditions are suitable.
- ☐ Start tender crops somewhere warm and bright.
- ☐ Prepare a well-lit, secure space for seedlings.
- ☐ Harvest overwintered crops before they bolt.
- ☐ Clear spent plants and refresh beds for the next crop.
- ☐ Check soil moisture and ventilate on suitable days.
- ☐ Keep fleece ready for cold nights.
MARCH POLYTUNNEL GROWING FAQS
Can I put tomato plants in an unheated polytunnel in March?
March nights can still be too cold for tender tomatoes. Grow young plants somewhere warm and bright and move them only when conditions are suitable, with protection ready if temperatures drop.
What should I do if winter crops start to bolt?
Harvest usable leaves promptly and clear plants when quality declines. Use the space for planned spring sowings or transplants.
How often should I water in March?
There is no fixed schedule. Check moisture in the bed or pot, then water at the roots when needed. Increase checks during sunny spells.
For advice on individual crops, see our Top of the Crops guide. Browse the monthly growing guides or continue with April.
