March - What To Sow, Grow & Do In The Greenhouse

March marks the real start of the growing season. Your greenhouse gives you a head start on vegetables like radishes, spring onions and tomatoes, plus flowers like cosmos and marigolds.

We've spoken to our gardening expert friends Ellen Mary and Ruth Darrah and here is everything you need to sow, grow and do in your greenhouse this month.

What to plant in March

Vegetables to plant in your greenhouse in March

March is the ideal month to get a range of vegetables started under glass, from quick-cropping radishes to summer staples like tomatoes.

Radishes

Seed to harvest within 3 to 4 weeks

Radishes are one of the quickest crops to harvest and come in a wonderful range of colours and flavours. Choose from mild varieties like Breakfast or go for something peppier like Icicle. You can select traditional red varieties, golden radishes, or eye-catching purple cultivars depending on your preference.

For sowing, space seeds at roughly 2.5cm intervals, pushing them about 1cm into the compost. They grow well in containers and will even thrive tucked between rows of other vegetables like tomatoes, making them perfect for maximising space. Keep the soil cool and moist to prevent bolting, and you will have radishes ready to harvest in just 3 to 4 weeks.

For more information on how radishes pair with other plants, see our companion planting guide.

Spring Onions

Seed to harvest in 8 weeks

For sowing in March, select a summer variety of spring onion, as these will thrive in your greenhouse during the warmer months ahead. Prepare your soil or compost by mixing in a good helping of fertiliser to give them the nutrients they need.

Plant seeds about 15mm deep and space them roughly 5cm apart. Spring onions prefer well-drained soil, so ensure your compost is not compacted. Keep the soil consistently moist throughout the growing period. After about 8 weeks, when the stems reach around 15cm tall, you can start harvesting them for use in the kitchen.

Tomatoes

Seed to harvest in 3 to 4 months

March is the perfect month to sow tomato seeds, ready for transplanting once the weather warms. Choose a vine variety if you plan to grow them in the greenhouse, or bush varieties if they will go outside. Fill seed trays with moist compost and press seeds just 1cm below the surface.

Keep your seed trays warm and away from direct sunlight whilst they germinate. Once the first seedlings appear, move the trays to a bright location. As your tomato plants grow, they may need staking or supporting with canes to keep them upright. With careful nurturing, you will have strong transplants ready for the greenhouse by late spring.

For detailed guidance on growing tomatoes from start to finish, see our tomato growing guide.

Broad Beans and Peas

Seed to harvest is 10-12 weeks for peas and 12-14 weeks for broad beans.

March is an excellent time to sow broad beans and early peas in seed trays within your greenhouse. The controlled environment offers frost protection whilst the plants establish themselves, giving you a real advantage over outdoor sowing.

Keep the compost moist but never waterlogged; overwatering can cause the seeds to rot. As the weather warms and the danger of frost passes, you can gradually acclimatise your plants to outdoor conditions before transplanting them to their final growing positions, typically in May. This method gives you a crop ready to harvest in 12 to 14 weeks for broad beans and 10 to 12 weeks for peas.

Learn more about protecting your beans over winter with our overwintering guide.

Salad Leaves

Seed to havest in just 4 to 6 weeks

Start sowing salad leaves like lettuce and radish in your greenhouse from March onwards. Using a programme of successional sowing, where you plant new seeds every couple of weeks, means you will have a continuous supply of fresh leaves ready for picking throughout the season.

Greenhouse staging makes seed sowing much easier, giving you a comfortable working height and better organisation of your seedlings. Sow your lettuce seeds directly into shallow trays or containers, keeping them consistently moist. Most salad leaves are ready to harvest within 4 to 6 weeks, depending on variety.

Discover our range of greenhouse staging to make your sowing easier.

Flowers to Grow in Your Greenhouse in March

Get a head start on summer colour by sowing cosmos and marigolds under glass this month. These cheerful flowers bring pollinator-friendly blooms to your garden and can be sown directly in the greenhouse with excellent results.

Cosmos

Blooms from July

Sow cosmos seeds in sectioned trays during March, pressing them roughly 3cm into the compost. They love sunlight, so position your trays in the brightest part of your greenhouse. Keep the compost well watered throughout germination and growth.

Once your seedlings have developed their first few true leaves, pinch out the growing tip at about three sets of leaves. This encourages branching and helps create bushier, more compact plants that will reward you with more flowers. By late May or June, your cosmos plants will be strong enough to plant out in their final position, where they will bloom prolifically from July right through to October.

For more advice on pinching out plants, see our guide on when to pinch out.

Marigolds

Blooms from June

Marigolds are among the most rewarding flowers to grow in a greenhouse. These cheerful blooms naturally deter pests, making them a perfect companion to grow alongside your vegetables. Some varieties are even edible, adding a peppery touch to salads if you fancy trying them.

Sow marigolds in seed trays in March. They prefer good drainage, so ensure your compost is well-aerated, and place them in a sunny spot where they can soak up plenty of light. Before planting them outside after the last frost, harden them off gradually by exposing them to outdoor conditions for increasingly longer periods each day. Once established, your marigolds will flower abundantly from June through to October.

To keep your greenhouse and garden healthy, read our tips on keeping your greenhouse pest-free.

Greenhouse Tips for March

From pest prevention to soil preparation, here is how to make the most of your greenhouse as the growing season begins.

Check Overwintering Plants

As temperatures start to rise and daylight hours lengthen in March, take time to review any plants you have protected with bubble wrap or fleece insulation over the winter months.

Check whether your insulation still needs adjusting or if you can begin to remove it gradually. Make sure your plants are receiving adequate moisture and light as they begin their new season of growth. Hardier varieties may be ready to move outside later in the month, though you should still protect them from any surprise frosts that can occur at this time of year.

On mild days, open your greenhouse doors and vents to allow fresh air to circulate. This helps prevent disease and strengthens your plants, though always be careful not to expose delicate young seedlings to sudden cold draughts.

Learn more about insulating your greenhouse with our guide on bubble wrapping.

Protect Against Slugs and Snails

As new growth emerges from your seedlings and overwintering plants, slugs and snails become increasingly active. These pests can cause significant damage to tender young leaves, so it is worth putting protective measures in place now.

• Wrap copper tape around the rims of pots and containers to create a barrier that slugs and snails are reluctant to cross.
• Scatter broken eggshells around your plants; the sharp edges deter these pests from approaching. Set up beer-filled slug traps, which are surprisingly effective at catching them overnight.
• For a longer-term solution, you can also apply biological nematodes to your soil, which are natural predators of slug and snail eggs.

For further strategies on managing pests, see our greenhouse pest management guide.

Prepare Your Soil

Before you start planting directly into greenhouse beds, take time to prepare your soil for the busy growing season ahead. Gently fork in a good 5cm layer of well-rotted manure to enrich the soil with nutrients and improve its structure. Well-rotted manure is ideal because it will have broken down fully and will not scorch tender plant roots.

If you have homemade compost available, this is an excellent choice. Alternatively, you can use pelleted chicken manure, which provides quick-release nutrients, or fish blood and bone, a traditional choice that provides a slow, steady supply of nutrients over time. Whichever soil amendment you choose, work it in thoroughly to give your plants the best possible start.

Choose the right compost for your needs with our compost guide.

Plan Your Greenhouse Space

Before you sow all your seeds, it is worth spending a few minutes thinking about how you will use your greenhouse space. Position your most sun-hungry plants, such as tomatoes, peppers, and aubergines, along the south-facing side where they will get the most light. Use shadier spots for plants that prefer dappled shade, such as lettuce and some herbs.

March is also the perfect time to challenge yourself and try 2 or 3 new varieties that you have never grown before. Whether it is an unusual tomato colour, an unfamiliar salad leaf, or an ornamental flower you have never attempted, pushing your boundaries makes gardening more rewarding and interesting.

For inspiration and ideas, see our guide to growing in a greenhouse for beginners.

Support Wildlife in Your Garden

Whilst you are gearing up for the growing season, remember that March is also a critical time for wildlife. As your garden comes back to life, pollinators and birds are beginning their nesting season and need our support.

Leave patches of dandelions and other wild flowers for early bees to feed on when few other nectar sources are available. Keep your bird feeders topped up with fresh seeds, and make sure water sources are available and clean, as birds need extra hydration during nesting season. Put out nesting materials such as twigs, moss, and pet fur so that birds have the resources they need to build their nests. Consider planting native hedges such as blackthorn, hawthorn, and dog rose, which provide food and shelter for wildlife year-round.

Learn more about creating a wildlife-friendly garden in our guide to bringing wildlife into your garden.

Pond Maintenance

If you have a pond in your garden, March is the month to start awakening it from its winter sleep. Remove any netting or protective coverings that you placed over it in autumn. Check the water level and top it up if needed, as evaporation may have lowered it during the winter months.

Inspect any plants around your pond and remove dead or diseased material that has accumulated over winter. If you have water plants in your pond that need dividing, March is a good time to tackle this job as the growing season is just beginning. Cut back the growth around your pond to allow new shoots to emerge, which improves the overall look and health of the feature. A well-maintained pond supports amphibians and insects that provide natural pest control throughout your garden.