Once the weather starts to warm up, hardy crops can be sown outdoors in milder areas. March can still be wintery though, so only sow outside if conditions are suitable and the soil is warm enough – a good indication that the soil is warm enough is when weed seeds start to germinate! Whatever the temperature outdoors, sowing half-hardy and tender plants can continue undercover in preparation for transplanting into the garden after the last frost.
More seed sowing advice:
Half-hardy annual climbers
Half-hardy climbers add dazzling colour to fences, walls and arches. Many can be grown in containers and will flower right through summer into autumn. These versatile plants can be sown under cover in the next few weeks and planted out in a sunny sheltered spot after the last frost.
Cathedral bells (Cobaea scandens), as its name suggests, has glorious purple-flushed bell flowers with a sweet fragrance, while Cobaea scandens f. ‘Alba’ has soft ivory blooms. Also known as the cup and saucer vine, the flowers look like open cups sitting on a ruffled saucer. Although a vigorous woody perennial in tropical America, Cobaea is usually grown as an annual in the UK. The lush foliage quickly covers fences and arches, producing flowers from July that are ideally grown beside or over a path to make the most of the beautiful scent. Sow seeds on their sides at a depth of 1cm and keep at 18-24°C until germination.
Black-eyed Susan (Thunbergia alata) is a vigorous perennial, often grown as a half-hardy annual. Quick to cover a trellis or to cascade over the edge of containers or hanging baskets, this is a climber that suits almost any garden. Thunbergia alata ‘Orange and Red’ makes a fantastic backdrop to a hot border, while paler varieties such as T. alata ‘African Sunset’ and T. alata ‘Sunrise White with Eye’ create a more subtle effect. Seeds should be sown into pots or trays, lightly covered with compost or vermiculite, and placed in a propagator to germinate at around 20-25°C.
Festooned with vivid yellow, blue, purple, white or even red flowers that open early and close up in mid-afternoon, morning glory (Ipomoea) adds wonderful colour and drama to any planting scheme. We love to grow I. purpurea ‘Dacapo Light Blue’ which has the palest blue trumpet-shaped flowers with a dark purple central star and luminous white eye. I. tricolor ‘Grandpa Ott’ is an old variety with striking deep violet-blue flowers and a red central star. The related climber Spanish flag (I. lobata) has an abundance of tubular flowers that fade from fiery red to creamy yellow.
Soak Ipomoea seeds for 24 hours before sowing. Sow at a depth of 5mm and keep around 20°C until after germination. Morning glory and Spanish flag thrive in a sheltered, sunny spot, and flower through summer and into autumn. Seeds and plants are toxic if eaten.
Purple bell vine
Purple bell vine (Rhodochiton atrosanguineum) is a long-flowering perennial climber, usually grown as a half-hardy annual outdoors in the UK. Its impressive tubular dark purple flowers hang down from soft magenta calyces. We love to grow this magnificent vine in a large container from which it twines up our trellis and covers the wall, but it also thrives in greenhouses, conservatories and in sunny sheltered spots in moist, well-drained soil. Sow seeds in pots and cover lightly with compost. Place pots at 15-18°C until germination, then transplant seedlings when large enough to handle.
With airy foliage and vibrant daisy-like blooms, cosmos are popular stalwarts in cottage garden borders, container displays and as a cut flower. These half-hardy annuals can be sown now and will be ready to plant out after the last frost. I always want to grow more varieties than I can fit in my garden, but I’m particularly fond of Cosmos sulphureus ‘Bright Lights’ with its mix of single and semi-double flowers in lemon yellows, tangerines and smoky reds; the softest peachy-apricot blooms of elegant C. bipinnatus ‘Apricotta’; the vintage pinks and crimsons of C. bipinnatus ‘Antiquity’; and the ‘Apollo’ series which produces single white, pink and carmine-red flowers on compact plants, ideal for growing in containers.
Sow cosmos seeds from March to the end of April in pots and lightly cover with compost. Place in a propagator or on a warm windowsill at around 21°C. Transplant seedlings to individual pots and grow on under cover until the end of May or beginning of June. Harden off for a week and plant out in containers or borders. Seeds can also be sown direct from the middle of May for fabulous colour until the first frosts.
Many panicum grasses are elegant half-hardy annuals with arching sprays of pale green tiny flowers from July to October. Easy to grow from seed, they are favourites in borders, gravel gardens and as foliage plants for cutting. Annual varieties include Panicum elegans ‘Sprinkles’, P. elegans ‘Frosted Explosion’, P. miliaceum ‘Violaceum’ and P. capillare ‘Sparkling Fountain’. Most reach 60-90cm in height and work beautifully in containers or in borders alongside late-flowering perennials such as echinacea and sedum.
Seeds can be sown indoors or in a greenhouse from this month until mid-spring, or direct sown in April and May. Sow onto the surface of moist, peat-free seed compost and cover thinly. Transplant small clumps of seedlings into pots and grow on undercover. Harden off seedlings and plant outside once the risk of frost is over.
Reduce the amount of new plastic coming into the garden by choosing durable plant labels made from recycled materials. Even better, make your own labels from old wooden spoons, garden pebbles, offcuts of wood, or old clothes pegs on sticks. Decorating and adding plant names to homemade labels is a fun activity to do with children. When my kids were younger, we decorated spare slates with coloured chalks and used them as labels for the vegetables and flowers they had grown from seed.
The first warm, sweet tomato from the greenhouse or vegetable patch is one of the highlights of summer. Easy to grow from seed and with so much choice, tomatoes offer a rainbow of new varieties alongside old favourites. We love the meaty taste of ‘Indigo Rose’, a dark purple-black tomato rich in antioxidants, and ‘Tumbling Tom Red’, which trails from containers and hanging baskets in a cascade of cherry tomatoes. The kids enjoy growing ‘Tigerella’ with its red and yellow stripes, and super-sweet ‘Sungold’.
If blight is a problem, try resistant varieties like ‘Crimson Crush’, ‘Lizzano’, and one of my favourites, the small beefsteak tomato ‘Oh Happy Day’. Before you buy seeds, check whether the variety you have chosen is a cordon or bush tomato. Cordons (or indeterminate) need to be tied into tall supports as they grow, whereas bush tomatoes (or determinate) do not need tying in and are ideal for pots.
Sow tomato seeds thinly in trays or pots of moist peat-free seed compost, ideally by the end of the month. Lightly cover the seeds with compost or vermiculite, and place in a heated propagator at around 18°C or on a warm windowsill, covering the pot with a clear plastic bag to create a mini propagator. After germination, remove seedlings from the propagator and place pots in a well-lit spot so plants do not become leggy. Prick out seedlings into individual pots once they are large enough to handle and grow on under cover. Harden off and plant out after the risk of frost has passed. Once pots of tomato seedlings fill our windowsills, it feels like the new growing season is underway.
Celeriac is a tasty, but highly underrated vegetable, fantastic in soups to warm up chilly autumn days. March is the ideal time to sow seeds as celeriac needs a long growing season before it is harvested from late September onwards. There are some great varieties to choose from including ‘Giant Prague’, an heirloom celeriac with a superb flavour, and ‘Monarch’, which has smooth skin and tender flesh.
Sow seeds under cover, sprinkling thinly on the surface of peat-free seed compost in seed trays or modules. Lightly cover with compost or vermiculite and place in a propagator or on a windowsill at 15-18°C. Germination can be rather erratic and it may take around three weeks before seedlings begin to appear. Transplant into individual pots when seedlings are large enough to handle and ensure the temperature stays above 10°C to prevent plants bolting. Acclimatise and plant out after the last frost.
With their mild oniony flavour, chives are an easy perennial herb to grow and can be sown under cover this month. Sprinkle seeds thinly on the surface of moist, well-drained peat-free seed compost in pots or trays, cover lightly with compost, and place in a propagator at 18-21°C until germination, which takes around two or three weeks. Transplant seedlings to 8cm pots in groups of four when they are large enough to handle and grow on in cooler conditions until the last frost. Harden off and plant outside.
Alternatively, sow seeds outside at a depth of 1cm from mid-spring in a sunny spot. Thin seedlings to 23cm apart. Chives are ideal to edge paths and vegetable beds, or to add to herb containers. Their pretty mauve pom-pom flowerheads are fantastic for attracting pollinators like butterflies and bumblebees to the garden.
Cold frames are invaluable in spring for warming the soil, hardening off seedlings sown indoors, and growing on young plants that need a little extra protection. They are also useful for raising cuttings and giving vegetables like salad greens and broad beans a head start. It is easy to make an inexpensive cold frame from pallets for the base and an old window for the sloping top. If you don’t have the materials to hand, they could be sourced from local buy and sell groups, or recycling websites.