Nasturtiums are good value if you want flowers tumbling over a wall, filling an untidy corner or brightening the edge of a vegetable patch. They germinate from fairly large seeds and grow quickly in favourable weather. The surprise for many new gardeners is that feeding them too generously can leave you with masses of leaves and fewer flowers.
The annual nasturtium most people grow is Tropaeolum majus. It is frost tender in the UK, so a little patience with sowing time pays off. You can start seeds under cover in spring or sow them directly where they will grow when the weather has warmed.
Work out whether you want a bush or a trailer
Compact bush nasturtiums are useful at the front of beds, in pots or window boxes. Trailing forms send long stems over basket rims, across borders or up suitable supports.
Check the expected spread on the packet. A trailing cultivar in a small mixed container can cover its neighbours, while a compact plant won't climb several metres up a trellis no matter how carefully you train it.
Many nasturtiums have edible flowers and leaves, but only use correctly identified plants grown and handled appropriately for food. Avoid treating nursery plants exposed to unknown ornamental pesticides as ready-to-eat garnishes.
Know when to sow outdoors
For direct sowing in Britain, late spring to early summer is a good window once frost is no longer expected and the soil has warmed. The RHS recommends sowing annual nasturtiums directly outdoors in late spring or early summer.
A few seeds sown too early into chilly damp ground can rot or emerge very slowly. Wait until conditions favour growth rather than trying to compensate with more watering.
If your garden is exposed, start some indoors as insurance. This lets you put out larger plants when temperatures improve without rushing tender seedlings into cold wind.
Starting nasturtium seeds indoors
Fill small pots or modules with suitable peat-free seed compost. Use one or two seeds per pot, following the packet depth, usually around 1.5–2cm depending on the supplier.
Water the medium gently and keep it moist without waterlogging. Provide a bright, frost-free growing position. Seeds may germinate at different rates; don't discard a sound-looking pot after only a few days.
When shoots appear, give them enough daylight to prevent weak, stretched growth. Nasturtiums don't appreciate unnecessary root disturbance, so individual pots are convenient for moving plants on.
Prepare a sunny outdoor position
Nasturtiums flower best in a reasonably sunny site. They can grow in poorer soil and often produce excessive lush foliage in heavily enriched ground.
Remove weeds and loosen compacted soil, but don't fill every planting hole with rich compost or routinely feed with high-nitrogen fertiliser. Good drainage matters, although young plants still need moisture while establishing.
A border alongside a path or the side of a raised bed is useful for trailing forms. Consider where the stems will spread before sowing: they may otherwise smother slower-growing neighbours.
Direct sowing step by step
Rake the surface to a fairly fine tilth and remove obvious stones and weed roots. Mark the intended planting sites so you know where to water and look for seedlings.
Place seeds at the depth recommended on the packet, usually a couple of centimetres, and cover with soil. Sow more widely for trailing types than compact ones, referring to the cultivar's mature spread.
Water gently after sowing if the soil is dry. Keep the sowing area lightly moist during germination. Once seedlings emerge, thin overcrowded groups rather than leaving half a dozen plants pressed into the same small patch.
Move indoor plants outside safely
Young plants raised in a protected greenhouse or windowsill need hardening off. Start with short daytime periods in a sheltered spot and increase exposure gradually.
Bring them inside during cold nights. Plant out only after frost risk has passed. A warm day doesn't protect the leaves from a near-freezing night.
Ease plants from their pots without breaking the rootball. Set them at the same level as before, firm the surrounding soil gently and water in. Give them time to establish before redirecting long new stems.
Growing nasturtiums in hanging baskets
Trailing nasturtiums can make a colourful hanging display. Choose a basket with adequate compost volume and a liner that retains moisture while allowing surplus water to escape.
Plant carefully around the basket, leaving each rootball enough space. The long stems will find the edge as they grow, so don't force young fragile growth through small holes.
Hanging baskets dry quickly in summer sun and wind. Check them regularly. A plant that tolerates lean garden soil may still wilt badly if its small hanging rootball goes bone dry.
What about window boxes and patio pots?
A wide container with drainage holes works for compact varieties. Use appropriate peat-free potting compost, remembering that the mix may be richer than open garden soil.
Don't overfill the planter. Leave watering space and allow room for mature foliage. If flowers are scarce but leaves are enormous, excessive feeding or too much shade may be a factor.
Pots offer the advantage of portability. You can move a young plant temporarily if an unexpected cold spell arrives, though a full trailing basket will be much less convenient to shift.
Watering and feeding through summer
Water newly planted seedlings when necessary while their roots establish. Once strong, plants in the ground often tolerate some dry spells better than those in containers.
Aim for steady growth without keeping the soil saturated. A wilting pot may need a thorough soak followed by good drainage; a constantly sodden pot needs the opposite attention.
The RHS notes that very fertile soil can encourage leafy growth at the expense of flowers. Resist the urge to apply extra feed whenever a plant isn't flowering. First check its light and the natural growth stage.
Slugs, caterpillars and aphids
Young nasturtium leaves attract slugs and snails. Inspect seedlings after wet evenings and use suitable wildlife-conscious protection where necessary.
Blackfly may gather on fresh shoots. Small colonies can sometimes be managed by removing affected tips or washing insects away appropriately. Large infestations should be assessed before using controls that could also harm beneficial insects.
Caterpillars of cabbage white butterflies may eat leaves. Some gardeners deliberately grow nasturtiums nearby, but claims that they reliably protect other crops are not strongly established. See them as flowers first, not a guaranteed pest shield.
Harvesting flowers or saving seed
Pick flowers only from identified edible varieties grown without unsuitable sprays. Wash gently and use them fresh; the petals can be attractive in salads.
Leave selected flowers to form seed if you want to collect it. Pick mature seed appropriately and dry and store it according to the packet or credible horticultural guidance. Offspring of hybrid varieties may differ from the parent plant.
Annual nasturtiums are normally killed by frost. Clear the remains in autumn and plan fresh sowings for the following spring. A few self-sown seedlings may appear if seed fell into suitable ground.
Why nasturtiums produce leaves but few flowers
The biggest culprits are rich soil and insufficient light. A shady position may support leaves but reduce flowering, while heavy feeding favours vegetative growth.
If seedlings turn yellow soon after planting, check for cold exposure, waterlogging and root damage. More fertiliser is not always the solution, and a weak plant may need warmer conditions instead.
A successful nasturtium patch is simple: warm sowing soil, modest fertility, sufficient light and room for the mature stems to trail. Let the plant do what it does naturally.