Nasturtium Care: Why Rich Soil Gives Leaves and Not Flowers

All leaves and no flowers means it is too well fed

The most common complaint about nasturtium is a huge, glossy, healthy plant with two flowers on it. That plant is not sick, it is comfortable. Nitrogen drives leaf growth, and nasturtium has no restraint about it: given plenty, it builds leaf indefinitely and never gets round to flowering. The cure is to make life harder, not easier.

Stop feeding completely. Stop watering except when the leaves actually droop in the morning. If it is in a container, move it into full sun and let it dry to the point of flagging between waterings. Three or four weeks of that and flower buds appear in the leaf joints. It is uncomfortable to watch a plant you have been nursing being starved, but it is the whole trick.

Never use a lawn feed, a leafy-vegetable feed or fresh manure anywhere near nasturtium. If you feed at all, use a high potash tomato feed at half strength, and only for a plant that is visibly starving.

Water only when it flags, and water the soil not the leaves

Nasturtium leaves are shaped like small shields and shed water off the surface, so overhead watering mostly runs to the ground and wets the crown. The plant wilts dramatically in afternoon heat and recovers on its own by evening, which fools people into watering a plant that does not need it. Judge in the morning: leaves still down at breakfast means it is genuinely dry.

In open ground an established plant needs watering only in a real drought. In a container it is different, because the plant is large relative to the pot and can empty an eight inch pot in a day at midsummer. Water containers thoroughly until it runs from the base, then wait until the top inch is dry again.

1. Check at the base, not the tips Push a finger two inches (5 cm) into the soil beside the crown. Damp at that depth means no water is needed, whatever the leaves are doing in the sun.
2. Water in the morning Evening watering leaves the crown wet overnight, and a wet crown under a dense canopy is where stem rot and slugs both start. Morning watering dries off by dusk.
3. Lift the runners occasionally Trailing stems root where they touch damp ground and start competing with the parent. Lift them once a month and let the ground under them dry.

Full sun, and no support

Six hours of direct sun is the minimum for good flowering, and eight is better. In part shade nasturtium survives happily but the stems stretch, the leaves get larger and the flowers hide underneath them. Heat is a separate matter: above about 86F (30C) flowering slows, buds drop, and the plant sits and waits, then comes back strongly when the nights cool in late summer.

Climbing forms have no tendrils and no twining stem. They climb by hooking their leaf stalks over things, which means they will scramble up netting, twiggy sticks or an established shrub, but they will not go up a smooth cane or a wall on their own. Give them something with cross pieces to hook onto, or let them trail and cover the ground instead.

Nasturtium in a hanging basket wants the same treatment: an ordinary compost with nothing added, full sun, and watering only when it flags. A basket fed weekly turns into a green curtain with nothing on it.

Deadhead unless you want seed

Each flower lasts a couple of days and then sets a three-part green pod. Once several pods are swelling the plant slows down and flowering tails off, so pinching the dead flowers off with their stalks keeps it going for weeks longer. Take the stalk right back to where it joins the main stem, because a stub left behind rots and pulls the joint with it.

That said, those pods are the caper crop, so most gardeners deadhead one part of the plant hard and let another part pod up. Decide which runners you are leaving alone and pick the rest clean twice a week.

1. Twice a week through midsummer Go over the plant when you pick leaves for the kitchen and take off every faded flower and its stalk with your fingers. It takes five minutes and adds a month to the display.
2. Cut back a tired plant by half By late summer the oldest runners go bare and yellow at the base. Cut the whole plant back by half with shears, water it once, and it regrows and reflowers in about three weeks.
3. Take out the yellowed inner leaves The center of a mound gets dark, damp and airless. Pull out the yellowing leaves from underneath every couple of weeks to keep air moving and slugs out.

Nasturtium sap stains and the stems are brittle. Snap the stalks off cleanly rather than tearing, and expect the broken ends to weep a peppery smelling liquid for a minute or two.

Record which stations flowered and which ran to leaf

Because the whole outcome depends on soil richness, the useful record is a map rather than a diary. Note where each plant went, what had been growing there the year before, and whether the plant flowered well or made a leaf mound. After one season you will know exactly which corners of your garden are lean enough, and you can sow straight into them without guessing.

Worth writing down as well: the date the first flower opened, the date blackfly first appeared and on which plant, and the date of the first frost that killed the plants. Those three dates repeat closely from year to year and let you time the trap planting and the caper picking properly next season.