Growing Lupin

Lupins are true all-rounders in the garden. They fertilise the soil as they grow, break up even compacted layers with their powerful taproots, and attract bees, bumblebees and butterflies with their vibrant flowers. No wonder they're considered one of the most effective green manure plants in organic gardening.

Part of the legume family (Fabaceae), three annual species are mainly used in the garden: the narrow-leaved lupin (Lupinus angustifolius), the yellow lupin (Lupinus luteus) and the white lupin (Lupinus albus). All three live in symbiosis with nitrogen-fixing bacteria (Bradyrhizobium lupini) on their roots, binding atmospheric nitrogen directly in the soil. Depending on species and location, they accumulate up to 15 grams of nitrogen per square metre — a solid dose of fertiliser for the next crop.

Their taproots reach up to two metres deep into the soil and leave behind fine channels after decomposing, allowing water and air to circulate more freely. This benefits all follow-on crops, especially root vegetables like carrots or parsnips.

Varieties fall into two groups: bitter lupins and sweet lupins. Bitter lupins contain alkaloids and are used exclusively as green manure. Sweet lupins (alkaloid content below 0.05 per cent) are also edible and deliver up to 40 per cent protein — one of the richest plant-based sources there is. Hildegard von Bingen already valued the lupin as a medicinal plant.

Lupins are sown directly into the bed; raising seedlings indoors isn't necessary for green manure. You can sow from March to September, with the ideal window being mid-March to early April, as soon as the soil is frost-free. Seeds germinate from as low as 3 to 4 degrees Celsius, with around 15 degrees being optimal.

As dark germinators, the seeds need a soil covering of 2 to 4 centimetres. Seeds left on the surface germinate poorly. Roughen them lightly with sandpaper before sowing and soak them overnight in water. This speeds up germination, which otherwise takes 7 to 14 days.

For green manure, broadcast sow at roughly 15 to 20 grams per square metre. If you're growing lupins on a plot for the first time, it's worth inoculating the seed with the right nitrogen-fixing bacteria. Without this symbiosis, lupins can't deliver their full nitrogen-fixing benefit. You can find inoculants at specialist suppliers.

Lupins love a sunny, open spot and tolerate neither waterlogging nor too much lime in the soil. They cope with partial shade, but flowering and sturdiness suffer as a result.

The three species differ quite a bit in their soil preferences: The narrow-leaved lupin is the all-rounder, thriving in light to medium, sandy to loamy soils with a pH of 5.0 to 6.8. It handles lime best of the three. The yellow lupin is the pioneer for really poor, acidic sandy soils (pH 4.6 to 6.0) where hardly anything else grows. The white lupin prefers medium soils with a good water supply (pH 5.5 to 6.8) and produces the highest biomass.

The narrow-leaved lupin dominates cultivation in Germany today and offers the widest choice of varieties. Among the determinate types, 'Boruta' provides even ripening and suits damp conditions. Among the branching types, 'Boregine' is particularly high-yielding. 'Probor' grows up to 120 cm tall and works well on wildlife plots too.

The choice of yellow lupin varieties is very limited today, as the fungal disease anthracnose has pushed most susceptible cultivars out of production over recent decades. The sweet lupin variety 'Juno' is still available.

The white lupin is experiencing a small comeback. Since 2019, the first anthracnose-tolerant varieties like 'Lublanc' and 'Ares' have appeared. This species has the highest protein content, making it especially interesting for grain use.

For pure green manure, cheap bitter lupin seed does the job perfectly well. You only need sweet lupin varieties if you want to eat the seeds — and check the alkaloid content on the packet.

Lupins are among the most undemanding plants in the garden. Once established, their deep taproots largely take care of watering for them. Only water during the germination phase and in prolonged dry spells. Like most plants, they can't stand waterlogging.

Nitrogen fertiliser isn't just unnecessary for lupins — it actually does harm. It inhibits nodule formation, makes the plants more prone to disease and can trigger root rot. Rock dust or calcified seaweed in the soil, on the other hand, supports the bacterial symbiosis.

For green manure, chop the lupins in just before or during flowering. At this point they've built up the most biomass and stored the most nitrogen. Cut the plants at ground level, let the material dry slightly, then work it shallowly into the soil. After a waiting period of about four weeks, you can plant your follow-on crops.

Anthracnose (Colletotrichum lupini) is by far the most dangerous disease in lupin cultivation. You'll spot it by twisted, curving shoots and brown lesions with orange spore masses on stems and pods. The white and yellow lupin are hit hardest, while the narrow-leaved lupin shows much greater tolerance. Prevention: only use certified seed, remove infected plants immediately and never compost them, and stick to a wide crop rotation.

Fusarium wilt (Fusarium oxysporum) causes lupins to wilt suddenly and die off from the base upwards. A consistent growing break is the main defence here too. Powdery mildew can appear in warm weather when plants are too closely spaced. Horsetail tea works as a preventative.

Among pests, various aphid species (lupin aphid, black bean aphid) are the most common visitors. Encouraging ladybirds and lacewings as natural predators is the best approach. Young plants also attract slugs, so keep an eye out right after sowing. The leaf weevil chews characteristic notches into the leaves, and its larvae damage the valuable root nodules.