Growing Persian clover

Persian clover is a legume and originally comes from the eastern Mediterranean and the Near East. Farmers have valued it for decades as a catch crop and fodder crop, but in the home garden it still leads a quiet existence. That's a shame, because as a green manure it has pretty much everything going for it: it fixes atmospheric nitrogen via root nodule bacteria, loosens the soil with its strong taproot, suppresses weeds by closing over the ground quickly, and draws in bees and bumblebees with its honey-scented flowers.

The botanical species name resupinatum reveals a charming quirk: before opening, the flowers twist through 180 degrees so that the keel points upwards and the standard points downwards. From May to September it produces pink to violet, ball-shaped flower heads up to 2 cm across. The three-part leaves with finely serrated edges look a lot like those of the better-known white clover. Depending on the site, Persian clover grows 10 to 60 cm tall, though it usually sprawls low across the ground and covers the soil like a living blanket.

In the vegetable garden, Persian clover really shines as a soil worker. It fills gaps between main crops, enriches the soil with nitrogen and, once it's killed off by winter frost, leaves behind a protective mulch layer. The following crop benefits from the nitrogen it leaves in the bed.

Persian clover is only ever sown directly into the bed; raising it indoors is neither necessary nor sensible. Broadcast the fine seed over a finely crumbled bed. A capped, crusted surface slows germination. Then rake the seed in lightly or press it down so it gets good contact with the soil. Sowing depth is 1 to 2 cm.

You can sow from April to August, as long as the soil stays free of frost. The minimum germination temperature is around 5 °C, and it germinates best at 15 to 25 °C. In mild regions you can sow until mid-September and use the Persian clover as a winter cover that dies off in the frost. The first seed leaves often appear just three days after sowing, which is remarkably fast for a green manure plant. You only need a few grams of seed per square metre. When undersowing beneath main crops, sow a little more sparingly than for a pure stand.

Persian clover does best in full sun but also tolerates partial shade. That makes it particularly interesting as an undersown crop beneath taller plants such as tomatoes or maize. It's pleasingly undemanding when it comes to soil. It likes nutrient-rich loam to clay soils most, but sandy, free-draining soils work well too. The only thing to avoid is persistent waterlogging. If you have heavy clay, loosen it a little before sowing.

The ideal pH is between 6.0 and 7.5. On acidic soil, liming before sowing is worthwhile because the nodule bacteria work less effectively in acidic conditions. Despite its Mediterranean origins, Persian clover needs quite a lot of water while young. Well-established stands, though, cope fine with short dry spells.

You'll find a few proven varieties at garden retailers. 'Gorby' proved particularly strong in Swiss trials: fast early growth, strong competitiveness and good resistance to black blotch. 'Laser' is fine-stemmed and leafy, suits chalky soils particularly well and is often used as a base for mulch sowing. 'Maral' is widely available in German-speaking countries and a solid all-round variety for green manure and bee forage. Since Persian clover mainly serves as a green manure, differences between varieties matter little in the home garden.

Persian clover is one of the easiest green manure plants to look after. During germination, keep the soil evenly moist, because the fine seed on the surface dries out quickly. Once the stand has closed over densely, it copes with less water.

If the Persian clover grows too vigorously, simply cut it back to about 5 cm with shears or the lawnmower. This encourages branching and stops it setting seed. If you don't want it to self-seed, cut it before it comes into full flower.

To work it in as a green manure, mow the plant mass shortly before or during flowering and incorporate it shallowly into the soil. As with all legumes, leave the roots in the ground to rot down; they add extra organic matter and keep loosening the soil. If sown in autumn, the stand dies off over winter and forms a natural mulch blanket that you can simply rake in come spring.

Compared with other clovers, Persian clover is fairly robust against disease. One clear advantage: stem and bulb nematodes don't attack it, which makes it a better choice than red clover on infested ground.

The most common fungal disease is black blotch (Cymadothea trifolii). Small black spots form on the underside of the leaves, while the upper side shows yellow patches. Infected leaves curl up and die. If you spot this, mow the area and remove the plant material. The variety 'Gorby' is considered not very susceptible.

Clover rot (Sclerotinia trifoliorum) can in principle affect all clover species, but it hits Persian clover far less often than red or white clover. Typical signs are whitish-grey fungal growth on the stems and patchy gaps in the stand. The fungus's resting bodies survive in the soil for years. The most important prevention: keep breaks of 3 to 5 years between clover crops and use certified seed.