Corydalis (Corydalis lutea) is native to all of Europe, including Britain. It is a branched, spreading, hairless perennial, twelve to eighteen inches high. The leaves are lacy and blue-green, looking rather like the leaves from a fern. The yellow flowers grow on stalked stems. They are tubular and spurred and grow to about 3/4 inch long in free flowering racemes somewhat nodding. They bloom from spring to late autumn. Corydalis is often used for rock gardens, borders, ground cover under shade trees, and old walls. Corydalis is hardy in zones five to seven.
Corydalis grows in part to full shade. It is tolerant of heavy shade, making it ideal for a shade garden. The flowers will add interest for most of the growing season. Corydalis can tolerate full sun but will sun scorch if it dries out.
Corydalis is happiest in rich, moist soils that never dry out. The drainage must be good as wet soils in winter can be fatal. To make sure that corydalis does not get wet feet, you can till up the site where you are going to plant it to a depth of six inches. Spread three inches of compost over the tilled area and till it in well. The addition of the compost will help the soil drain well after a rain while feeding the plant the nutrients it needs to grow well. Corydalis prefers a neutral or alkaline pH.
Corydalis does not like hot and humid weather so doesn’t grow well in the deep South. It grows best where it is cool and dry.
This plant is an aggressive self seeder so may be a bit of a nuisance in the flower bed. If the foliage looks bad in the summer, you can shear it back to the basal leaves.
Corydalis has no major pest or disease problems. It is deer resistant.
Corydalis can be propagated by seed, division, or separation. Purchased seeds often show poor germination. Seed should be collected from the corydalis and immediately planted in containers. The seed will over winter in the containers and germinate in the spring. Keep the soil moist in the containers but not soggy. Otherwise, they need no special care.
Since corydalis self seeds so freely, you can dig up some of the new plants and transplant them elsewhere. Corydalis is a perennial, so you can dig it up and divide it in the early fall or spring.
If you do not want the corydalis to self seed, deadhead the blooms religiously. This will also encourage re-blooming. If the corydalis is spreading too aggressively, mulching them with a two to three inches of coarse mulch will make it more difficult for the seeds to grow and spread.
In the spring, before the buds open, fertilize corydalis with a good balanced fertilizer such as a 10-10-10 or some compost spread around the plants.
Corydalis may die back when it is cold or at the height of a hot summer. This is normal and the plant will grow again after the temperature is more to its liking.