Corydalis flowers come in almost all colours. Although a couple of yellow ones are grown widely in gardens in the UK, quite a number of others would also do well. Until the blue-flowered Corydalis flexuosa was introduced in the late 1980s, the bright yellow C. lutea was the best known species in our region. This grows almost anywhere in the sun or light shade, but probably best in the rock garden or in cracks on walls. The related C. ochroleuca, with creamy-white flowers, grows in similar places. Both like limey places, but tolerate anything not too acid. Typically they grow as annuals, but if the soil is deep and fertile, C. ochroleuca in particular persists for several years and has a long flowering season, sometimes with a few flowers right through mild winters. Both seed around freely, but do not appear to hybridize. Incidently, both C. lutea and C. ochroleuca have been separated to the genus Pseudofumaria because of considerable differences in the flowers. However, Plant Finder and almost all nurseries stick to the old names.
Corydalis nobilis is very different - a long-lived perennial, which emerges in mid-March and disappears in late June and increases slowly by narrow rhizomes. Although my two plants bought from Paul Christian seven years ago are now large, they are not yet large enough to risk dividing. The huge number of plants scattered around the grounds of Gothenburg Botanic Garden form a striking cover of yellow in late spring and have often been written about. However, the plant does not quite live up to its name when seen close-up in a garden bed. The flowers are over all too soon and the first ones to die turn brown and spoil the general appearance.
Corydalis cheilanthifolia is the best of all yellow ones for gardens in our region. Its ferny leaves are attractive all the year and the flowers form a mass of yellow in March and April, and often occasionally at other times of year. It grows in any deep humusy soil, typically persisting for three years, but an extra year in very deep soils. It seeds around in sufficient numbers to maintain it in the garden, though not prolifically; seedlings often develop next to walls or rocks. If you decide to transplant it, even more than other corydalis, the roots need gentle treatment. C. ophiocarpa, which is somewhat similar in behaviour and can sometimes become frequent enough to approach weed status, also has attractive leaves, but the flowers are an undistinguished pale yellow and green mix.
There are at least three other annual or biennial yellow-flowered corydalis which are becoming increasingly common in gardens: C. heterocarpa, C. pallida and C. ochotensis. Gardeners often have one of them without realizing this, because they are frequently sold or distributed under the wrong name, usually C. nobilis. One major seed seller, several nurseries and all the seed distribution schemes of the main garden plant societies (including HPS) have been at fault in the past couple of years. C. heterocarpa is the best for gardens. Ideally seed is sown in early autumn and the plants then start flowering in late April and keep flowering for many months. The size of the plants varies enormously, depending on how deep and fertile the soil is. Large plants initially look fine when free-standing, but eventually get straggly and are best left to grow among perennials or small shrubs. Seeds are large and, unlike many other corydalis, can be stored dry for a few months and germinate easily.
Two other yellow-flowered species are also being grown more widely. C. chaerophylla is supplied by several nurseries. Its attractive flowers can persist from June to November, but the plant must not only have deep soil, but be kept moist all the time in summer. In nature it probably grows by streams and I have had the best results when standing a large pot in a bucket of water.
Corydalis bracteata is tuberous and rather like a large yellow-flowered C. solida. In some parts of eastern Europe it is an important garden spring flower, but has been less of a success outdoors in the UK. It is particularly susceptible to alternating frozen and mild conditions. Some C. solida cultivars have the same problem, but, as there are many which do not, hopefully one day someone will provide a C. bracteata cultivar which also stands up to the frequent climate shifts of late winter in the UK. A number of hybrids of C. bracteata and C. solida sold as Corydalis x allenii grow vigorously in our gardens, but the typical mixed yellow and purple of the inflorescences do not have the same appeal.
First published in the North Yorkshire Group Newsletter, March 2009
and subsequently in Cornucopia Issue 26.
© Copyright for this article: Brian Whitton
This article was taken from a copy of Cornucopia that was published in 2010. You could be reading these articles as they are published to a national audience, by subscribing to Cornucopia.