Sunday, 20 April 2014

Why does my blue Corydalis flexuosa die?



How to grow Corydalis flexuosa or perhaps try Corydalis elata instead!


About twenty five years ago this exciting new herbaceous perennial was introduced from China and created quite a stir. I used to help man the Askham Bryan College stand at the renowned Harrogate Spring Flower Show. Each year people would depart through the gates with armfuls of this wonderful plant. They were almost  all destined to die. (The plants I mean, although we all go in the end!). We used to call certain subjects ‘nurseryman’s plants’ because they were easy to propagate and were so beautiful that when the customer wrongly blamed their own incompetence when they died they bought some more. People catch on and you never see Corydalis flexuosa at the show anymore. 

Before writing this post I thought I would check out existing cultural advice on the world wide web. There is nothing to tell you why they are difficult to grow! 
The key factor is that they need to be grown in a pot. Then they are easy! My pot grown Corydalis have flowered freely for a six to eight week period between January and May for twenty years now. (It depends on the winter weather exactly when it comes into flower). Every time I try popping a few in the ground they die the following summer. Now here’s the thing, all my tub grown plants are outside throughout the year and contain merely my own fertilised acid garden soil.

When I lived in Bolton Percy, where the soil is slightly alkaline I grew them in tubs of multi-purpose peat based compost fortified with extra slow release fertiliser. I understood at that time that my blue corydalis needed acid soil and I omitted any lime. This is not confirmed by the literature that says they can be grown on acid, neutral or alkaline soil. I wondered why such advisory sites just do not  just say the pH does not matter! Perhaps they don’t know?

What is special about growing in pots? I have concluded that it is regular  summer watering that prevents them drying out. In late Spring Corydalis flexuosa dies down and goes dormant for about six months. Their  perennating vegetative structures are shallow surface rhizomes, These are extremely delicate and if they dry out the plant dies. We get long periods of irregular drought here in York but my pots of dormant plants in my little nursery next to my greenhouse get a splash of water every time I water! In the open ground when dormant, one forgets where they are and neglects to water them when they need it. Worse, as I have explained in a previous post my own sandy soil is hydrophobic when dry and the water runs away, but significantly not in pots where it is retained by the rim and soaks in.

You never stop learning in gardening. In writing this post it occurs to me that if my reasoning is correct and my success in pots is because the rhizomes do not dry out in summer then there might be another answer. What if I plant Corydalis flexuosa under a two inch gravel mulch? A little project this year! Hot off the press I have noticed today a small plant flowering in the leaf litter of an emerging iris in my bog garden. It has ‘well drained moisture’ and is covered by a natural organic mulch.

I have cheated with this picture, this is the very similar Corydalis elata. It shows dying rhizomes exposed on the  soil surface. With elata they form in winter and they are usually deeper. It  is best to spread a little extra soil over them if they are exposed.

Two years ago I did a couple of lectures in South West Scotland where it rains quite a lot and the well drained soil is water retentive. My host had a beautiful clump of flexuosa in the open ground. Later on, on my busman’s holiday, I noticed some at Calley Gardens at the foot of a moist north wall.

I teased C. flexuosa sprouting rhizomes apart in late December and later published the photograph of a rapidly growing clump at the end of January. I regret to say my plant was rather swamped by the emerging hyacinths that flowered in March.



 C.flexuosa amongst the Spring bulbs

Corydalis elata 





This plant is almost identical to flexuosa but is twice as high and grows well in the open ground and is a normal, easy, long-lived summer flowering herbaceous perennial. My own are in moist soil and full sun to medium-heavy shade. It is a much easier plant than flexuosa but does not have the advantage of flowering in late winter.

The book says they may be carefully divided in spring, Is this what they mean? I divide mine with a sharp spade any time I fancy.

The hybrid between flexuosa and elata is of median height and flowers well through the summer.

The rabbits have tried to help me divide my corydalis but have done no serious harm!

Other corydalis

I am very fond of all corydalis, it is related to dicentra of which I hold the National Collection.
The genus is a much larger one than dicentra, it is genetically close, looks similar but has a much greater range of colour. Corydalis may be annuals or short lived perennials and their lifecycle may be dependant on seed or vegetative structures such as tubers, rhizomes and roots. Most of them are easy to grow. Some unfortunately are too easy and seed everywhere.

Corydalis chelianthifolia has delicate fern-like foliage, seeds itself  freely but only if you have well drained soil (or gravel or a wall!)


Brenda banned lovely Corydalis lutea from our garden because it seeds too freely. I grow it in Bolton Percy churchyard garden where it loves to seed in a wall!

There are many fine named varieties of Corydalis solida available when you buy your Spring bulbs. If they like you and you have freely drained soil they will come every year.

This warm spring, the above pink Corydalis solida has set seed. The seedlings will be variable but I am dying to try them!

This is the ‘wild-type’ of Corydalis solida collected in a chalky Cotswold garden fifteen years ago where it so liked the conditions it had taken over. At home it has merely seeded enough to give me a dozen clumps!

Corydalis ochroleuca is common, lovely, very free seeding and somewhat invasive. The plant on the label is very rare - if it actually still exists - and looks completely different. Dicentras are frequently mislabelled in public gardens. I must send them an e-mail. 

In common with the rest of the UK their Dicentra eximia is actually Dicentra formosa!


My own Corydalis ochroleuca seeds everywhere. Brenda would have banned it had she known! 

Dicentra cucullaria flowers at the same time as Corydalis flexuosa and also likes pots!

12 comments:

  1. They are lovely plants and a beautiful blue. I had to smile at your nurseryman's plant comment as I often think the same tactics are employed when selling tender bedding and tomato plants very early in the year. A when the plants dies the punters will buy some more!

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    1. Yes it is a problem for nurserymen. If they behave responsibly their competitors beat them to it and there are other considerations, some gardeners have greenhouses and it is sensible to buy small bedding plants and grow them on.

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  2. I have bought a number of corydalis flexuosa over the years and lost them all. I also have (still have) corydalis spinners which is a cross between elata and flexuosa (according to www) and I would thoroughly recommend it - I bought it in 2010 and it has gone through wet, cold and mild winters and has gradually spread to make a really attractive and well behaved plant.
    I like the solidas too, I have just rescued a couple that were around the edge of a conifer and in danger of disappearing completely.
    I grow a pale pink dicentra formosa in combination with snowdrops and astilbes which gives a truly maintenance free bed that has year round interest (except Nov and Dec)

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    1. Thanks for your insights Pauline. It is great to get readers experiences.
      As to your last comment I gather there is a new fashion called mingling.
      People like you and I have always mingled but I must say after Brenda yesterday made rude comments about one of my own Dicentra formosa popping up in the middle of an emerging astilbe I shall watch this 'accident' with interest. Perhaps your dicentra and astilbe don't mingle so intimately!

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    2. Oh and thanks Pauline for reminding me the hybrid between elata and flexuosa is called 'Spinners' and is available on the net. I think with its lovely delicate green foliage, it is my favourite.

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    3. Sorry it's me again Pauline
      Looking at the lovely foliage on most of my 'Spinners' today It dawned on me that in my picture that I described as a cheat for flexuosa rhizomes, they are only on this hybrid which is not surprising on an intermediate between flexuosa and elata. Where the few Spinners surface rhizomes on a few of my plants had died new shoots are appearing at the margins from deeper down and in a few weeks will be as good as their companions

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    4. I have just been looking at this old post Pauline and I thought I might mention for the benefit of folk reading it to find how to grow corydalis that my Corydalis 'Spinners' flowered for a very long time May through to October this year. You might remember I featured it in my second post on hybridity as a wonderful example of hybrid vigour (long flowering) and intermediacy (season).

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  3. Well Roger it turns out that I have an interesting observation, several years ago I planted two varieties of Corydalis flexuosa, 'Pere David' and 'China Blue' within 3' of one-another, please don't ask why, now one thrives and the other disappeared within two years of planting, they were both in my "woodland section" which actually gets irrigated if things become too dry and there didn't appear to be any plants adjacent which were more likely to deny either plant an adequate supply of moisture in the soil. I do realise that this is an extremely small sample but we know genetically some plants are not destined to live long and are examples of short lived perennials particularly if they are producers of abundant seed, however, be that as it may your experience provides a very logical explanation for the lack of success. I grew Corydalis ophiocarpa from bonus society seed and although it is an extremely robust plant it unfortunately lacks flowers of any significance and I have just consigned all but one to the compost heap.

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    1. Yes corydalis ophiocarpa is nothing special!
      I expect your irrigation in your wood and the advantage of the shade of the trees is the secret to your flexuosa growing so well. Trees without irrigation for many of us leaves it too dry, but of course the plant does like shade.
      I have never known mine set seed(I know you are not claiming that) but I would love it if they did!

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    2. Please don't take this the wrong way but why do we ALL (me too) insist on keeping just one of a plant that we don't really rate when there are often much better varieties around. I have finally removed an over vigorous iris sibirica from the garden (I have several much better ones). I certainly wouldn't keep an old pair of socks that had holes in the heels but I noticed some of this iris in a pot - just in case? but in case of what?

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  4. Very interesting post and assessment Roger. I don't have experience planting them, but keeping them in pots sounds like good advice. I like the description of ‘nurseryman’s plants’. It is so true here and people keep buying and buying.

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    1. Good to hear from Niagara Donna! I saw in your own recent post you were warning about gardeners planting their bedding plants too early.

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