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Plant of the Month Corydalis curviflora rosthornii ‘Blue Heron'

I like corydalis, and over the years have bought many and have grown several from seed from seed distribution schemes. Many of the latter are relatively short-lived but seed freely, so that now I have many plants without the slightest idea what they are. However they are welcome as long as they do not overpower their neighbours, in which case they pull up easily.

‘Blue Heron' is a much more sophisticated plant than its cousins. I bought it last year from Edrom Nurseries. We had a minor drought here last summer and it went dormant. It has just come back into growth (as have many of the other summer dormant corydalis in the garden.) The name refers to the dark blue, relatively large flowers that should be produced in the spring, but could equally well apply to the excellent blue-green foliage which brightens up a winter day. It is ‘bluer' to the eye than to the camera.

It was collected by Dan Hinkley in Sichuan and has not been in cultivation very long. It likes a relatively moist but well-drained site in cool shade.

Cornus canadensis – Dwarf dogwood

Peter Williams has sent in the following article on Cornus canadensis, a lovely, but occasionally temperamental plant.

I tried to grow this plant about 25 years ago when I ran a small, part-time nursery specialising in acid loving plants. I bought stock of Cornus canadensis on a number of occasions and each time potted about three-quarters for sale and planted out the remainder. None ever seemed to do well. The potted plants sulked in the compost I used for rhododendrons and other acid lovers, and those planted out into what I thought were ideal soil conditions all disappeared. I gave up on the species.

About 10 years after my last attempted introduction, I was lying flat on my stomach trying to get at the base of a weed that was growing through a large and very dense rhododendron, when I saw a tiny rosette of Cornus canadensis. It was so shaded under the rhododendron that it was almost dark and the soil was very dry.

I eased the tiny specimen out of the soil and planted it in a very shady spot at the edge of a north-facing woodland bed. I had little expectation of it doing well but the plant had clearly ‘learned its lesson' and grew away rapidly. In the first full growing season in its new location, shoots appeared all around the plant and a few flowers were produced. Eight or nine years later, it now covers about 20 square metres and flowers profusely in June and July. It is even invading the edge of the lawn! I now dig out ‘chunks' in early spring for friends and sell vigorous potted specimens at our charity open-garden days. I do not know why this plant was so temperamental for so long and then flourished.

As its name indicates, this species' home is Canada together with North America, Siberia and southern Greenland. In all locations the conditions are cool and wet and soils are base-poor and acid, very similar to my own soil. Its natural habitat is usually montane dwarf shrub communities but it is also known to invade montane grassland and is obviously quite a vigorous spreader when ‘playing at home' because it is a weed of lowland blueberry crops where the fields are recently cleared woodland. Interestingly, it has a close relative that occurs in similar locations to C. canadensis but which extend to Europe and even moorland areas of the UK.

This is C. suecica the Dwarf Cornel and it is of very similar appearance to C. canadensis. North Yorkshire marks the southern edge of its distribution in England and I have seen it growing in Hole of Horcum on the North York Moors. Where the two species grow together hybridisaton is possible and the resultant hybrids are said to have intermediate characteristics. The common name for this species in North America is the Bunchberry because of the bright red fruits produced each autumn that are used for pies and jellies. My clone has never produced fruit suggesting that it is not self pollinating, but it does have good autumn colours before it is largely covered by leaf fall in autumn.

The flowers of this dogwood are typical of the family – the true flowers are surrounded by coloured bracts that look petal-like. Incidentally, the common name ‘dogwood' comes from a corruption of the French word dague meaning dagger because the stems of the larger dogwoods were used for skewers. The flowers have one very special feature – they open ‘explosively' to release their pollen when touched by a large insect. This can be seen in a fascinating video sequence recorded at 10,000 frames per second on You Tube at https://www.youtube.com/watch?v=aFR17bX0noI. The advantage of having such explosive release is that it reduces pollen predation and increases the chances of the pollen becoming attached to the hairs of pollinating insects. It also helps in wind pollination of adjacent female flowers.

So why did this species fail for so many years in my garden and by report, those of many other shady gardeners? Some studies in North America suggest that it cannot tolerate mean summer temperatures higher than about 18°C so perhaps it needed fairly dense shade to prevent thermal damage. Another possible suggestion is that it was attacked by vine weevil – but I was not aware of this happening in the areas of my garden where it failed, and I never found vine weevil larvae in the compost of the potted plants. My planting advice would therefore be to get the plant established in a very shady area where it gets no direct sunlight. If it becomes established it may well be able to spread out of the shade and become far more light and temperature tolerant.

The lush growth of this species in my garden might now become its downfall. In North America it is eaten by deer and moose and we are having increasing trouble with Roe deer grazing!

Dealing with Dry Shade. Part 2: Very Dry, Very Shady

The sort of site I have in mind is under the canopy of an evergreen tree, to the north side of a large wall, or beside a tall evergreen hedge. Such sites get very little direct sunlight, and receive moisture directly only during heavy showers. Of course they get diffuse light and moisture via diffusion through the soil from other parts of the garden but they can be quite dark and dry, particularly close to hedges and trees which are competing for what moisture there is. That they can still support plant life is reinforced by the fact that ‘weeds' grow quite happily there. I have such a site beside a tall leyland hedge. In front of it is a less dry shade garden under some birches. Woodland geraniums, foxgloves, Lunaria redidiva and Trachystemon orientalis have all seeded themselves under the hedge, which was already carpeted with ivy and, in one place, ground elder trying to get in from next door! However, I can assure you that if I tried to transplant one of the geranium from under the birches into this site it would struggle to survive.

It is all about the balance between root growth and top growth. The seedlings that have planted themselves have grown slowly as their roots found moisture. When we try to plant something in a dry spot, the roots and top growth are in balance for a much more benign regime where the pot is watered regularly. I think this is one reason that people find dry shade difficult. You have to look after newly planted specimens carefully. They need enough watering to stop them dying, but not so much that they never adapt to their new life. They are like drug addicts that need to be gradually weaned off the water!

When dealing with such sites you must be quite clear, and above all realistic, about how much effort you are willing to put in and what results you want. If all you want to do is weed and tidy occasionally then all you can expect is reasonable year-round ground cover with perhaps some flowers in spring. If you are willing to put a lot of effort into soil improvement and to install a simple watering system then plants from the temperate evergreen forests of East Asia are possible. I have a friend who grows cypripediums in such a site!

Let's deal with these two extremes, although I think most of you, like me, will end up somewhere in-between. In either case the thing you must not do is to try to cultivate the soil. Digging only serves to break the roots of the canopy plant, which starts them into new growth, and sucks even more water from the soil.

If you are ‘low effort' then dig a small hole, pore water into it until it drains away only slowly, and then plant into the hole. Note again the point made above. Go for small plants with lots of root and not much top growth. You may even consider removing some of the top growth from herbaceous plants in order to reduce the water demand. You should keep a good eye on new planting and water if they start to look stressed. It is better to water the soil around the plant to encourage roots to grow out.

Now to the problem of what to plant. You need to select from plants that come from woods and woodland edges in areas with relatively dry summers such as Europe, Asia Minor and the Western USA. Some of the more robust, summer dormant mediterranean plants may also work (eg Arum creticum). What you choose will depend on personal taste. I would select a few of the evergreen ‘Western' epimediums such as E. pinnatum subsp. colchicum, x cantabrigense, perralderianum, x perralchicum etc. but steer clear of East Asian species, including grandiflorum. I would always include Vancouveria hexandra. I would certainly use Lunaria rediviva. Ivies make good evergreen ground cover, but can be a bit too vigorous. We have had success buying pots of small leaved ivies sold for bottle gardens. These have several rooted plantlets that can be separated, grown on until they have plenty of root and then planted out to give a cheap, colourful carpet. Do not be afraid to scatter seeds into the area and then thin out later. Foxgloves, honesty, hellebore (fresh) seed will germinate in-situ. You will not get the big, blowsy plants that you will in a sunnier, moister site, but they will grow and look good, both in flower and leaf. Be willing to try most summer dormant bulbs. I think there is one simple principle: buy (or better still grow your own) small, cheap plants and be prepared to accept losses. Do not waste money on woodlanders from East Asia and Eastern North America. They are stunning, but they are unlikely to be happy.

If you are willing to put in a lot of effort, then you should take the stance that the site gives excellent shade and drainage, and undertake to provide the soil and moisture yourself. You need to build up a layer of about six inches of good woodland soil. This should be a mix of roughly equal parts of leaf mould, loam and some aerating medium such as mini-chipped bark. However, be prepared to improvise. If you have garden compost and well rotted garden chippings, a mix of these will work just as well. I have read recommendations to put down a layer of semi-porous fabric or plastic with small holes in it, before building up the soil layer. This is said to prevent tree roots coming up into the planting layer and slow down water loss. I have not tried this, but it sounds reasonable. Once you have the planting mix in place, lay a ‘leaky-hose' over the surface, with a connector to allow you to plug in the garden hose when needed. Then top the whole area with a mulch. Bark looks good but is expensive; rotted garden chippings work as well and soon look natural. Water when the medium under the mulch is no longer moist. I find it useful to have ‘indicator plants' to tell me when to water. I use Deinanthe bifida, which hangs its leaves before the other plants show signs of stress, and also recovers without trouble once watered.

Such a bed will allow you to grow any of the deep shade loving plants from the summer wet woodlands. I will not give a list; there are too many and the choice is a matter of taste. Take out a second mortgage and visit Crug Farm or Edrom Nurseries!

Available Seed

If you are a paid up member of the Shade and Woodland Plant Group and would like any of the seeds listed below, please send a SAE to S.J.Sime, Park Cottage, Penley, Wrexham LL13 0LS.

If you have late ripening woodland seed to donate, please send it to the same address.

Akebia quinata
Arisaema candidissimum
Arisaema ciliatum
Arisaema consanguineum
Gentiana asclepiadea
Geranium psilostemon
Kirengeshoma palmata

Actaea cordifolia ex ‘Blickfang'
Adenophora takadae var. howazana
Astilbe rivularis CC6857
Cardiocrinum giganteum var. yunnanense
Cornus capitata
Fothergilla major Monticola Group
Hydrangea paniculata ex ‘Tender Rose'
Iochroma australis (both blue and white forms)
Phytolacca japonica

Shade Charade

Guess the variety.

Genus 4 syllables.
● 1st syllable: a county in the Western Highlands
● 2nd syllable: Sebastian Lord ….
● 3rd and 4th syllable: … half a cry of parliamentary approval.

Species 4 syllables.
● 1st two syllables: cat repeats sound of contentment
● Last two: the second half of the parliamentary cry above

Variety 2 words
.. sounds like a communists Nepalese soldier.
The solution to last month's Charade was Saruma henryi. This is a woodland plant from dense moist forests in China. It is a genus closely related to asarum, the name being an anagram. It has similar, heart shaped leaves, but the flowers are not tubular, having three, essentially distinct, yellow petals. It flowers intermittently for most of the summer and is easy to grow in any site that does not get too dry. It can be propagated from seed or from divisions.

Joe Sime