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A Northumberland Alpine Gardener's Diary

This entry: 24 August 2008 by John Richards

Northumberland Diary. Entry 88.

Glasshouse shading

Whether or not to provide alpine houses with shading, and if so how, is a hot topic of recurrent interest whenever alpine gardeners meet. For many alpines grown under glass, heat and dry air are the biggest enemies, so different to the cool moist conditions they enjoy on the mountainside. Shading cools the house and increases humidity, but alpines are also adapted to the high levels of light and ultra-violet they experience in the wild, so that shading causes the plants to be drawn, out of character, more susceptible to disease (especially basal rots) and poor flowering.

For many subjects, the ideal solution is to plunge them outside for the summer, and then bring them under glass for the winter. I find this works particularly well for porophyllum saxifrages. After this very wet summer they have never looked healthier in a plunge where they receive full light until midday but are shaded from the afternoon sun.

One of the problems is to make sure that sufficient space is left in the alpine houses for the plunge material to be returned under glass in the autumn.

By removing outside those subjects that are liable to scorch under glass and which enjoy summer rainfall, I find I am able to get by without shading the glasshouses. The benefits are that the subjects receive good light (or as good as this cool, north-facing garden can manage) and the bulbs receive a good baking. I am not advocating this policy for everyone. As noted above, this is not a typical garden, and a shade-free strategy is more likely to work in a poor summer such as the last two experienced here. Also, to maintain alpines under glass in full light is more likely to suit the retired gardener who can tend to his or her plants every day, watering and spraying whenever needed, and moving vulnerable subjects to a cooler spot in an emergency. Also, it is vital that plants under glass are plunged, so that roots stay cool in all circumstances. One of the perils is red spider which flourishes under glass, but which I never see in the open garden. However infestations rarely if ever reach a point at which the plant is threatened.

The two glasshouses are in the sunniest part of this rather dark garden and only become shaded by trees in the evening. Nevertheless, they lie against a hedge to the north. This remains uncut throughout the summer and as well as the beech twigs there are some large plants, a hedgerow elm and a hoheria that fill quite a lot of the northern skyline of the alpine houses in summer. In this way, incoming light levels are reduced but precious sunlight is not.

However, as the days shorten about now, I cut the hedge (with a good deal of perspiration and profanity I should add as the areas are not easy of access), bringing the hedge level down to the ridgeline of the glasshouses. In this way I can maximise the autumn light-flux. Here is one of the houses just after the cut.

 

Two Buddleia davidii in the hedge behind the glasshouses are not cut back until rather later, after flowering has finished. It is vital that I do so, otherwise buddleia seedlings will appear all over the garden. This is a real measure of climate change by the way. Buddleias never used to self-sow here as the seed was unable to mature before autumn frosts. In the North-East our railway sidings and embankments used to be innocent of the butterfly bush. No more.

Our two main bushes are white-flowered. We prefer this colour to the normal rather insipid lilac, as do the butterflies. We know this as there is a lilac plant next door, rarely favoured. The dark forms are lovely, but for some reason we lost our 'Black Knight'. Probably it was cut back too low.

The autumn hatch of the hibernating butterflies has proceeded apace, and we have enjoyed up to 20 Peacocks together, with a smattering of Commas, Small Tortoiseshells and Red Admirals. Our Commas are another sign of the changing climate. Only twenty years ago they were rarely seen north of Derbyshire.

 

Autumn glories
We are starting to get the benefit of late-flowering subjects. Here are two of our favourites that we grow in several places, often together. The scarlet Zauschneria angustifolia sets no seed but creeps around vigorously. Salvia bulleyana does seed around, to the extent that I try to deadhead it. It has curiously lacklustre flowers, but combines well with the zauschneria.

Autumn glories

The salvia is a roadside weed in parts of Yunnan, as is the blue-flowered annual houndstongue, Cynoglossum amabile. As the name suggests, the latter is indeed loved here, as it perpetuates itself with no external help beyond scatterering the prickly nutlets in open patches of ground when they mature in the autumn.

I posted Lilium 'Cover Girl' this time last year. However this group has grown dramatically in the damp, and at a height of 2.5 m definitely demands another outing. Growing in a loose organic soil, they are distinctly top-heavy, and I welcome additional support from some of 'nature's binders', Calystegia sepium. This will horrify purist gardeners, but bindweed is part of life here, does not put in a serious appearence until late summer, is easily pulled off where not wanted, and has, it must be admitted, very handsome flowers!

Having almost finished the hedges (at the cost of sore and prickly arms), I am eyeing up the next challenge, one of the 'D' beds that I intend to pull apart and start again. Whether Actaea pachypoda, a gift from Alan Newton five years ago, will survive the onsalught is another matter. Rather, it is in the wrong place, being somewhat leggy, but the fruits are undeniably handsome.

A good Banks-er

Many of you will know my neighbour and friend Alan Furness as a gardener of discernment, so when he actually stops and mentions one of my plants during a perambulation, I know I have something special.

Celmisia mackaui is a remarkable species, endemic to sea-cliffs on the Banks Peninsula, east of Christchurch. A big species with wonderful smooth, sea-green foliage, it is rarely seen in cultivation, perhaps because it is not the hardiest of subjects. Remarkably, this large clump only germinated from Ross Graham's seed in the spring of 2007. Slow at first, it spend the winter under a frame-light used to cover the nearby petiolarid primulas. However, it is now well into its sheltering peatblocks and has loved the monsoon summer. Flowers (and they are nothing special, the usual white daisies) would almost spoil the pristine foliage.

A good Banks-er
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