Cambridge University Botanics Mountain Plants Diary
This entry: July 2013 Living with a heatwave by Helen Seal and Simon Wallis
It's July 18th and already this month we have experienced temperatures in excess of 25 degrees on eleven days. The heat has moved down the soil profile reaching a surprising 16 degress at a depth of one meter. The showers have all missed Cambridge, so the monthly rainfall total to date is a miserly 2.5 mil. The result is parched, straw-coloured lawns and the gardeners have all but melted under the sunhats and layers of sun-block. But on the rock garden some plants are thriving without shade or irrigation.
It's no surprise that this Hirpicium armerioides is flowering in the heat: the seed was collected in 1995 by an expedition from Edingburgh Botanic Garden to Lesotho where it was found growing on a shrubby, rocky hillside. What is more surprising is that it survived last year's record rainfall.
We also have to thank Edinburgh for this Helichrysum montanum which is spreading from a small planting pocket to cover the rocks. The seed was collected from the imaginatively-named Devils' Knuckles also in Lesotho.
Close by the bright flowers of the South African bed, the New Zealand bed displays muted pale flowers (but a rich variety of foliage textures and colours). In the first two weeks of this month, a sparse leguminous shrub bucked the trend with cascades of bi-coloured lilac and purple small blooms. Under the flowering arches, a label indicated a 1956 planting of Carmichaelia enysii. It fooled us: this Carmichaelia is a true dwarf of 10 cms, whereas our shrub grows to 1m 60cms. Perhaps it is a Notospartium species? This certainly needs further investigation.
New Zealand beauty
Across the lake, the South American bed also grows some heat-tolerant mountain plants. Baccharis cf magellanica makes mounds of small, tough evergreen leaves and tiny compositie white flowers. This too was sourced from Edinburgh nearly twenty years ago.
Baccharis cf magellanica
The Haplopappus have survived unprotected on the rock garden far better than when we trialled them in pots under cover. Haplopappus coronopifolius was planted in 1957, and came originally from the famous plantswoman Valerie Finnis. The Alpine Encyclopedia gives this name as a synonym of Haplopappus glutinosus. A recently acquired plant of this grows close by, and the comparison is interesting. The old plant is more vigourous, and altogether larger, with bigger, paler flowers. Note the edges of the leaves - the new plant is more finely toothed. The species has a wide area of distribution and this may expalin the variation.
Haplopappus glutinosus 2009
Haplopappus coronopifolius (syn glutinosus) 1957
A new addition to the Western European bed is another composite, grown recently from AGS seed. Andryala arghardii is reputed to be susceptible to winter wet and best grown in the alpine house, but planting on a well-drained scree slope helped it survive last year's exceptional wet and cold.
Although this is a prime flowering time for members of the Asteraceae family, there are some other families represented. A dwarf Dipsacaceae, Pterocephalus perennis, thrives in the full sun and survives the occasional tramping when visitors wander off the narrow path.
Pterocephalus perennis close up
It is surprising that the fleshy-leaved gentians haven't wilted in high temperatures. Gentiana cruciata now grows in the shade of a Pinus mugo. It came to the garden from Wisley in 1960. It is especially valued as few blue gentians flourish here : the alkaline soils of pH 7.7 or so are a partial explanation.
In contrast, Gentiana dahurica came to us from Kevock Nursery four years ago and is now established in full sun in the Asian bed.
A happy combination
Tall herbaceous plants are unexpected in the rock garden, but are just as much a part of mountain flora as the diminutives we treasure. Our local expert on Lathyrus is always offering to share her plants, and this Lathyrus tuberosus has complimented the striking and tall Epilobium dodonaei from the Caucasus, supplied by Blooms in the 1980s. It is safe to let this seed as too few germinate to become a bothersome weed. How we wish we could say that about the native willow herbs!
Shade is one of the benefits of the anomalous champion tree in the centre of the rock garden, Catalpa x erubescens 'Purpurea'. A bed of European saxifrages and a few other alpines have thrived since they were moved under its protection. This Campanula coclearifolia was grown from seed, wild collected by the Italian gardeners at Giardino Botanico Alpino 'Saussurea'.
Catalpa x erubescens 'Purpurea'
But the heat has wrecked havoc with this comfortable arrangement. This morning a limb of the Catalpa cracked and split, an example of the phenomenom Sudden Limb Drop. When the garden's arboriculturist has taken the chainsaw to the branch and removed it, the bed below will be exposed to full sun and the alpines will probably wilt and bleach. Remedying this will test our ingenuity! (The Catalpa is not compromised by this loss and can continue to delight our visitors with its velvety, wide leaves and late July panicles of flowers.)