A Northumberland Alpine Gardener's Diary
This entry: 27 July 2008 by John Richards
Northumberland Diary. Entry 84.
Latent heat of evaporation
It has taken until the last week of July for summer to arrive here, but for the last four days temperatures have soared to 26C or so, and are set to go even higher. I am writing at 10 am on an overcast morning and its 25C already. These temperatures may seem even cool for those of you gardening in the south-east (or in a country with a more continental climate), but they approach the stressful for my favourite subjects, asiatic primulas, androsaces, meconopsis and the like.
In these conditions I find it helps to spray vulnerable subjects once or twice a day. The reason that humans sweat (or in some cases 'glow') in hot weather is that evaporating liquids take energy (in the form of heat) from their immediate environment, thus cooling it (and us). Plants 'sweat' (transpire) too, primarily to maintain turgor. In doing so, they take water from the soil, so hot plants face the dilemma of either cooling themselves by transpiration while becoming too dry at the root, or saving water but cooking to death. If they are gently sprayed, they cool without transpiration, and by humidifying the atmosphere, transpiration rate reduces.
In such weather, it is vital to spray plants lightly only, as long as watering regimes are adequate. Overwatering in warm conditions is the easiest way to set up lethal root-rots.
The last statement is not universally true. For years I have struggled with a few subjects that grow in very wet alpine habitats; usually at the margin of, or even in, mountain streams, spring-fed mires etc. For three of these subjects, I have experimented this summer by putting the 'long-tom' plastic pots in which they grow into the margins of my small ponds, submerging them to about one-third of their depth. For two of these, the Bulgarian Primula deorum and the Greek Crocus pelistericus the results have been dramatic. The plants have grown rapidly after years of stasis, and have never looked healthier. In the third case, Saxifraga hirculus, the jury is still out, but they were submerged later, and possibly too late in the season to have much effect this year. We will see.
The trick will be to judge when to remove the pots from the water and transfer them to a relatively dry environment, probably plunged in sand in the alpine house, to mimic the winter freeze.
Late summer plants for the rock garden
Late summer is rarely the most rewarding time of year for the rock garden, but I am gradually discovering plants that perform well at this time of year, and I am featuring six of these.
Perhaps my favourite is the Greek Pterocephalus perennis. This is a common plant growing on hot limestone rocks at intermediate altitudes, so it is perhaps surprising that it thrives and flowers well in this cool rather shady garden, but grown from seed collected by the MESE expedition in 1999, it has proved a solid banker here.
Another Greek subject that has a long flowering season here is a form of Centaurea triumfettii. Unlike most forms encountered in the wild, this stays admirably dwarf. The AGS Encyclopedia of Alpines makes a wise comment here, inferring that this plant which is often known under the name 'stricta' is not that, which is as the name suggests a taller plant. I suspect that the plant we grow may be a Turkish form for which the correct name might be cana. In any case, it is a good rock plant, creeping around gently, although beloved of snails which can damage the foliage.
Staying in Greece, the 'pink dandelion', Crepis incana, featured last year, is at its best now. A superb long-lived plant from the valley of the Styx.
I hesitate to include the next subject, Sedum spurium, as it can be very invasive. However, it has no equal for covering the ground in open gravelly places, and it is very easily removed from where it is not wanted. We use it to soften edges around the pond, the troughs etc. We have several colour forms from white to a deep rose.
Another excellent ground cover for gravelly places is Campanula carpatica. Some of you may regard this as too good a plant for such cavalier treatment, as it is commonly given pride of place at the front of the border. Here it is used as a common work-horse for difficult rather shady places beside the front path. An excellent plant as it is, it rises above such indignities. The white-flowered form is particularly good, shining through the evening gloom.
Finally in this section, a plant I find I featured in the very first issue of this diary, back in August 2006. Then I had not grown Erodium petraeum for very long, but it has since gone from strength to strength, so it might be worth comparing the two pictures.
Diary habitues (if any) will know that we escaped the worst of the Northumberland winter in January and, not for the first time, headed for the KZN Drakensberg. Seed collected near the Sani Pass Hotel germinated in April, and, amazingly, two subjects have already come into flower. Of course, these have yet to be tested by a winter, but growing at about 1600 m, they experience some frost and snow in the wild and should prove to be moderately hardy. Here is the campanula-relative Wahlenbergia cuspidata. Only one flower has yet opened but there are plenty to come.
So far I am less impressed with Diascia cordata. Interestingly, this species was not being offered according to my four-year old copy of 'The Plant Finder', plants so-named being forms of D. barberae. I encountered the latter species much higher up the Sani Pass, so it may be hardier and in any case tends to be a far more attractive plant. Now that breeders have worked hard with diascia, with sensational results, it is unlikely that this rather straggly form of D. cordata will have much impact, except with enthusiasts.
A Diascia National Collection holder, Chris Boulby, lives in Northumberland, and, in exchange for the perennial hardy Nemesia rupicola that I collected seed of on our first Drakensberg trip, she gave me N. sylvatica, which like the former has flowered all summer in one of the alpine houses. Under cover the winter poses no problems for this plant, but I suspect it will need cutting hard back after flowering if it is to perennate.
Another subject that has created an impact in the alpine house for many weeks is Helichrysum heldreichii. This rare endemic of Cretan gorges was first equated with the Mallorcan H. ambiguum when it was first introduced, curiously as it has scant resemblance to the latter. Since, as an ideal subject for silver-foliage classes it has become a staple at AGS Shows. The mid-summer flowers are rarely seen on the show bench, and the purist exhibitor probably cuts them off, but I find it an attractive subject at this time of year.
...formerly known as 'Belle Etoile'....
When we arrived here in 1989, we found a philadelphus rather buried under a lime tree, engulfed in snowberry. It was too big to move, but it has received a modicum of TLC over the years and now consents to flower sparingly most summers. It has wonderful large white flowers, and a very faint pink stain at the base of the flower as it opens. Exactly the same plant grows on the campus of Newcastle University, and talking to the Head of the Ground-staff, Ian Walker a few days ago, he thinks it was received as 'Belle Etoile'. Neither of us thinks it actually is 'Belle Etoile' as that fine plant should have a heavier purple mark on the flower. Any opinions?