As observed during March 2008
4/3/2008
New life after a harsh winter
After the ravages of winter and the alternating low and rising temperatures, growth is now beginning to move on plants. In the borders, damage on overwintered growth is showing as discolouration on the exposed foliage, most prominently where early morning sun has caught a frosted corner. The fast thaw of the sap within the plant tissue expanding on thawing causes cell walls to rupture, and so tissue damage occurs. This image shows frost damage on the evergreen foliage of Magnolia grandiflora - on mature plants of this size the damage is disfiguring but not fatal.
In woody plants where you suspect there may be no life, you can detect signs of life by scraping at the bark layer with a thumb nail. Where there is a sign of green colouration beneath the scraped bark, there is hope of bud burst. Never be too impatient to rip out plants.
Plants with silver / grey leaves or those with hairy leaves have experienced a bitter winter through excessive rainfall with freezing and thawing temperatures. Leaf rot on these plants will be the least of our problems this spring; basal and crown rotting will be severe. The benefit of a free-draining root zone for this group of plants is evident.
The 19th and early 20th century were the glory days of plant collecting. New plants were flooding into the country. The recipients of these highly valued specimens were not sure of their hardiness, so initially consigned them to stove houses. As knowledge of these developed, many were moved to sheltered aspects within gardens and estates. We now have a greater awareness of the hardiness of many plants and the ability to select from clones known to survive our winter conditions.
Hardiness zones were developed to help predict where a plant may reasonably be expected to survive a minimum temperature. The USDA system is well used in America, from Alaska to Hawaii. In Britain the RHS and the European Garden Flora developed two different zoning systems. However, they are only guidelines as to where in the country a plant will survive, be it outdoors or with protection.
They give a guideline as to the minimum temperature plants will experience in a set zone. Other factors will play a part in a plant's winter hardiness: microclimate, where the plant was collected, altitude or geographical variation of type, soil (whether well drained or heavy and waterlogged) and plant association.
Thinking back to the severe winters experienced during my lifetime, the deep snow of Christmas 1962 and January 1963 was enjoyable. The snow blanketed plants in an insulating layer during the long winter of sub-zero temperatures. At the time, the container plant industry had not evolved. Plants which had proved themselves hardy in the locality were obtained as cuttings, divisions or seed from family, friends and neighbours. Nurseries sold plants bare-root, with autumn as the preferred season for planting.
The winters of 1978 - 79 and 1981 - 82 also highlighted the benefit of good plant selection. By this time, containerisation and the garden centre industry were well developed. With a ready supply of container plants, the planting season extended the year through, with spring most popular. Many southern hemisphere plants succumbed during these two winters... and will again due to this winter's harsh weather. We can hope for a traditional spring of sunshine and showers with no late frosts to boost plants into life and encourage growth.
Two select members of the Ericaceae family that we continue to provide protection for are presenting flowers now.
Epigaea gaultherioides is native to temperate South-West Asia. This shallow-rooting member of Ericaceae is found colonising the corner of a shaded north-facing frame adjacent to the Alpine House, where it appreciates a well-drained, cool root run. To flower successfully, it needs protection from the severe cold of the winter, but ironically must experience a cold spell to initiate flowering.
From Equatorial Ecuador at 2000+ metres we grow Cavendishia grandiflora. Collected on the old road from Quito Santo Domigo in the Pichincha province, the terminal flowers hang down, reminiscent of a scene in a dairy farm milking parlour. It is potted in an open bark mix, growing in the back-up glasshouses where the night minimum temperature is 10oc or above.
10/3/2008
The colour of spring - Forsythia
This is a plant everyone is aware of - in the gardens of suburbia, filling borders in country gardens, competing well in mixed hedgerows, this bright yellow unruly shrub spills into colour in every municipal park throughout Britain.
What makes Forsythia so popular? It is tolerant of all soils and conditions, but prefers full sun to shade and loves neglect; in fact when left alone to become a conglomerate of twigs, the plant becomes an ideal nesting site for the bird population. It roots easily as softwood, semi ripe and hardwood cuttings.
The genus was named after William Forsyth, a Scot, credited with constructing the first rock garden in England and a founder member of the RHS.
The F. x intermedia hybrid has many named cultivars 'Lynwood' and 'Spectabilis' being the most popular. These are selected crosses between F. suspensa and F.viridissima. Established specimens of both these cultivars are growing near the North Gate.
There is a collection of species and other cultivars growing in the south-west corner of the Garden, where the minute differences in colour and flower form can be observed, compared and contrasted. Observe, for example, an original Reginald Farrer collection from 1930, F. giraldiana, with prominent pin-like stigma and style. In contrast the cultivars have prominent anthers. In all of them, last season's growth is covered in pointed flower buds encased by fresh green sepals.
14/3/2008
Little and large
Rhododendron praevurnum is a huge evergreen species reaching 4 metres across and 3 metres high and is characterised by the distinctive blotch of wine-red colour on the inner corolla at its upper base. Bud development is deep pink on development, opening and ageing with style to a shade of white.
The flower bud formation is quite regal in the way it develops and expands throughout late February, providing the best show of flowers during mid-March.
This rhododendron is native to Central China, notably the provinces of Szechuan and Hupeh, from where it was introduced to Britain in the late 1800's.
The RHS give this species a zone 7 marking in terms of hardiness zones, which relates to the USDA plant hardiness mapping. In Britain, this species will survive where the average minimum annual temperature is down to -12.3 to -17.8 degrees Celsius. Although fully hardy, the flowers can be devastated by early sun after an overnight frost.
As a contrast, the alpine trough to the west of the Temperate Palm House is home to a miniature floriferous power pack. Saxifraga oppositifolia was collected in eastern Greenland by George Argent in 1974. Deep purple petals open and fade almost to white. Its dense, ground-hugging cushion - only extending 20mm in height when in flower - is a necessary habit considering its geographical range through the arctic and high mountains of Europe. Other forms are cultivated in the Rock Garden scree where again a free-draining root zone is provided. There is a pot grown example, growing in the north alpine frame. In this sheltered microclimate the growth is very straggly.
This perennial herb is hardy to -45.1 degrees Celsius or zone 2 by USDA standard. Though at home in arctic bog and tundra, it adapts well to cultivation. It is a British native, so if you miss it at the Botanics, look out for the splash of purple when walking in the Scottish hills.
20/3/2008
Woodland floor to hedgerow; a selection of the best British natives for Easter.
Primula vulgaris and its close relative the Cowslip, P. veris, are well known spring indicators. Spreading by seed and the scratching activity of mammals which inadvertently split clumps apart, resulting in the propagules rooting into the surrounding vegetation. The Primrose has been developed by generations of plant breeders and is now marketed aggressively as spring bedding. Flower colour ranges through white to red and blue. Resist temptation and plant or sow the true yellow species; where a damp shady corner is available these plants will thrive. Look to see the difference in pin eye and thrum eyed flowers. Pin eye is self evident; observe the prominent pin headed stigma. In the thrum eyed flowers the multiple anthers protrude slightly from the petals.
Two others to keep an eye out for are Coltsfoot, Tussilago farfara, a British native with an invasive root system. The yellow composite flowers ride on a scaly stalk. When seen this is a reliable indicator that the soil is warm enough for weed seedlings to germinate. Should the weekend be dry go armed with a hoe and reduce the germinating population sprouting up through bare soil before they become a problem. One seedling that rapidly colonises bare soil is Lamium purpureum, the Red Dead Nettle. Flowering early in the year it is a provider of nectar for early bees who need sugar after their winter sleep.
This Easter has coincided with Daffodils in full bloom. Look out for Narcissus pseudonarcissus. Flowering in the Rock garden with yellow corolla surrounded by much lighter petals. A coloniser of damp ditch banks but as with the Primrose will appear in most areas in various forms.
At various locations through the garden are swathes of Wood Anemone, Anemone nemorosa, a coloniser of woodland glades. The brittle match thin roots form mats through the leaf litter. On a sunny day the myriad of buds suddenly open transforming large patches of ground into a white or pink carpet.
Also in the family Ranunculaceae is Pulsatilla vulgaris the Pasque Flower, found growing in the rock garden the purple buds protrude upwards from the central clump, opening bell shaped and drooping down into the dissected foliage. Covered in minute hairs this gives the foliage a silver appearance. Close by is P.v. ssp. grandis slightly earlier to open and with larger flowers and the same bright yellow anthers, a native to Central Europe. Both prefer dry grassland, preferring a limestone base where they clump up successfully producing lacy seedheads during summer.
Harry Lauders Walking Stick was discovered in a Gloucestershire hedgerow two centuries ago, Corylus avellana 'Contorta' is hanging with yellow catkins filled with pollen. Look closer at the deciduous shoots to see the female flowers, small buds with slightly protruding red stigmas ready to catch the wind blown pollen grains.
Finally two parasitic species formerly in the family Orobanchaceae but with the family revisions following the adoption of the Angiosperm Phylogeny Group (APG) within the Herbarium is now in Scrophulariaceae. Lathraea clandestina is found near the pond at the base of host trees. A mass of purple parrot bill shaped flowers arise from the leafless clump. In contrast the flowers of L. squamaria, the Toothwort, are arranged on a short stalk, the whole is devoid of chlorophyll, insipid pink in colour and would pass as a shrimp if put on a dinner plate.
25/3/08
After the excess of chocolate at Easter; Turkish delight.
Our plant of Omphalodes cappadocica was collected as seed by the late Douglas Henderson, Regius Keeper of the Garden from 1970 until his retirement in 1987. The twelfth in a long line of eminent academics to have held this Royal appointment since 1670. It was during a visit to Turkey in 1961 that seeds of this "Navelwort" were collected near the town of Pazar on the Black Sea coast.
An evergreen rhizomatous perennial that covers ground in shade or semi shaded situations. Growing in the rock garden in dense shade beneath a coniferous canopy it is now pushing up masses of clear blue flowers on terminal cymes. Juniperus rigida is providing shelter from heavy rain which O. cappadocica dislikes and when subjected to will die out.
In the family Boraginaceae it has large leaves covering the ground which shrivel brown as new growth is produced from late March onwards.
For previous years' highlights during this month, see the March Garden Highlights Archive page.