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Cambridge University Botanics Mountain Plants Diary

This entry: 28 August 2013 - Stone-Breakers: Potting-on the Sa by Lucy Rowley, Simon Wallis and Helen Seal

This month we have been focusing on tending to our National Collection of Saxifrages. The majority of the collection are European species, including representatives from various sub-sections such as Silver Saxifrages (Ligulatae), Dwarf Cushion (Porphyrion), London Pride Saxifrages (Gymnopera) and Mossy Saxifrages (Saxifraga).

This two-yearly process involves re-potting the entire collection in order to allow space for root growth as well as refresh the potting compost. We do it at this time of year as most of the plants have flowered by August, giving us the opportunity to remove rosettes that are dying back after flowering and generally clean the plants up. We pot into clays rather than plastic and plunge pots into sand beds under our canopy. The sand plunge creates insulation from the winter cold and protects the roots from the summer heat, as well as retaining moisture in drier periods. Some Saxifrages actually prefer pot cultivation, such as Saxifraga x hornibrookii ‘Riverslea’, a small cushion plant with tightly-packed rosettes that dislikes the winter wet.

A potted-on Saxifraga cochlearis 'Probynii'

A potted-on Saxifraga cochlearis 'Probynii'

In terms of growing media, our general Saxifrage mix is made up of a loam-based compost mix, horticultural grit, sharp sand and a dash of general fertiliser; this creates an excellent free-draining, neutral medium for successful cultivation. When dealing with some mossy Saxifrages, woodland-dwelling Irregulares and some acid-loving Porphyrion species, leaf mould and fine bark chips are also added to the mix in order to increase acidity and retain a bit more moisture. A mulch layer of horticultural grit helps to stop the pot from drying out, suppresses weeds and also helps to support the cushion form.

The Saxifrage mix and some of the tools of the tra

The Saxifrage mix and some of the tools of the tra

Re-potting also gives us a chance to inspect the roots and look out for the dreaded vine weevil, the most prolific pest of Saxifrages in pot cultivation. During the last re-potting session two years ago, we found many of the plants had fallen victim to vine weevil attack. We removed all visible grubs by hand and treated the rest with the nematode Heterohabditis bacteriophora and the fungal bioinsecticide Metarhizium anisopliae var anisopliae. On highly infected plants, chemical treatment was also necessary (Thiacloprid). More rigorous and frequent pest & disease checks were then carried out to keep the pests at bay. Thanks to this, not a single vine weevil was found on this year’s re-potting!

We also study the collection whilst we are re-potting, checking the labels and making notes on the condition of the plant compared with previous inspection. Sometimes (and this will come as no surprise to our regular readers!), we come across Saxifrages which we are not sure are labelled correctly. For example, we have a plant that came into the garden labelled as Saxifraga henrichii¸ which, after closer inspection, Simon suspects may actually be Saxifraga callosa subsp. australis ‘Lantoscana Superba’. Good spot, Simon!

Any plants that are in a poor condition, or that we only have one of, we will propagate in order to preserve and expand the collection. We propagate by taking rosette cuttings using a mix of grit, sharp sand and perlite. The cuttings are kept moist in propagators until roots are produced. Less frequently, we also sow seeds from fresh. Sometimes, we are lucky to find well-rooted offsets have formed, especially if the Saxifrage has expanded into the plunge sand.

Some rosette cuttings hopefully rooting-on

Some rosette cuttings hopefully rooting-on

Cuttings growing on

Cuttings growing on

Most of the Saxifrages grown in our alpine yard will go out into the display mountain glasshouse between January and June. They are plunged directly into raised sand beds, allowing visitors to study the fascinating and intricate flowers and foliage at a more accessible level. We also have a permanent display of Saxifrages grown on tufa in the display glasshouse, which benefit from growing in the sheltered crevices of this porous rock. When watering, it is best to water the tufa rather than the plant, (especially in winter), to prevent fungal infections and rot.

The Saxifraga Display in April

The Saxifraga Display in April

Some colourful Porophylum Saxifraga species and hy

Some colourful Porophylum Saxifraga species and hy

An impressive Saxifraga fortunei 'Rubrifolia'

An impressive Saxifraga ‚??Rubrifolia‚??

Out on the rock garden, as mentioned in the last blog, we lost a large limb from the Catalpa which provided necessary dappled shade over the Saxifrage bed. As a result, the plants have suffered from scorch, and furthermore to drying out caused by prolonged periods without rain combined with hot temperatures. Extra irrigation and the addition of more rocks and tufa will be needed going forward to remedy this problem, and we are currently propagating to replace the lost plants.

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