Bananas
Proteaceae

 

growing
Exotic Succulents & Cacti
outside & unprotected in the UK

 

Furcraea longaeva - flower spike
A night shot of a developing flower stem on a Furcraea longaeva. This plant belongs to the agave family or Agavaceae and, like Agave species it is monocarpic (dies after flowering). Also, like some other members of the family, furcaeas produce bulbils - miniature plants - as well a seeds on the flower stem after flowering. The new plants from the bulbils are genetically identical to the parent. Furcraea is related to the agaves and at first sight appears to be ones of them; however, its degree of 'succulence' is a good deal lower as is obvious from the thickness of the leaves.

 

 

General

In this section I describe what I consider to be some of the main contributors to the atmosphere of exotic plantings - succulents whose foliage, flowers and general character have enhanced our garden. My experience, as indeed is the case with all these sections, is limited to our locality - the milder south-west of England - and to the plants I describe.

Agave potatorum growing in a raised bed in very gritty soil at the foot of a south-facing wall. The overall position is on a south-facing slope of some 30 on average. This agave is not generally reliable outside and tends to be attacked by rot during winter. The next-to-ideal conditions provided here have ensured its successful over-wintering during worse than average winters. To its right is Aeonium urbicum, a comparatively tender aeonium which survived the 2010-2011 winter (temperatures down to minus 4.5C in the open) with minimal damage.

Succulents, of course, are associated both in the mind and in their natural environment with dryness and therefore any attempt to 'twist' reality for example, by placing a sub-tropical succulent, in a bog garden will end in a double failure: firstly, the juxtaposition will appear incongruous even ludicrous and, secondly, it will be followed shortly by the demise of the plant. An extreme example perhaps but I have seen something similar time and time again in its milder manifestations in the gardens of friends and acquaintances . The main rules are, therefore, 'keep the plants dry' and 'ensure that it looks dry where they grow'.

The word 'succulent' is used to describe are plants which are able to store and conserve moisture and can therefore withstand long waterless periods that occur, for example, in deserts. Succulents are also often tolerant of high temperatures. Thick leaves and stems are both characteristic features of succulents. When adequate moisture is available the plants are moist and watery inside but during dry periods they can lose many times their weight in water. In this sense cacti are also succulents despite being generally referred to and described separately.

In this section I shall use a very broad definition of succulents and concentrate in those hardy plants which are members of the following and related genera or families: Agave, Aloe, Euphorbia, Senecio, Crassulaceae, Cactaceae, and Aizoaceae. These groups comprise a large number of good garden plants which I have found to be hardy even during worse than average winters in the coastal area where I garden. Although individual species respond differently to low temperatures and other unfavourable conditions by and large they all they all require almost the same treatment in the garden.

 

Requirements

Opuntia - unknown species, Devon, UK
A clump of hardy opuntias (details of species unknown) growing on a well-drained bank on the south-facing terraces in the rear garden. Under such conditions the opuntias are fully hardy here in Torquay. Prior to planting they had been knocking around and flowering in soilless buckets and pots for a couple of years. I obtained them from an enthusiast who had been growing outside in a very cold part of Torquay for about forty years.

Of all the various conditions generally mentioned for the growing of succulents (both in pots and in the ground) all fall into two main areas: soil type and position. While all plants are in variously affected by soil and position, otherwise hardy succulents can easily and quickly be killed quickly and effectively by not getting these two absolutely right.

In principle, the requirements seem quite easy to meet. Soil: very well-drained, dry in winter, preferably neutral or slightly acid. Position: as much sun as possible but at least for most of the day, as much heat as possible, shelter from wind and driving rain, preferably on a south-facing slope and against a wall of a house. The vast majority of failures of the plants on this list result from problems with these requirements - even the rock-hardy sempervivum will struggle and eventually give up in damp shade; and this can mean even in the shade of other plants. The general difficulty with these requirements is that they often require a degree of hard work in order to achieve the optimal state.

As a a general guideline for soil I would recommend a 50:50 mixture of grit (anything along the lines of horticultural or Cornish) over an area of at least 18" (45 cms) in diameter and around each plant all to a depth of some 18" (45 cms). If the soil is limey, add a couple or three shovelfulls of moss peat to the mixture; if the soil is clayey add 50% more grit. The grit should ensure that the roots at least in the immediate vicinity of the plant are never waterlogged. Added to this, the plant itself will benefit if it is planted directly into a small 3-5 litre pocket of pure grit. The roots will, of course, penetrate the surrounding soil in their search for nutrients.

One danger to look out for is to ensure that the whole pocket of gritty soil does not itself sit in a pool of water. This can easily happen in a clayey soil, especially during the winter months. Such a situation will mean that the gritty soil is not able to ensure that air circulates around the roots and consequently invites rots and moulds to take hold. Here some sort of drainage must be installed or the level of the soil built up and a raised bed created. In any case, growing succulents in raised beds or, much better still, on terraces on a south-facing slope further increases their chances and promotes faster, better and healthier growth.

There are no hard and fast rules here and I have seen many people growing agaves and aloes successfully in ordinary soil. 'Suck it and see' the saying goes. However, the chances of survival and successful growth are vastly increased by taking such advice seriously.

Agave salmiana angustifolia 'Variegata' on Devon, UK
Agave salmiana angustifolia 'Variegata' during the winter 2010-2011. Although the cold spell was relatively severe and lasted long this agave suffered only minor damage and was again looking its best as early as June.
One of the most successful and visually pleasing ways of growing exotic succulents is in the gaps between the stones of a south-facing wall. Here Echeveria is growing next to a Drosanthemum hispidum in a retaing wall in the rear garden.

Although most gardens do not have the landscape features to enable the immediate enjoyment of exotic succulents some modifications can be made. Raised beds, filled with a mixture of soil and grit (and peat if necessary) immediately spring to mind. It is important to ensure that water can easily drain out of the whole bed itself. Another, more interesting garden modification is to build a wall (at least 9" thick) which will support a south-facing bank which can, in turn be terraced1. If the wall rises well above the top level of the bank it will give the plants additional shelter and heat. The reverse side of the wall can be used in many ways: to support purpose built compost bins, to create a lean-to garden store or even to form one wall of a garden shed.

Most succulents like a lot of light and heat. It is as well to understand the essence of the problem we have in the U. K. regarding both of these resources. Light (and radiant heat directly from the sun) is less intense here than in lower latitudes for several reasons. Firstly, sunlight strikes our latitudes more obliquely than it does further south and therefore the concentration of heat and light per given area is less. Secondly, in the U. K. we have to contend with a greater number of relatively dull days when light levels are low. Thirdly, for much of the year the deviation from the vertical increases greatly while the daily period of illumination decreases, also greatly. This is not offset by the longer days during the spring and summer months simply because during early mornings and late evenings again light strikes the U. K. very obliquely.

While all this sounds very pessimistic there are advantages which the U. K. has over many places in similar latitudes. These consist of the maritime climate, the warming effect of the Gulf Stream and the presence of the warmer part of the Atlantic Ocean upwind from the U. K.. Together these effects give a degree of winter warming, additional to the sun's radiant heat, and compensate somewhat for the dull winter months.

There is not very much that we can do about long winters and dull weather apart from moving to more favourable locations in the south or to cities where human activity keeps temperatures a degree or so warmer than the surrounding countryside. However, by gardening on both natural and artificial south-facing slopes we can easily increase light and radiant heat to levels similar to those of warm temperate and even tropical latitudes. I am not alone in noting that south-facing slopes make a vast difference to plant performance.

Shelter in gardens is generally best provided by the planting of appropriate shrubs and trees which break up and slow down the wind rather than by solid objects like fences and wall which often funnel wind currents, cause local increases in wind speed and produce undesirable eddies. However, with succulents stone walls are desirable because they store and reflect the sun's heat. This makes it even more important for shrubs and trees to be dotted around in order to alleviate any new wind problems.

 

A Succulent List

The following is a list of (mostly) succulents which I have grown outside with no protection (other than that afforded by closeness to walls in the case of ground-grown plants and perhaps also a roof overhang in the case of pot-grown plants) at Southern Comfort. They are all worth considering although some have more merits than others. All have passed my 'success' test meaning they suffered no major setbacks during the winter period and grew steadily during the warmer part of the year.

Many other plants passed the 'survival only' test when growing outside. I do not consider such plants 'hardy' in any meaningful sense although during milder winter they may succeed and even make acceptable growth the following summer. However, I believe that more could be added to the 'success' category by being absolutely ruthless with the soil and the position of the plant. It must be remembered that in the wild it is only those plants which manage to seed themselves in exactly the right conditions survive and prosper - all the others struggle and fail. We observe only those which have succeeded but we can never know to the last relevant detail what it was that made the plant survive in that position and fail in a another seemingly identical one close by. The canny gardener who wants a plant to succeed will try it here, there and somewhere else as well as changing soil composition and what have you before finally giving up. We can therefore be optimistic that some of the 'survival only' and worse cases may be turned into stories of success.

Cactaceae

Epiphyllum (sp.)
Opuntia (large, unknown species)
Opuntia (syn. Nopalea) cochenillifera
Opuntia verschafeldtii

Crassulaceae

Aeonium arboreum
A. urbicum
A. arboreum 'Zwartkop'
Cotyledon
Echeveria peracockii
Echeveria rosea

Echeveria other species
Crassula sarcoacaulis
Sedum sp.
S. spectabile
S. prealtum
S. dasyphyllum
Sempervivum
sp.


In pots:

Crassula ovata
C. arborescens
Sedum falcatum

Agavaceae

This list is only a small sample of the more commonly available hardy agaves. The selection offers sufficient variety for a solid agave contingent in an exotic garden but would never satisfy a serious collector. Mary and Gary Irish (Agaves, Yuccas and Related Plants, Timber Press, 2000) list almost 50 Agave species and varieties which are hardy to -4C and 36 which are hardy to -7C. Many are hardy to -15C. However, since their total list contains only about one-fifth of the total number of species it is not unreasonable to assume that the total numbers which are hardy to the above temperatures is at least a few times greater.

Agave americana
A. americana 'Variegata'
A. americana 'media-picta alba'

A. celsii
A. salmiana angustifolia 'Variegata'
A. salmiana spp. ferox
A. parryi
A. potatorum
A. filifera
A. 'Sharkskin' [possibly A. victoria-reginae x A. scabra or A. fernando-regis x A. scabra]
Beschorneria septentrionalis
B. yuccoides
Cordyline australis
C. indivisa
C. kaspar 'Bronze'
Furcraea longeava
Dasylirion texanum
Yucca elephantipes

Y. recurvata

In pots:
Agave attenuata
A. montana

A. stricta
A. utahensis
A. victoria-reginae
A. xylonacantha
Manfreda variegata

Aizoaceae (Mesembryanthemums)

Aptenia
Carpobrotus
Lampranthus


Asphodelaceae

Aloe striatula
A. arborescens
A. aristata
A. humilis
A. brevifolia

In pots (some with winter protection):
A. bainesii
A. ciliaris
A. claviflora
A. cooperi
A. ferox
A. longibracteata

A. maculata (syn. A. saponaria)
A. marlothii
A. mitriformis (syn. A. perfoliata)
A. mutabilis
A. peglerae
A. plicatilis
A. polyfilla
A. pretoriensis
A. reitzii
A. striata
A. thraskii
A. variegata
A. vera
A. zebrina

Euphorbiaceae

Euphorbia characias
E. mellifera
E. myrsinites
E. rigida
E. stygiana

Senecio

Other

Trichocereus (now Echinopsis) terscheckii
Agave utahensis
Kniphofia (various)


1. Such wall then becomes a 'retaining wall'. There are rules of thumb which apply to the relationship between the height of the supported earth and the thickness of the wall. For normal conditions this is 3:1 (level of earth:thickness of wall) . Ignore those at your peril. In addition, there may be local planning regulations to satisfy.

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