Soil preparation for succulent and spiky planting

[AndyC]

Between our drive and the road ouside is a south-west facing raised bed backed by Cornish hedge. It is about 10M long and 1.2 M wide at its widest.

The soil is quite free draining and the hedge takes out quite a lot of available moisture. The old tree stumps are being ground out next week.
I intend covering the soil with weed suppressing membrane and gravel, with holes cut through the membrane for the plants.
I've never devoted a bed for succulents and spikies before so would welcome some advice as to what soil preparation I should undertake prior to putting the membrane down. I have been given 150 litres of old horticultural grit which I thought I would dig in around individual plantings, is that likely to be enough for drainage?
I have some Aloes, Lampranthus, Nolina nelsonii already with Furcraea longaeva and Echium amoenum 'Red Feathers' growing on. Will probably add some Sedum and an Agave or two.
I also have had a Agave attenuata in a pot for a few years and would like to plant it permanently on the raised bed. Would it have any chance of survival here if I gave it a winter rain shelter? Thanks in advance.

[Paul S]

Andy - if it were my border I would be inclined to increase the proportion of inorganic drainage material. With the high levels of rainfall you get over your neck of the woods that soil-rich mix will remain quite claggy, even if well-drained. Plants like yucca, Beschorneria and Dasyliron might do ok - most likely also the Nolina you have - but I would imagine aloes and agaves could do with something a bit sharper. I don't know how well you know Steve Mules but he has some lovely raised beds around his house with a great selection of plants. Aloe polyphylla and Agave geminiflora do particularly well.

Agave attenuata - not a chance, I am afraid. I struggle to keep it alive in a 'just' frost-free greenhouse.

[AndyC]

Thanks Paul. I have about half a ton of 'dirty' 20mm gravel, would that do for digging in?. What depth of mixture (soil and gravel) should I aim for and what proportion of soil to gravel?

[Paul S]

I'd go for as much as you can fit in, really. Over here on this dry side of the country (I get around 500mm a year, I guess you'll be 2-3x that) I plant things like agave into pure 'all in' ballast or 20mm gravel. Treat 'em mean! To an extent your extra rainfall is offset by milder winters - stuff can tolerate being wetter at the root if it doesn't freeze so much - but even so it is easier to add water if necessary than to take it away. Plus even you SW Cornwall gardeners aren't immune to the odd nasty winter! Folks like Rob Senior, if you know him, would be able to give you some very good advice based on years of local succulent-growing experience. Also Steve Mules, as I said.

Having said all of that - a thought just occurred to me - last time I was in Falmouth (some years ago now) there were some fabulous Agave salmiana var ferox in various planters and borders around Queen Mary's Garden. I can't remember what the soil is like there but they seem to do ok - maybe I am over thinking it!!

[AndyC]

Thanks again. I don't know the two gentlemen that you've mentioned but I have missed a trick. I bumped into the head gardener at Gyllyngdune gardens the other day and should have asked her what their Agaves are growing in, I'll accidently bump into her again and ask the question.

[kGarden]

East Ruston Old Vicarage, on the coast in Norfolk, relatively mild being very near the coast, low rainfall, being Norfolk, put 600 tonnes of stone into their "Desert Wash" ...

http://www.e-ruston-oldvicaragegardens. ... t-wash.htm

If it were me, and given it slopes downhill somewhat, I would also put a land / "French" drain in. Trench, perforated drainage pipe (comes on a roll) in the bottom, cover with gravel, fill the trench back in.



If you think tree / hedge roots will get into the pipe wrap it in a "sock" - stuff a bit like fleece. Lets water get to the pipe, but not roots. (Weed suppressing membrane might also do for that job)

[charlie]

Thanks again. I don't know the two gentlemen that you've mentioned but I have missed a trick. I bumped into the head gardener at Gyllyngdune gardens the other day and should have asked her what their Agaves are growing in, I'll accidently bump into her again and ask the question.

Steve Mules at Lower Kennegy Nursery is still going strong and is the go to guy for these sort of plants but I am afraid Rob Senior is quite frail now and spends most of his time looking after his wife Dawn so sadly we dont tend to see him at plant related events anymore.

The planting at the edge of the carpark at Truro Crown court used to be largely arid loving plants, may give you a few ideas

[AndyC]

Thanks both, sounds like I might have some serious civil engineering to do if I want a 'proper job'. I will get some expert local advice as well before diving in.

[kGarden]

An alternative might be to keep the rain off? Can't remember where, but I've seen corrugated plastic sheet formed into "outside privy" style frames/boxes and put over plants to keep winter rain off.

Would a permanent sloping corrugated clear-plastic roof, supported just from the back, along the whole length be out of the question?

Or even necessary? <ScratchesHead>

[AndyC]

All done now. I hope I've improved the soil drainage sufficiently, time will tell.
I haven't got a lot more to plant in it apart from a few Sempervivum and Lampranthus. I would like to try an Agave, I like the dark blue/green ones so I'd appreciate a recommendation for one that doesn't get too large and is hardy. Thanks.