Raised planting beds in a greenhouse

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Paul in Essex
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Raised planting beds in a greenhouse

Post by Paul in Essex »

A recent slipped disc/hernia whilst moving big plants followed by a week of reflection in Tenerife has decided me to stop my joint-straining annual in/out regime, completely clear out my main greenhouse, make (or, rather, get someone to make) some raised beds and simply grow the best plants in these beds.

The greenhouse is 20ft x 12ft and built on a dwarf concrete block wall around 3ft high. There is a solid concrete floor with no chance of breaking into the ground beneath, so these raised beds will contain as much soil as the plants will get. My plan is to make a front retaining wall out of concrete block to around 18" high in whatever shape I settle on but, ideally, taking advantage of the headroom at the apex for the tallest growing plants. Soil level will slope from back to front at the sides. So far so good.

Soil and drainage. I can't really decide what to do. Possibly put some drainage holes in the bottom of the retaining wall and a layer of drainage material over which I place the soil - with membrane between? And the soil mix - I am guessing it can be a bit richer than the mix I used for potting. Plants will be a mix of cacti, succulents, cycads, xeric ferns and bromeliads, temp either unheated or 'just' frost-free.

I welcome any advice, hands on stories, what went right, what went wrong, what I need to do, what I need to avoid etc :)

Muchas gracias!
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Re: Raised planting beds in a greenhouse

Post by DaveW »

Hi Paul.

I have a similar centre bed setup in my 18ft x 12ft 6inch greenhouse. It is 2ft high (double row of 1ft concrete blocks). I cut a couple of 3 inch drain holes with a cold chisel in the concrete floor underneath the bed so excess water drains into the hardcore under the concrete which acts as a soakaway. You will probably find it is harder to keep wet than over wet though since a deep bed dries out quicker at the surface than 2 inch pots on the staging's due to the Perched Water Table effect where water sinks down to the lower layers much quicker than for shallow pots.
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Paul in Essex
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Re: Raised planting beds in a greenhouse

Post by Paul in Essex »

Thanks Dave - drainage holes in the concrete sounds like a plan - do-able and a neat solution.
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Re: Raised planting beds in a greenhouse

Post by Phil_SK »

I'm giving up with my small area of free root run as I just can't keep it wet enough and anything that isn't naturally massive never seems to get its roots down enough to thrive.
Phil Crewe, BCSS 38143. Mostly S. American cacti, esp. Lobivia, Sulcorebutia and little Opuntia
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Re: Raised planting beds in a greenhouse

Post by JaneO »

I have raised beds forming part of patio and struggle to keep them wet. Think of it as container growing, water every day even after rain. We moved from clay to chalk and I missed camellia, magnolia and azalea so we filled stone built walls with ericaceous compost. I top them up twice yearly and only use rain water from butts to water. None of this will apply in your case, other than stressing the need to water a lot more than you think. The bottom of retaining wall is always bone dry. They are three feet high. Hope you succeed.
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Paul in Essex
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Re: Raised planting beds in a greenhouse

Post by Paul in Essex »

Thank you, both. I hadn't imagined the problem would be lack of water! :)
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Re: Raised planting beds in a greenhouse

Post by Herts Mike »

At least you got a slipped disc from honest exertion. I got mine from loading the dishwasher!
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Paul in Essex
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Re: Raised planting beds in a greenhouse

Post by Paul in Essex »

:grin:

More evidence, were it needed, that such tasks are best left to women...* :shock:

* Disclaimer .. said for comic effect and in no way representative of the distribution of tasks in my household or, indeed, any misogynist views on my part...
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Re: Raised planting beds in a greenhouse

Post by agavedave »

Perhaps something along the lines of the self watering pots that have a reservoir at the bottom and capillary action to draw some moisture back up.

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Re: Raised planting beds in a greenhouse

Post by topsy »

Hi Paul,

Well we have had plenty of experience of open beds but not one which was on concrete and there are many things we would do differently if we were to do it again. Firstly whatever height you have it will not be enough, so choose your species carefully or be prepared to take cuttings regularly which often results in the remaining plants looking a bit tatty, at least for a while. Secondly you will need to support taller growing plants, particularly as they may well start to grow more quickly and will consequently not develop a central woody core to support themselves. Tall plants under glass do not get stressed by winds as they would outdoors and tend not to be able to support themselves. We thought we should have used a framework at about 6 - 8 feet off the ground through which the taller plants could grow and be supported.

Thirdly, not that I am suggesting your plants have pests, you will need to be able to get to all your plants for inspection, insecticiding and weeding. A top dressing will help to keep weeds to a minimum and a glysophate based weed killer will not harm cacti but could be lethal to succulents.

Our open bed was located on the vegetable plot and so was reasonably fertile, but we still used a good mixture of mediums for the plants to grow in. Grit, cocoa shell, triple XXX - well rotted manure, sand, top soil, sedge peat and finally some chippings for the surface after planting. We had no problems with the bed drying out as it was based on the veg plot, in fact we had the oppposite - too wet, particularly once one of the trichocerei had broken through the roof to the elements, it let in the rain and let the heat escape and still it didn't flower because it didn't get any winter rest, didn't know what season it was. Also the garden sloped down to the greenhouse and the water table was never that far down.

We received a good piece of advice at the outset, don't bed out any euphorbias because if you have to prune them you will have a real mess of latex over other plants. We didn't heed it!

It still beat carting the plants in and out of the greenhouse each spring and autumn.

Good luck, Suzanne & Tony
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