Raised bed on concrete?

Lillydee

Hi guys,

Thinking of starting a raised bed on my back patio (as it's the only place I can control weeds ie the dreaded horsetail!)

Has anyone any experience in putting a raised bed on concrete instead of directly over the ground. Any issues I should worry about or advice :-)

Cheers!

pwurple

Your issue is going to be drainage. You will need to break up that concrete before putting anything like soil on it. Otherwise you will get waterlogging and rotting plants when it rains.

redser7

So long as you have enough dept of soil it should be fine. Think there was a thread recently where somebody suggested 12 inches would be plenty for most things. What are you making it out of? If it's brick you should make weep holes to let it drain. If its just wooden sleepers I'd say the water will run out the bottom easily enough.

Sonnenblumen

Agree with Redser, in fact i would say RBs are ideal for hard surface areas.

A few years ago we completed a major project of installing of raised beds in a corporate complex near Dublin Castle. Over 200m of Raised Beds were built with varying heights (225-775mm) on elevated (10-15m above ground level). It was as close as one could get to doing vertical gardening but on tiered ledges ranging from 0.9-1.5m deep. We also installed automated irrigation systems. A wide range of plants continue to thrive including trees silver birch, flagpole cherry (p.amanogawa) trees, as well as bamboos, rhododendrons , shrubs from 0.3-1.8m tall (choisya, prunus lusitanica, lavenders, acuba, viburnums, pieris, santolina, euphorbia etc etc) and a vast array of herbaceous planting.

On eof the key concerns is managing water and achieveing a balance to ensure plants get enough but not too much and making provision for dealing with excess water. So in order to avoid a flooding mess, you need to have aplan for managing run-off, I would also recommend that you put a layer of pea gravel at base of planter.

Good luck

Poulgorm

Lillydee wrote: »

Hi guys,

Has anyone any experience in putting a raised bed on concrete instead of directly over the ground. Any issues I should worry about or advice :-)

Make the concrete using 8 mm aggregate only - no sand. Then you will have a base that is porous (i.e.free draining) and will be impervious to weeds coming through it.

About a 4 to 1 mix - 4 buckets of 8 mm aggregate to 1 bucket of cement.

Ordinary concrete will be watertight, leading to a build up of stagnant water in the bed => very unhealty / dead plants.

Lillydee

Thanks for the advice guys! The concrete is already in so can't be changed. So would the best thing be to build a high bed, with pea gravel at base and maybe drill some holes near the bottom for more drainage?

Sonnenblumen

If you want to create a discrete gap/wedge for drainage purposes. Simply place a piece of folded material (ideally one which will not rot) under one corner where you wish the water to drain.

It is highly unlikely you will require to do this, as the gap between the underside of raised bed and concrete surface is most likely not water tight.

Poulgorm

So the concrete base is in. If you are using timber sleepers for the sides of the bed, use bits of floor tiles under the first sleeper to create a drainage outlet for the excess water. If you don't have any, go into any tile shop and buy a few of the cheapest ones you can find. Minimum 6 mm thick. Break them up and use them.

My Potatoes

pwurple wrote: »

Your issue is going to be drainage. You will need to break up that concrete before putting anything like soil on it. Otherwise you will get waterlogging and rotting plants when it rains.

+1
Break the concrete.
If you create a gap under the boards, you'll have a film of soil leaching out from the RB onto the concrete. Unsightly, potentially slippery, and leave it there and weeds will grow. Or you'll be sweeping it away every couple of weeks.