The principal In principle these beds are very simple.
As with any other raised bed there is a layer of soil or compost in which to grow your plants.
But beneath this there is a reservoir.
The reservoir is filled with rainwater.
This could be the rain that falls on the bed and drains through, or it could be rain collected elsewhere and diverted to the bed for storage.
It is the latter which I will describe here.
How does it work? You know how a rain barrel works - you have a diverter connected to the downspout from your roof.
When it rains water is diverted from the downspout into the rain barrel until the latter is full, then the excess rain just goes down the drain.
Now, imagine there is a box full of compost on top of your rain barrel that you can grow plants in.
Further imagine that there is a solar powered pump which pumps water from the barrel and waters the plants growing in the box.
The pump is automatic so it waters regularly and applies the water the plants need.
If it applies a bit too much this just drains back into the barrel for re-use later on.
Now imagine that instead of the raised bed being built on top of a rain barrel, that the rain barrel is built into the raised bed - out of sight, but exactly where you need it.
How much water do I need to store for a square metre of bed? There is no simple answer to this question.
But a few things to consider are: -
I have beds which hold 120 litres per square metre, but have needed to top these up in prolonged dry spells.
Another bed containing 180 litres per square metre has never needed topping up.
Local climate will make a big difference - in hot dry areas a considerably larger reservoir is likely to be needed.
How big can a reservoir be? There is no absolute limit, but there are practical and economic considerations.
Where the reservoir must be completely contained within the framework of the bed, the capacity will be dictated by the maximum acceptable height of the bed, and the minimum acceptable depth of growing material.
Where it is possible to dig below the framework capacity can be increased.
One practical way of doing this is to dig out sufficient soil below the frame to then put it back in on top of the reservoir to grow in.
To go to extremes, the reservoir can cover the whole garden with a number of beds stationed on top.
In this instance capacity would be very large and the catchment area would also be increased.
In my garden I have found the best compromise to be a reservoir which is sunk about 20-30cm (8-12") into the soil and rises about 15cm (6") above it.
The top of the reservoir being above ground level makes the connection of a rainsaver much more practical.
Where a soil depth of 30cm (12") is needed this would give a bed height of 45cm (18") which many people find comfortable to work with.
What will I need?
Mark out the reservoir hole by cutting around the inside of the frame with a spade.
Move the frame well out of the way.
Dig the hole, setting the soil to one side.
The deeper the hole and the steeper the sides, the more your reservoir will hold.
In loose soils steep sides and great depth will not be possible, but as long as it holds together until it is finished, the reservoir filler will stabilise things.
Put the raised bed frame back into position.
Put in the liner.
If there are any stones, cut root ends etc you will need to cover these with old carpet or a layer of sand to protect the liner.
Tack the liner around the inside top of the frame, but ensure that folds are made in such a way that all cut edges are at the top to avoid leaks.
This can be done by ensuring the bottom polythene is flat and that it extends up the sides and ends flat too.
There will be excess liner in the corners.
Pull the top of this towards the middle of the bed - the most excess is at th top, tapering to nothing at the floor.
The excess can be laid along the side of the frame and tacked in place.
Decide where the maximum water level in the reservoir should be.
Using a spirit level mark this on your downspout.
Taking account of the rainwater diverters design cut the pipe and install the diverter.
Using the connection kit connect to the reservoir.
The pipe from the diverter should slope down slightly so that water will flow along it.
Even so, the higher up the connection the better as this reduces the impact of connector leaks on the reservoir.
Install a "well tube".
This is a tube from above the top soil level of the bed to the bottom of the reservoir.
The solar pump inlet can be lowered down this.
It can be rigid or flexible pipe, but must be at least 30mm (1 1/4") bore Pack the reservoir with inverted pots.
These will greatly increase water holding capacity and reduce the amount of gravel needed, but if any parts need great structural strength, omit the pots in that area.
Fill with pea gravel, ensure the pots are held in place and carefully fill in the gaps around them.
If needed to make up depth put gravel over the tops as well.
There should be 5cm (2") of gravel above the maximum water level to ensure that the bed is well drained.
Put the mypex or capillary matting over the top of this.
Fill with soil.
Take the opportunity to mix in compost or conditioner so that the soil you are using is the best possible.
Install a WaterWand Heliomatic solar pump with either a seep hose or dripper system.
Your raised bed is now primed to collect rainwater and to reapply it to your plants.
As with any other raised bed there is a layer of soil or compost in which to grow your plants.
But beneath this there is a reservoir.
The reservoir is filled with rainwater.
This could be the rain that falls on the bed and drains through, or it could be rain collected elsewhere and diverted to the bed for storage.
It is the latter which I will describe here.
How does it work? You know how a rain barrel works - you have a diverter connected to the downspout from your roof.
When it rains water is diverted from the downspout into the rain barrel until the latter is full, then the excess rain just goes down the drain.
Now, imagine there is a box full of compost on top of your rain barrel that you can grow plants in.
Further imagine that there is a solar powered pump which pumps water from the barrel and waters the plants growing in the box.
The pump is automatic so it waters regularly and applies the water the plants need.
If it applies a bit too much this just drains back into the barrel for re-use later on.
Now imagine that instead of the raised bed being built on top of a rain barrel, that the rain barrel is built into the raised bed - out of sight, but exactly where you need it.
How much water do I need to store for a square metre of bed? There is no simple answer to this question.
But a few things to consider are: -
- What you are going to grow - big plants need more water than small ones, drought adapted plants use much less than others.
Vegetables often need a lot of water. - How often does it rain? in your location do you get year round showers or is rain seasonal.
You will need far more storage for the latter. - How big is your catchment? If you are capturing rain from a large roof, a relatively small amount of rain will fill your reservoir so dry season top ups are more likely.
- Do you have a back-up? If you are totally dependent on the reservoir it needs to be big enough to cover every eventuality.
If you can top up with a hose pipe in an emergency it's not so important.
I have beds which hold 120 litres per square metre, but have needed to top these up in prolonged dry spells.
Another bed containing 180 litres per square metre has never needed topping up.
Local climate will make a big difference - in hot dry areas a considerably larger reservoir is likely to be needed.
How big can a reservoir be? There is no absolute limit, but there are practical and economic considerations.
Where the reservoir must be completely contained within the framework of the bed, the capacity will be dictated by the maximum acceptable height of the bed, and the minimum acceptable depth of growing material.
Where it is possible to dig below the framework capacity can be increased.
One practical way of doing this is to dig out sufficient soil below the frame to then put it back in on top of the reservoir to grow in.
To go to extremes, the reservoir can cover the whole garden with a number of beds stationed on top.
In this instance capacity would be very large and the catchment area would also be increased.
In my garden I have found the best compromise to be a reservoir which is sunk about 20-30cm (8-12") into the soil and rises about 15cm (6") above it.
The top of the reservoir being above ground level makes the connection of a rainsaver much more practical.
Where a soil depth of 30cm (12") is needed this would give a bed height of 45cm (18") which many people find comfortable to work with.
What will I need?
- Raised bed frame (sides & ends but no bottom)
- reservoir liner - Regular butyl rubber pond liner or some heavy duty black polythene films work.
Must be absolutely water tight! - Rain water diverter with connector kit.
Filtered versions such as the guttermate are best. - Lots of 10 litre plastic pots or old plastic buckets, fairly strong.
Bulk buy from horticultural sundries man. - Mypex or capillary matting to separate soil from reservoir.
- Pea gravel.
- Well muscled friends, spades, scissors, tacker, drill, saw, spirit level.
Mark out the reservoir hole by cutting around the inside of the frame with a spade.
Move the frame well out of the way.
Dig the hole, setting the soil to one side.
The deeper the hole and the steeper the sides, the more your reservoir will hold.
In loose soils steep sides and great depth will not be possible, but as long as it holds together until it is finished, the reservoir filler will stabilise things.
Put the raised bed frame back into position.
Put in the liner.
If there are any stones, cut root ends etc you will need to cover these with old carpet or a layer of sand to protect the liner.
Tack the liner around the inside top of the frame, but ensure that folds are made in such a way that all cut edges are at the top to avoid leaks.
This can be done by ensuring the bottom polythene is flat and that it extends up the sides and ends flat too.
There will be excess liner in the corners.
Pull the top of this towards the middle of the bed - the most excess is at th top, tapering to nothing at the floor.
The excess can be laid along the side of the frame and tacked in place.
Decide where the maximum water level in the reservoir should be.
Using a spirit level mark this on your downspout.
Taking account of the rainwater diverters design cut the pipe and install the diverter.
Using the connection kit connect to the reservoir.
The pipe from the diverter should slope down slightly so that water will flow along it.
Even so, the higher up the connection the better as this reduces the impact of connector leaks on the reservoir.
Install a "well tube".
This is a tube from above the top soil level of the bed to the bottom of the reservoir.
The solar pump inlet can be lowered down this.
It can be rigid or flexible pipe, but must be at least 30mm (1 1/4") bore Pack the reservoir with inverted pots.
These will greatly increase water holding capacity and reduce the amount of gravel needed, but if any parts need great structural strength, omit the pots in that area.
Fill with pea gravel, ensure the pots are held in place and carefully fill in the gaps around them.
If needed to make up depth put gravel over the tops as well.
There should be 5cm (2") of gravel above the maximum water level to ensure that the bed is well drained.
Put the mypex or capillary matting over the top of this.
Fill with soil.
Take the opportunity to mix in compost or conditioner so that the soil you are using is the best possible.
Install a WaterWand Heliomatic solar pump with either a seep hose or dripper system.
Your raised bed is now primed to collect rainwater and to reapply it to your plants.