vegetable patch design
raised bed heights and widths

raised bed heights

Illustration of a raised bed showing increased evaporation at the edges of the bed.

ILLUSTRATION - High Raised Bed
The higher the walls of a raised bed the higher the evaporation from the sides and edges.
The higher the sides of a raised bed are the more moisture is lost from the bed due to evaporation.

This is especially so if the sides are made of wood as timber placed next to moist soil will draw moisture out of the soil and into the timber as the outer side of the timber is being dried by the sun.

But metal sides also increase evaporation.  While water cannot pass through metal as the metal sides of a raised bed are heated by sunlight it passes the heat through to the soil, making the edges of the bed warmer than in the centre. The warmer the soil, the higher the evaporation level.

ideal height for a raised bed
The ideal height for a raised bed is 15 to 20 cm.  This is high enough to the level soil as described on the Merits Of Raised Vegetable Beds webpage but not too high to cause excessive amounts of evaporation.

Unless you have back problems that make it difficult for you to bend over there is no reason to build a raised bed higher than 20 Centimetres.

The other exception is if you are building a wicking bed as you need a minimum height of around 40 cm to build a wicking bed.  For information on how to build a wooden wicking bed see the How To Build A Wooden Wicking Bed webpage.

building beds higher than 20 cm
PHOTO - Woven weed mat
Plastic woven weed mat lining the inside of a raised bed.  The plastic has been turned down at the top to prevent fraying.
If for whatever reason you wish to build a raised higher than 20 cm then you should line the inside of the the bed wall with a plastic weed mat.  This will reduce the amount of evaporation that occurs.

If the timber you are using is treated pine the plastic will also help stop any chemicals used to treat the pine from leaching out into your garden soil.

raised bed widths

A raised garden bed should be no wider than the width that you can comfortably reach into the centre of the bed without having to step on it.

Ideally you should rarely  step on your garden bed.  This is because regular stepping on the bed will gradually compact the soil, which creates less than ideal conditions for growing vegetables.

So a raised garden bed should be no wider than the width that you can comfortably reach into the centre of the bed without having to step onto it.

This width will depend on how long your arm is, but for most people this will mean a bed of between 90 centimetres and 1.2 metres wide.  The most common width being a metre wide. 

Any wider than about 1.2 metres wide will force you to step onto the bed in order to get to it's centre.