Dear David

Many thanks for your recent message regarding the depth of timber raised bed required to grow certain vegetables. We design, manufacture and supply timber raised beds in allotment, standard and superior versions of timber raised beds and although the size of the superior timber raised bed planks is larger than the other two (creating as deeper bed), the information below regarding tiers is still relevant!    

Firstly, I’d refer you to our Raised Bed Information Pages for details of the number of tiers (and depth of soil required) to grow common garden vegetable varieties, including most of the plants you wish to produce. You can find the Planting Guide on any of our raised bed product pages.

Back to your query and the answer to the questions not provided in the guide are below;

1. Leeks would benefit from a two tier deep bed along with parsnips, carrots and peas. To produce the blanched white stems, leeks need earthing up or planting at least 15cm deep so 2 tiers (30cm) will be ideal.

2. A folding willow obelisk may be a tight fit in a 1.2m square raised bed. I’d probably suggest the willow obelisks or the Maypole Climbing Frame for supporting your runner beans. A single tier bed will be deep enough but as beans are both greedy and thirsty plants make sure the soil contains plenty of organic matter. 
   
3. Rhubarb crowns are planted near the soil surface so a single tier (15cm) deep bed would fit the bill nicely. Just make sure you have enough room in a 0.6m square bed for a forcer if you want to produce earlier stems. 

4. Again, a single tier bed provides enough soil for the salads, tomatoes, courgettes and beetroot you intend to grow.

It’s worth remembering however that all crops can grow in a two tiered bed but carrots and other root crops will struggle in a single tier version, so we always advise you to build two tiers or more if possible. This gives you greater flexibility with planning where your crops can grow and, more importantly, with crop rotation.

I hope this information is of help; many thanks once again for your enquiry and the best of luck with your raised bed project.     

Martin