A question that is always asked here, especially from new prospective raised bed gardeners and that is, how deep should raised beds be ? The answers again are varied and will depend on a number of factors. A lot will depend on what we are growing, what quality is the soil, what is the prevailing climate like, is the soil arable and workable and what depth are the raised bed frames that we are working with. In essence the rule of thumb is, the deeper the better. In case that there is some confusion, a similar question that is asked in a roundabout way, and that is, what height should my raised beds be ?
The height or depth issues that we attempt to unravel today will be teased out and hopefully some clarity will emerge by the end of this post. Lets start at the beginning and work our way through the different scenarios and tackle each issue seperately. A good rule of thumb is that we leave the soil level at least 2-3″ below the top of the timber frames sides. This is because we may need to add some bark mulch at a later date and if we leave some room for this, we will ensure that the mulch don’t fall out over the sides. Birds may also scatter this mulch about, so we need to take this into consideration.
What are we growing;
This is the most important of all the questions that is asked on a regular basis and politely I would suggest in most cases, its entirely up to the person themselves. They would work with what they have at their disposal, in terms of a raised bed frame and also work within their budget. So for beginners, and for the purpose of this exercise, we will work our way through the different and most popular vegetables and cover the soil depth requirements for each briefly. A standard raised bed frame measuring 4ft x 8ft and say that the sides are 8″ tall, then that is a great place to start with vegetables that will comfortably grow in this space and depth. Some of these would be, Cabbages, Lettuce, Strawberries, Onions, Leeks, and most of the common herbs like chives, spinach, celery, mint and so on. All or most herbs and salads would only need a few inches of soil to flourish and are really easy to start with. So in terms of work preperation, you can see very little is required for these varieties.
Deeper rooted vegetables:
Moving on to the next category of deeper vegetable requirements and we will firstly look at how to have a better soil depth and how that is arrived at. If needs allow, a taller sided raised bed frame would accomadate these varieties and would require a little more prep work and planning. Again the soil is the common denominator and this is crucial for the successful growth of these veggies. The depth of soil we would be looking for would be in the region of 10 – 16″ at a minimum. The deep rooted vegs that we would be planting in this example are, Potatoes, Peas, Beans, Peppers, Carrots, Parsnips,Kale, Swedes and Squash and others in that vein. It’s easy to understand the soil requirements if we have been gardening prior to now and for the beginner or novice gardener, we will learn as we go forward. Experience will come with practice and for other reassuring measures, the growing instructions come with the veg. packaging. There are gardening experts at all the nurseries and garden centres, who will offer the best of advise at all times.
Other deep rooting variety requirements;
There are a few species that require a greater depth of soil, such as, Tomatoes, Asparagus, Artichokes, Pumpkins, Watermelons and when we name these few, its easy see why these need that extra growing room, ie, when we see the fruits that these produce, its easy to understand their needs in this terms of garden room and space. How do we arrive at depths to accomadate these few vegetables that require a greater depth that the others ? Answer , double digging or really high sided raised beds. What is double digging, I hear you ask ! This is where we dig the soil within the bed frame area before we actually locate and assemble the raised bed. We would dig the soil to a depth of 12-16″ and then add some topsoil when the bed is in place and this would give us the deep soil measurements that we would need for these special varieties. A little more preparation than normal and a little extra hard work in the beginning, but the pay-off and rewards come later.
Basic soil preparation;
The first layer of soil, say to a depth of 6″ is where most of the vegetables get their nutrients and growing requirements, so this is the most critical part of a raised bed or garden. Having this area well cultivated and fertilized, will pay us back by the bucket load. Ensuring that the soil is well tilled, aerated and also that it has good drainage will return high quality and good quantities of harvest. The beauty of raised beds is that we can improve the texture and quality of the soil at all times. Every year before planting, its a case of adding some good quality compost, farmyard manure or well rotted leaves or hedge trimmings. Just fork in and mix with the existing soil and we are good to go. Better still, if we could add this mixture just after harvesting and leave over the winter months to break down and integrate with the existing soil, that would be ideal.
If by chance that we would like to plant some of the deep rooted and short rooted vegetables in the same bed, a good tip is to plant the tall growing veggies towards the centre of the bed and the shorter ones nearer the sides or outside. This will ensure that the tall plants will not block out the sun or smother the smaller varieties.
I hope that this post will answer some of your questions and queries and if not, please use the comments box down under and we will reply to all your issues in due course.
Happy gardening to all who try and succeed,
Cheers Phil Browne.
” There can be no other occupation
like gardening in which,
if you were to creep up
behind someone at their work,
you would find them smiling “.