From flat to raised trench beds

One of the most noticeable changes in the kitchen garden over the last decade or so is the advent of raised bed systems. Whereas in the past the vast majority of all vegetable plots were very similarly run; on the flat and managed by an annual dig and a path and row system now there are a host of approaches and methods; many not on the flat and often without any annual dig. I somtimes wonder whether we're following reason and logic -or maybe it's just a fashion?

Now I must admit to having a vested interest as I've used raised beds in most of my gardens and naturally I'd be reluctant to think I might have made a mistake and wasted a lot of effort or damaged yields inadvertently with poor methods. However in a spirit of enquiry over the last few years I have been experimenting and modifying and have now 'progressed' (well so I believe) to what can ungainly be called a rotated raised trench bed system.

Originally I read extensively and tried many possibilities, I have grown on the flat, angled to the sun, on ridges, in trenches, in beds and even once upon a time traditionally and by rows. I grew in raised beds with wood sides, plastic foam, slabs, tyres and even in cold frames made from recycled pinball bodies. In every case it becomes apparent that soil drainage, aeration and temperature are just as crucial to success as good watering and fertility. If our plant's roots are comfortable then they do well, if not they do badly.

I used raised beds in my previous garden where I'd had a very thin layer of scrappy soil over chalk -a situation where few other approaches than container growing or raised beds could work. By having the paths dug out and all available soil mounded up a sufficient depth could be obtained. There I learnt that using wooden or any other edging to keep up the sides was counter productive. Although the edging helped conserve the soil and moisture, and mulches stay put better on the top than on any slope or side, still the cost and pests taking refuge soon ruled replacing the rotting edging out. Of course if you need seriously raised beds of a couple of foot or more then edging is another matter but for economy and hygiene bare soil beds and paths with sloped sides to the beds works well, and cheaply.

I chose, when I came here over twenty years ago, to again set up a raised bed system. The primary reason here was winter flooding; I garden an old river bed at the 'bottom' of a long gentle slope (it is Norfolk after all). The area is prone to flooding most winters and I chose that part of the garden for the vegetable beds as by raising them this flooding would do least harm. This has made that area productive and the excavated paths serve in place of drainage pipes though necessitating wellies to harvest winter crops.

Of course I was swayed by all the other arguments. (See Raised bed methods) The raising does mean hand work is a little closer, and this helps the eyes as well as the back! Raised beds certainly dry out and warm up more quickly than beds on the flat meaning earlier crops are achievable. Aeration is improved as the greater surface of soil exposed should allow better air penetration. And when water was cheap and freely available I flooded the paths to soak the beds sideways and underneath. This is far better than sprinkling as it kept the surface dry discouraging weeds, pests and evaporation, it gave fantastic yields but used inordinate amounts of tap water. Now I use only rain water and I have to question the sanity of having raised beds on a sandy soil in the driest village in the UK.

For indeed raised beds do dry out and warm up more quickly than the flat. And keep on doing so. With very little summer rainfall (plenty of grey days and damp but little actual precipitation most years) raised beds can soon become as dry as dust. I have tried various ways such as different spacing, block planting and many sorts of mulches to conserve moisture and maintain the crops. I had great success with thick straw (see Straw) but as it became hard to find in small bales I was forced to give it up. Thick mulches such as straw worked well at conserving the moisture but they did keep the soil cooler which made for later crops and did not suit some such as sweet corn. Thin mulches are ineffective and all of them are hard to use on the sides. Plastic and woven sheet mulches are useful for a few crops especially for preheating the soil before French beans and melons but cost rules them out from widespread use and the plastic ones soon make the beds even drier.

Now I only use water when sowing or transplanting and try to do with out entirely where possible which works well for most roots, brassicas and even peas and beans. However although most crops succeeded it was noticeable that some such as potatoes become noticeably low yielding when grown in very dry conditions (though they keep well) and leeks and celery simply expired, if I was lucky they bolted first. The combination of very light soil, warm but dry conditions and low rainfall was fatal to these in particular. And I'll say nought about spinach....

So I wanted to grow these, and celeriac, turnips and a few others, a lot better, well at all in some cases. Simply adding water to their beds was tried but failed- they still did not prosper as the water seemed to disapear too quickly, just sucked away. Now I used to have a raised and angled to the sun salad bed which had a trough at the bottom end and this had grown these difficult crops well. Traditionally celery was grown in a trench but 'modern' practice and varieties now supposedly needed otherwise, but they had failed on the flat. So I decided to go for trough cultivation again, but the original spot was now taken by another feature. I could not use the dug out paths on the raised beds as the crop would be in the way -and would drown in winter. But I could put a trough in a raised bed. It would have to be in the drier end ones of my forty beds as otherwise the crops would still not stand out of the water. But it could be done.

My beds are roughly four to five feet wide and fifteen sixteen feet long with paths nominally a foot wide between and two foot wide at the ends. They normally stand proud of the path by about a foot. I excavated a trench roughly one third the width of the bed and used the spoil to further raise the sides and ends of the bed. The spoil was all top soil as I only needed to dig out less than a foot to make a foot deep trench so it was no problem spread about. However the subsoil, at the level of the path and below was not so good so this was broken up and well enriched with compost.

Naturally it was easy to keep such a trench well fed and watered and the extra cool damp and wind shelter really helped all the crops I'd intended it to benefit. I've no complaints and can only recommend you do the same if you have the same dry light conditions to contend with. But I found there were other crops benefiting I'd not expected. I've often gone on about the benefits of micro climate and soil aeration. These all work on every scale and I found not only did the intended crops do well down in the trough but others were doing better up on the ridges. (And it wasn't just the extra water as I tried comparisons.) French beans, Swiss chard and beetroot in particular did extremely well and were noticeably earlier and germinated more successfully on ridges than others grown on the flat nearby.

Well ridges are known to warm well, so I tried sowing sweet corn along the ridges. This did really well, and on ridges around an unwatered trench! It emerged a week or more earlier on ridges and made bigger plants, what is more, more emerged successfully from ridges than from the flat. Sweet corn did not shade out the celery or celeriac grown in their trench or even seem to affect them much but killed them rapidly where the trench was not watered. (Leeks in a trench however needed full sun to themselves.) Another good combination is melons or ridge cucumbers in the trench with the sweet corn around. Glass or plastic sheet turns the trench into a long coldframe and gets them off to a flying start and makes the roots of the sweet corn even warmer.

To fit in with a rotation it seems sensible to use the trench for a crop of potatoes the following year -after all it is already dug. The soil is warmer than you'd expect as being in the bottom of the trench it is sheltered yet exposed to the sun all winter and spring. I plant the sets just under the surface and as the sprouts emerge fill in the trench with the soil and weedy flushes from the beds and paths around and finally I earth up with grass clippings. After the spuds are dug out the bed is easily remade level again for the next crops. To follow the spuds the loosened soil suits first the legumes and after them the brassicas which I lime for well before at the same time as levelling after spuds. After the brassicas are gone the soil is firmly consolidated and ready for onions and roots just as with a standard rotation.

The same ex celery or leek trench plus a covering or better still a row of cloches is also a good way of getting a huge crop of early potatoes. By only partly filling initially they can be got going earlier than on the flat and the cloches give them enough headroom to fully mature without ever needing the cloches removing. They can be started off as early as February if the cloches are given a blanket on frosty nights.

Although I've never done a regular autumn dig this trenching now turns over at least a third of each bed very effectively every fifth year or so. And I have decent celery, celeriac and leeks at last, and sweet corn weeks earlier.