Last year I made a smallish bed raised high enough to grow carrots without having to protect against carrot fly – which doesn’t fly above around 2ft 6in, 3ft, or a metre, depending on which source you read. To be on the safe side I made my bed over a metre high. Also, because I felt that an awful lots of good organic matter was being buried deep in the base of this high box and going to waste, I created it as a tiered bed – essentially the high-rise carrot bed was a box set in the middle of a more conventionally sized raised bed. In this tier around the outside of the carrot box, I grew strawberries.
This two-tiered bed was a great success with both excellent strawberries and carrots – and then after the carrots were harvested, I transplanted radicchio and chicory seedlings to mature overwinter. It turned out that there were other advantages I hadn’t foreseen in having a raised bed inside a raised bed, and fired up by this, I made a three-tiered bed to go in a part-sunny, part-shady area in my garden this year.
Why make a tiered bed as opposed to a simple raised bed?
1. Carrots. The original tiered bed was designed to be high enough to grow carrots without them being troubled by carrot fly. Had I simply grown carrots in a 1m high box, a lot of soil would have been wasted filling the bed and it would be difficult to reach the middle of it to plants, weed and harvest.
2. Better access to light. The higher tiers get more sun. In my garden the back of the bed would be a damp and murky place were it not raised up to catch more sun.
3. Crop variety. It’s easier to adjust the soil to suit the crop in each tier. With carrots again, the soil in the top tier where the carrots are grown can have sand added to it which doesn’t affect the soil in the lower tiers which may have bonemeal added for strawberries, or manure for Chinese broccoli.
4. Crop rotation. Following on from variety above, crops can be ‘micro-rotated’ within the tiered bed with ease.
5. Warmth. The raised sides of each tier can reflect warmth on to the growing crops. This can be accentuated by painting the sides white, or even coating with silver foil.
6. Weed-free. Maybe this is down more to ground preparation but I find the tiered bed is to all intents and purposes weed-free, more so even than a conventional raised bed.
7. Aesthetically pleasing. A tiered bed does look smart, even a bit of a talking point. And in my narrow garden, the tiered bed gives the impression of width. I’m sure an artist could explain why.
Now, out of sheer laziness, I find myself beyond the point of no return in building a massive three-tiered bed on the allotment. I measured up a corner of the bed just under the asparagus where the slope makes it shady and prone to poor drainage – both problems which will be nicely alleviated by a raised bed – and then decided I couldn’t be bothered to cut all the gravel boards to fit, so might as well just use 2.4m boards for the base uncut. That base has now gone down and it’s HUGE. This year as well as carrots in the top, I’ll plant out the courgettes in the lower tiers – I can fill these with lot of kitchen compost which the courgettes like.