Friday 15 January 2021

Extend the growing season with a hotbed

Other people's gardens are always full of good ideas and inspiration, and few more so than Barnsdale, the garden of the late garden writer and presenter Geoff Hamilton. This 8-acre site is home to Hamilton's extensive kitchen garden and around 38 demo gardens or garden rooms. On my last visit I was particularly taken with the hotbeds in the kitchen garden.

I've been interested in hotbeds since reading of the Victorians' enterprise in building them to grow tropical fruit such as pineapples. But Barnsdale also demonstrates something else: the use of hotbeds to extend the growing season, getting seedlings off to an early start; as a nutrient-rich environment for hungry crops (courgettes, squash, beans, cucumbers) later in the summer, and then as a generator of well-rotted manure for the following seasons.

I've recently embraced the no-dig system of vegetable gardening with all the zeal of the convert, which means I need more and more compost, or manure, or organic matter generally, to cover new beds and top up existing ones. Buying it in each time would become very expensive very quickly. The obvious answer is to generate one's own. The vegetable compost heap is a rich source of - free - organic matter, and a hotbed adds an efficient - and free, again - source of manure.

The base of the bed is a four-sided box - I made my first one from leftover composite decking and a second one from gravel boards. You can then place a cold frame on top - again, either one you have already, or a DIY job. My DIY cold frame was made with more decking - softwood, this time - with the side boards cut diagonally lengthways to create a sloping lid. You can just see this in the picture, and the cold frame is positioned so that the angled top faces south to catch the best of the sun. I saved our two big double-glazed bathroom windows from our house renovation and these make a well-insulated glass lid. (Although they are rather heavy.)  

Position the cold frame on the base so that the glass cover faces south, if possible, to get as much light and warmth as possible in colder months. 
Hotbed number two, made with deckboards and lined with black plastic to help with heat retention.

The base is then filled with fresh - not rotted - manure. You may be able to get a local farmer or stables to deliver this, but for me it's a visit to the local stables to load the car up with bags full of fresh manure from the muckheap. Again, it's free, which is largely the point of doing it this way, but I do have to factor in the cost of having the car valeted afterwards. The smell lingers on a bit.

Once the base box is full of fresh manure and you've tamped it down to ensure the box is properly full, put the cold frame on top and fill with compost to a depth of around 5cm. Then fit the lid over the top and leave the manure for a week or so to settle. The contents will heat up under the glass and begin to rot down. You will probably find it sinks quite a bit in just a fortnight.

Top up with more compost to about 10cm depth, and from mid_January you could be ready to start sowing seeds for an early harvest. Sow thinly directly into the hotbed compost and keep the glass lid  on until they have germinated. Depending on the weather, I will start lifting the lid to ventilate the bed once the true leaves are showing, but will replace it at night and if temperatures drop.

I've sown salad leaves, lettuce, rocket, lamb's lettuces, radishes and Chinese broccoli in the hotbed in the third week of January and been eating fresh salads 4-6 weeks later. The beauty of the cut-and-come-again salad leaves is that you can keep cropping them all the way through until the summer.

Once the salad leaves begin to bolt in midsummer, you could re-sow, but instead I pop in a couple of courgette plants and watch them romp away as the roots reach the manure gently decomposing under the thick compost layer. Two courgette plants will grow to fill a 100cm x 75cm hot bed and provide plenty of courgettes throughout the summer and autumn.

 Other greedy crops such as squash, cucumber and beans could also go in the bed in summer. 

In autumn, once the courgettes have gone over, lift out the plants and chuck them on the compost heap. In the hotbed you should now find a thick layer of fabulous well-rotted manure ready to be dug out and used on the next season's growing beds.

And once the hotbed is empty, you can start all over again with a visit to the local stables ...