A hot box is a propagation box, which makes heat without the use of electric.
Many a keen gardener will be like me at the moment…Every windowsill and flat surface in the house is full of tender seedling. Tomatoes, peppers, aubergines, and the first courgettes. But now things need potting on, out of their little modules into bigger pots. Time to move them on and out of the house. Although the days are toasty, the nights are freezing, far too cold for all these tender plants.
Time to build a hot box…
I normally do all my seed germination in my hot box, not the house. This year, with the corona virus shut down in full swing it has been difficult to get muck. So a later start than normal. This week i took delivery of a trailer full of steaming fresh muck.
I have a lovely solid wooden raised bed in my greenhouse, that i built a couple of years ago. Previously i have made the box out of old hay bails, roped together. You can use what ever you have. You then fill this with fresh muck, woodchip or compostables; good and stinky. As this breaks down and composts, it heats up and keeps your greenhouse warm.
Once the box is full of muck i walk on it to press it down to level the surface. Fresh muck can be rather lumpy with clumps of straw, so I then top it with something finer to give a level surface. You can use woodchip, sawdust, sand or compost. This year i topped mine with a few bags of old compost i had left over from last year.
You can then start using the hot box as it is. I cover mine with some woven geotextil fabric (mypex), to keep everything clean. Then i put a layer of capillary matting to prevent seed trays from drying out too quick.
Already it will be heating up, great for germinating seeds. All that is needed now is a lid, to retain the heat.
I will eventually get around to making a tidy wooden frame hinged for ease. In the meantime, an easy quick option is a mini polytunnel type structure. Alcathene pipe is cut to length and used as hoops (you can often get lengths of this for free, as off cuts). This is then covered with plastic (of cuts from our new polytunel) This can be pulled up to inspect plants and water. It can be left open on hot days to reduce temperature and vent the space.