Neil's Recipe for a Hotbox - Brilliant for rooftop gardens

The trick in hot-box composting is to first generate the heat and then retain it.


If the temperature of the material can be raised high enough (say 55-60 deg C) then the material will start to break down and weeds and weed seeds will be killed. At 55 deg the rate of breakdown can be such that the volume of the material can be reduced by 50% in 1 week. This is a useful to avoid huge piles of partially composted material building up over the year. Once the hot-box has stopped 'cooking' the material can be removed and used as a mulch or stacked for later use in a normal compost heap. The process is a 'batch' process with the box being filled and emptied about once/week.


Any pile of fresh grass cuttings will get warm after a few hours but it is only the interior that gets slightly warm -the outside stays cold due to the heat loss and the generation of heat stops due to a lack of oxygen in the pile of material as it compresses down.


In hot box composting the heat is retained by placing the material in an insulated box - these can be made from whatever is available -in the Fairlie garden these are wooden boxes built from scrap wood and plywood with a 20mm (approx) layer of expanded polystyrene insulation between two layers of wood. (The inside layer of wood can be dispensed with if necessary however the insulation will eventually get damged and crumble)


The box should have a sufficient volume to ensure there is enough material to generate the heat. The wooden hot boxes in the Fairlie garden are approx 600mm x 600mm x600mm. The boxes also have lids and bases lined with the insulating material and a lid to keep out the rain.


An alternative (and much cheaper!) hot box can be made by obtaining 3 polystyrene fish boxes (with lids) from your local fishmonger/supermarket - These boxes are approximately 800mmx 400mm x25mm and are normally 'dumped'. Knock/cut the bases out of 2 of the boxes and stack them (the boxes are moulded to stack on each other) so that you have a larger box the height of 3 of the individual boxes - this is to ensure that you have sufficient volume of material. Tape the 3 boxes together with duct tape. Retain the lids (which are usually slightly thinner material and cut 2 of them down so that they can slide down inside your pile of boxes. The third lid is fitted as normal. This gives a better insulated lid which can slide down as the volume decreases (a stone will help!) and an insulated rain-proof lid.


The wooden boxes can be painted as required and the fish-boxes can be painted with masonry paint to allow them to 'blend in'. The wooden boxes will last quite a while but the fish boxes will eventually crumble and break but they are free and easily replaced and are ideal to try out the technique for little or no cost.


The mix of material that is placed in the hot box is fairly critical. FRESH grass cuttings are the key with other garden composting material added (about 50% by volume) to allow air to be retained instead of it all packing down - the pile of material should be slightly damp but not wet (the natural dampness of the grass-cuttings is usually sufficient) and full of air which is required for the process. The material should be turned over to aerate it about once/day - this will let some heat out but once it really starts to cook it will soon heat up again. Some experimentation with the proportions of the mix may be required.


A small insulated box (about 400mm x 400mm x 400mm) at our demo garden at the Scottish Garden Show was started on a Wednesday evening for the show and by Saturday morning had reached 65 deg F and halved in material volume. It was so hot you could not place your hand inside....

Neil's Recipe for a Wormery

Materials for a wormery


Basically a wormery is a wooden box with a lid to keep out the rain wtih drain holes in the base. No rocket science here!


The best material to use to build a wormery is scrap wood e.g pallets. (make sure the wood hasn't been treated though, as this can damage the worms). Plastic boxes can be used as in commercially available wormeries but these suffer from condensation and it is very easy to end up with the worms 'flooded-out'.


Tiger/Brandling/litter worms can be purchased or found in a mature compost heap or in another worm bin.


Making Your Wormery


Make the box about 2ft x 1 ft with a depth of about 18 inches (not critical!) Make a lid to cover the box or just use a plastic sheet (old fertiliser bag?) weighted down with wood to keep out the rain.


Make the base with with wire mesh or if it is made out of wood bore plenty of drainage holes around the bottom. The worms won't leave as long as there is food available. This gives drainage if required but keeps out moles etc.


Fill the box with 8-10cm layer of coarse sand or gravel and on top of this place a layer of moistened bedding material, such as mature compost, manure or leaf mould.


Put at least 100 worms into the bedding and add a litre of chopped food to one side of the bin. Cover the bedding and food with a well soaked newspaper. Put the lid back on and leave for at least two weeks without disturbing it, to allow the worms time to settle.


Caring for your wormery


The worms main source of food should be kitchen scraps and waste, although this should be combined occasionally with garden waste and weeds to vary their diet. Avoid feeding them meat, citrus peel or fish.


Once the worms have settled-in feed them by making a small 1 inch deep trench in the bedding and bury the kitchen waste.


You can kill worms by overfeeding them, so don't feed them until the previous batch of food has decomposed. Keep the wormery covered to keep out fruit flies and never allow it to dry out or get too hot.


You may produce more kitchen waste (at least to start with) than the worms can consume - just compost the excess waset as normal. Once they are thriving if you still have too much kitchen waste then make a bigger/another box and transfer you workers.


If your wormery begins to really smell, then it is probably too wet. and you should mix-in some brown (dry) material - dry leaves/straw.


If your wormery seems too dry mix-in some green material (wet grass cuttings). As a guide you should just be able to squeeze 3-4 drops of water out of a handful of the material.


Gradually the box will start to fill up - once it is full (6 months? depending on box size and how active the worms are then the top layer (6 inches?) will contain all the worms and uneaten food and the bottom will contain the worm compost. Scrape off the top layer into another container and harvest your worm compost. Add new bedding material to the wormery and put the old top layer (with all your workers) into the wormery and start again.


This worm compost can be sprinkled on beds and raked-in or mixed with other soil (max 10% by volume). Any excess compost that is produced can be put into sacks in a cool dry place and be stored ready for use.