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I've just finished raking, shredding and bagging all the leaves (including some moss, pine needles and cones) from last fall (and few years before). All this material inside bags is wet/moist, bags are closed and I've made few holes on the bottom and sides of bags.

But there is small 'problem' - I now have 23 × 240 liter plastic black bags laying around and my compost pile is already full...

Is there a way to speed up composting inside bags so I can have compost ready before next autumn?

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Would have been better if you had them bagged up actually in autumn, but you've already done everything you can to speed up the process of making leafmould compost by shredding the contents first; what you describe doing is perfect, with the possible exception of the pine needles and cones, depending on the ratio of those to leaves (though I'm guessing they were mixed in with the leaves when you collected them). Further confirmation/information that what you've done with your leaves is absolutely appropriate here https://www.rhs.org.uk/advice/profile?PID=478

Where you live, I think you get hot summers, so that should speed the process up too - but the pine needles and cones won't rot down like the leaves will, you might need to extract any much larger pieces of the cones when the leaves have rotted down to black soil like stuff. You need not worry about the pine needles, unless there is an excessive amount left in the mix at the end, when it's ready for use, but they're not a massive problem.

I used to stack mine as far as possible once the bulk in the bags had started to shrink down if there were a lot of bags - that seemed to speed the process up too, though maybe we just happened to have good summers those years. If you do have a good, hot summer, with luck, it should be ready for use next autumn - here in the UK, with our variable summers it takes 1-2 years, but that's using unshredded leaves which do take a bit longer.

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    It seems leafmould is formed by the action of fungi (mostly yeasts?) rather than bacteria. That's much slower, but it means you don't need to worry about turning it (it's anaerobic), or maintaining the temperature of the heap (fungi are happy at low temperatures). – Tom Anderson Jan 10 '18 at 11:34
  • Agree wholeheartedly on both counts! I shall add the RHS link – Bamboo Jan 10 '18 at 11:49
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    @FalseIdentity I doubt the yeast we use for baking and brewing is the right yeast for breaking down leaves, sadly - they evolved to eat ripe fruit. Wikipedia links to this paper about the fungi which break down leaves on the forest floor (which is the process leaf mould is exploiting), which says that most of the work is done by basidiomycota. Domesticated yeast is an ascomycete, so not even the right phylum! – Tom Anderson Jan 11 '18 at 16:03
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    @FalseIdentity from the question, it seems you also collected leaves which had been laying around from previous year/s - in which case, yeast will already be present anyway – Bamboo Jan 11 '18 at 16:55
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The enemy of composting in winter is the cold temperature. Composting requires the correct temperature to allow the progression of bacteriae (psychrophilic, mesophilic, and then thermophilic) to appear and decompose the materials.

A minimum size for a compost pile to work correctly is 1 cubic meter to allow sufficient mass to allow the heat to not be lost by dissipation. In winter you'll need to compost inside an insulated container. Since this is presumably going to be a recurring problem for you, perhaps you might want to build an enclosure out of cinder blocks, and also use a tarp to keep the snow off the pile.

cinder block pile

Since your base product is mainly dried leaves which is considered a brown or mainly carbon source, you'll need to add nitrogen. High nitrogen sources include fertilisers for lawns, and urine. Once the composting is finished, you could pull the cinder blocks down in summer to regain the space.

Winter composting guide

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Unless you poke tons of holes in those bags, you will not get aerobic decomposition. I would simply make another area for this stuff, spread it out in layers, add nitrogen, keep layering. In a few days take a pitch fork and turn it all. Get air in that mess.

This should easily make some nice compost by spring but you have to dump it out in an area with some elbow room. Nitrogen will energize the decomposers. That is what they eat for energy to do their very important job. Leaving that stuff in the bags will mean you are doing anaerobic decomposition which is very stinky, slimy...ugh. That debris might just be liquid slime by the spring.

Do you have anymore green adds for your compost? You've got mostly brown. Dumping kitchen scraps would help but then you might have a few critters. Best would be grass clippings but I am thinking you are done with that for awhile?

I haven't been able to find this kitty litter made from alfalfa in the stores but I don't go shopping much. Pure nitrogen and green addition to get your compost cooking. Makes adding nitrogen to your compost easy peasy.

Dump it out, mix it, rake it again into a pile, make sure you've added nitrogen and cover it loosely with a tarp so it doesn't blow back into your yard. Once a week go out and turn it, add moisture if it is dry and cover it back up. Loosely to allow air yet hold down the leaves from wind. Leaving it in the bags

great article on leaf compost

another article on leaf composting

anaerobic versus aerobic decomposition

  • Keeta, thank you very much! I check these things closely, I thought. Nice to know someone was reading them!! – stormy Jan 11 '18 at 2:35
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Composting inside the bags with the materials you've got will be a lot of work. Below I'll describe some optimal conditions for composting and how your bag compare to them and some solutions.

  1. Airflow, to avoid anaerobic breakdown of the material.
  2. Mixture of carbon and nitrogen but you've mostly only got a source of carbon.
  3. Heat, which will have a hard time developing in a typical trash bag. One cubic meter of volume is considered to be the smallest optimal size. This problem is made worse in winter.
  4. Microbial life, nematodes, and arthropods which often arrive from the pile being in contact with existing soil.
  5. Moisture to make the pile feel like a wrung-out sponge. It will be hard to monitor and add moisture in the bags.

Points 1 and 2 are required conditions for the composting process. The inability to build up heat will dramatically slow the process. Your bags probably have some microbial life that was on the organic material when you put it in, so point 4 might be OK.

So, what can you do?

  1. As others have pointed out, putting holes in the bags or transferring the material to a different style of container or pile.
  2. You'll need to add a high-nitrogen source to the bags and mix them up again. Coffee grounds or chicken manure are good low cost sources of nitrogen. You must add nitrogen and mix it in even if you combine your bags into a new pile
  3. I'm experimenting with a shredded leaf and coffee ground compost in a 70 liter plastic open topped bag in an unheated area that is a warm microclimate: an ikea bag in my garage. This experiment seems to be working as the pile is getting hot and breaking down. My winter climate seems to be similar to yours, so maybe you can find a place that is warm to stack the bags to offset this problem. Or you can combine your bags into 1 or more piles.
  4. Putting a handful of soil that is rich in microbial life should help. You could empty the bags into a compost pile outdoors on top of the soil.
  5. You'll just have to monitor the bags and periodically add water which will be tedious to do to each bag. The alternative, again, is to combine them into a single pile that you will still have to monitor and potentially amend with water but it will be easier to do.

Is it possible to compost in the bags you've got? For sure. The addition of nitrogen seems like a critical step, the rest of the elements for composting exist but it may not happen fast enough for you.

Two other possible uses for this material:

  • Shredded leaves can make a good top dressing for garden beds to keep moisture in the soil and reduce weed pressure by crowding out the sun. If you keep the leaves dry until you are planting you could use some of the material for that purpose.
  • Most people who compost have an abundance of carbon in the fall time (leaves) and a steady stream of nitrogen through the rest of the year (kitchen scraps, lawn clippings). If that matches your scenario then having some carbon material available to mix into the pile as you add nitrogens throughout the year will be helpful. If you want to do this: keep the leaf piles as dry as you can so they will be ready to add to the pile in the spring/summer next year.
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So, I had a similar issue with too many leaves and not enough compost-bin space for years. I have since moved and have even less space, but using some helpful strategies in addition to the excellent advice given above you could probably use up more of your backlog of leaves.

  • Build a special compost pile for all the extra leaves is mentioned above. I always found the problem was it filled up too quickly and I still have four times as many leave as fit into it. This was a problem until I took a 4ft piece of 4x4 and put a gate handle on one side and used it as a ram to smash leave into the pile. This compressed the pile and I could get three to four time the number of leaves into the pile every year. Plus I used the pile as a carbon source for my primary compost bins, so over a year, I used up more of the material.

  • Actually, sift your compost and use it up by dressing your beds in the spring and the fall. In the spring once the compost pile is thawed I sift it and age it in 55-gallon trash cans until dressing the beds once the perennials are coming up.

  • Run an additional compost pit under the vegetable and annual planting beds. Basically just bury the problem, and it will go away. I dig out 1 to 2 feet of a plant bed dump six inched or more of leaves into the hold along with sticks and logs, and fill it back in. It will decompose in 18 months and supply nutrient to deep root for 4 or 5 years. Trust me this will not cause any problems, fungus, worms and other critters will dispose of this fast.

  • Use the leaves for mulch on top of the compost you just laid down. The way my grandmother used up leaf mulch was interesting and counter to the way we are told to do things now. Under all mulch, bark or leaf, put 1/2" to 1" inch of compost minimum. This will help the 3 to 4 inches of mulch above to decay over the next two years. I volunteer in a professional garden and using this mulch and compost method works great.

  • Plant a green compost cover crop on a bed for the winter or spring and when you dig it in add 6 inches of leaf mold or leaves to help it rot. You said you already shredded the leaves so digging in this much carbon with a green compost would work out great since the compost is heavy on nitrogen. You might want to use a test bed until you figure out the method that works with your soil and cover crop.

  • Use your shredded leaves instead of peat moss, there is a trend in gardening to use local renewable resources as soil amendments, instead of peat moss.

Hope this helps, remember to mimic things that happen in nature.

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