When constructing a new compost pile we as gardeners can be very anxious for it’s temperature to rise and for the pile to get hot. And, if you are new to composting you may be wondering just how long you need to wait for a compost pile to get hot. Well, in this article I will cover some of the principles of making a hot compost pile and show a few examples of piles I’ve constructed, their size, and how long it took to get hot.
In short, a properly balanced moist compost pile that is roughly 50/50 green to brown material (by weight or volume), and is at least 3-4′ diameter and tall should reach temperatures above 120F within 7 days during the Spring, Summer, or Fall seasons.
But overall the time it takes to get hot will depend on four primary factors. Let’s explore those in detail, and then we can look at a real-life case study in my backyard where I will list ingredients/quantities, and document the temperature rise versus time.
Factors that influence compost temperature/speed
When it comes to generating heat in a compost pile, there are several factors that will influence how hot a pile can get. By controlling these factors, you can guarantee a successful hot pile.
Having a pile that is balanced between green and brown materials is key to making a hot pile. You can balance it by going 50/50 to 30/70 green to brown, and still have a good compost pile. However, if you want a hot pile, keep the ratio closer to 50/50 by weight or volume. For a list of possible materials that perhaps you hadn’t considered, see here.
But a balanced pile should have a carbon to nitrogen ratio (C:N) of approximately 25.  If this ratio goes higher, the temperature will rise more slowly, whereas if it goes lower it will get hotter more quickly. But, as I always say, keep it simple. Focus on a well mixed and balanced compost pile based on whatever green/brown ingredients you can. But, if you are interested in a few particularly potent green materials, read on.
Types of green ingredients
One other item I need to say on the green ingredients. Not all ‘green’ ingredients are created equal! Some have very low C:N ratios while others are much higher. For a few examples of very potent green ingredients that are often available:
- Grass Clippings – Grass clippings are often available to homeowners. And a few lawn bags of grass clippings as a primary green ingredient can get a very hot pile in less than 24 hours. But, with grass clippings you really need to mix it well, and keep mixing it to prevent it from ‘matting up’. Personally, when I use grass clippings I mix it every 24-48 hours to prevent the ‘matting up’ (which will eventually turn into sludge).
- Manure – various forms of manure are available. And they are all potent sources of green material, with chicken manure being one of the hottest. But, you need to make sure you either get your pile above various temperatures for a certain time to kill any pathogens, or apply your finished compost well in advance of harvesting any crops. See the USDA website for specific guidelines on this.
- Shrimp Shells – shrimp shells have a very low C:N ratio of 8:1, making them a material that will get decompose very quickly and give off plenty of heat. So, if you are at a shrimp boil or at a large gathering where shrimp are served….you may want to ask if you can have the shells (I have).
- Green yard waste – fresh green yard waste of any kind, weeds, leaves, unwanted shrubs or flowers will all break down very quickly similar to grass clippings. But if they have seed heads make sure you get the pile hot enough to kill off any weed seeds.
Size of the pile
The size of the pile is the single most important factor after having balanced ingredients. The larger the pile, the faster it will heat up. The reason for this is that the bacteria and microbes breaking down the material generate heat. And that heat is slowly lost to the environment. But a larger pile will have a smaller surface area to volume ratio, meaning we can have more heat-generating microbes working for a given area, slowing the loss of heat.
A balanced pile that is very large will have all the bacteria generating heat throughout the pile, and as we get closer to the center of the pile, the temperature will reach it’s maximum. While at the outer layers of the pile, the temperature will be closer to ambient. The thicker the pile, the better insulated it is from the outside temperature.
So, when we make our piles bigger, we are basically allowing the microbes in the center of the pile to get even hotter, which in-turn activates even more microbes to generate more heat. It’s a great feedback loop.
The location where we place our compost pile or tumbler will have an influence on how hot it can get. If we place it in a location with full sun, we will be able to take advantage of the heat generated from sunlight. This will warm the outer layer of the pile, thereby insulating the center even more. This will also help the pile heat up a bit quicker.
The outdoor temperature will influence the temperature of our pile. Although this concept is fairly intuitive, I felt I should mention it nonetheless. Since a compost pile will exchange heat with the outside air, we should touch on the concept of heat transfer and convention.
Essentially the rate at which heat will be exchanged between two different bodies will be directly proportional to the temperature difference between them. So, the bigger the temperature difference between the compost pile and the air, the faster the heat will leave, thereby lowering the temperature of the pile. Another way to put it is that it will take longer for a pile to heat up in cooler temperatures.
That being said, it is possible to hot compost in Winter. But you need to modify your methods by following these principles.
Examples of how long it took to reach temperature on two compost piles
Ok, so let’s take a look at a couple of piles I made (really one pile that kept evolving) and how the temperature at the center of the pile changed over time.
First, let’s look at the ingredients. I built this pile on October 9th using approximately 20 bags of coffee grounds from Starbucks, 7-8 gallons of kitchen scraps, and a lot of sawdust and some shredded cardboard.
The final size of this pile was about 4′ diameter and 2-3′ tall. The original temperature at construction was about 68F (ambient). After approximately 42 hours the temperature had risen to 80F, which is a nice slow/steady rise.
Based on my years of composting experience, this pile I believe it would have hit around 120F within a few more days. But that was not to be, as I got a text from my neighbor that his wife was cutting down all plants (both live and dead) and asked if I wanted them. I said ‘Yes Sir!’
With the use of my wheelbarrow we brought over about 6 trash bags full of green and brown plant material. These are mostly intact plants. So, I would need to go after them with garden sheers to reduce the size and allow a better mix. I had to use up all the shredded cardboard I had to build the pile and keep it somewhat balanced.
With the addition of these ingredients, my pile had now swelled to about 6′ long windrow that was roughly 3′ wide. When I finished construction, the internal temperature was the same as ambient, 68F.
I came back out to the pile the next morning around 8:30 am. For reference, the outside temperature at that time was approximately 48F. The new modified pile had now been sitting for approximately 17 hours. I didn’t even really need to take the temperature as I could already see steam rising from the pile. When I inserted my 24″ compost thermometer I was pleasantly surprised to see it had reached over 120F!
So, from 68F ambient to over 120F in 17 hours, that is pretty good if I do say so myself. But, the thing we need to remember about piles that are built with plant material as a green ingredient, they will compress on themselves. Now, these are regular flowers, and they won’t compress upon themselves as bad as grass clippings. But they should be turned a bit more frequently none the less. So, I went back out and mixed on the morning of 15 October and found that the internal temperature had risen to over 150 F.
The time it takes for a compost pile to heat up will depend on the ingredients you use, the size of the pile, the outside temperature, and also somewhat on the pile location. We can fully control all of these factors with the exception of the outdoor temperature.
That being said, a large balanced pile can get hot within 12 hours to seven days. While it may be tempting to use a potent green material like grass clippings or manure, we have to balance that choice against the frequent turning (for grass clippings) or account for the risk of pathogens with manure.
My own personal preference is to scrounge up as many coffee grounds from Starbucks and store up kitchen scraps, but at the end of the day I usually will just use whatever I have available. Because I’m going to make that compost no matter what, and I don’t let anything get in the way. Plus, I know that I can always make a cold pile hot later one if new green materials become available.
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