We’ve all read conflicting advice on what we can and can’t throw in the compost bin. Some people are more cautious than others and avoid composting any diseased plants, seeds, weeds, lemon rinds, and all kinds of materials. Today, we’ll be focusing on tomato plants – are they compostable, and if so, what are some gardeners so afraid of?

To me, the answer to “can you compost tomato plants?” is simply YES! But in order to explain myself, first let’s look at some of the three reasons why people may advise against composting tomato plants.

  1. Disease and contamination
  2. Longer break down
  3. Avoiding volunteers

Disease and contamination while composting tomato plants

This seems to be one of the main reasons given against composting tomato plants and it’s understandable since many tomato diseases are technically soil-borne. However, this argument doesn’t acknowledge the high probability that many infections are blown into the garden from the surrounding area.

Gardeners who have experienced any disease in their veggie patch should definitely be more vigilant about making sure they compost diseased plant material properly, but in most cases, this shouldn’t mean throwing it into the landfill.

In general, giving the compost pile enough time and water, “brown and green” matter, as well as increasing the temperature and turning the pile to maintain enough oxygen all help to break down and kill off any pathogens in the compost pile. Unfortunately, this doesn’t happen in one year.

In the first year, pathogens are still feeding on the host plant as it starts to break down within the compost pile. Both “good” and “bad” fungi are prolific at this stage due to the drop in the pH. In the second year, the pile is half decomposed and the spores are being starved of any new plant matter (this is a crucial step; do NOT add any more plant matter during the second year of decomposition). During this stage, the pH is beginning to neutralize so bacteria species are more prolific. By the third year, the proportion of fungal spores and bacteria are minimal and no greater than the surrounding environment, so your compost is now ready to use in the garden. 

In large-scale horticultural situations, farmers plow in any diseased crop to limit their spread and follow simple crop rotation cycles to control many diseases. Spores in the compost can become airborne, so it is good practice to cover diseased plants with more plant matter as soon as you toss them on the pile.

Common tomato plant diseases:

  •  Late blight (Phytophthora infestans) is a soil borne water-mold whose spores are blown by the wind to infect tomato plants. The spores require moisture to survive, so they can be killed by a single, dry, sunny day.
  • Early blight (Alternaria solani) and Septoria leaf spot (Septoria lycopersici) are both fungi which infect tomato plants, including their seeds.
  • Tomato leaf mold (Cladosporium fulvum) is a fungus. The wind dispersed spores require living plant matter and moisture to grow, but the fungus can over winter in seeds or dead plant matter for up to a year.
  • Bacterial wilt (Ralstonia solanacearum) is spread via contaminated water or via sucking insects, but not via wind dispersal. Research shows that bacterial wilt can be managed by decreasing the pH of the compost pile to between 4 – 5 during the summer. The amino acids that initially form already decrease the pH of the compost pile and adding oak leaves and pine needles can decrease the pH further, whereas anerobic composting can make the pH drop severely. 

Longer break down time of tomato plants

I guess if you want your compost to be ready within 6 – 1 2 months, then you probably have a giant list of things that “shouldn’t” go in the compost, including fruit, seeds, weeds, and anything with lignan and fiber; so pretty much all plant matter! Is this still composting?

To me, the whole point of my compost system is to avoid sending anything to landfill at the same time as building healthy soil for my garden. I suspect those who advise against putting anything in their compost bins due to the time it takes to break down aren’t adhering to the standard three-bin system and are probably emptying their compost bins after less than a year.

An easy trick to employ to help any plant matter decompose quicker is to cut them into smaller pieces. I tend to do this with a pair of shears, but a small backyard chipper or shredder would be ideal.

Some people recommend using a compost starter or additive to speed things along. Some commercially available compost starters are high-nitrogen fertilizers in order to produce more heat. You can save yourself the money by peeing in a bucket of water and throwing this on the pile or mixing in some chicken manure. Other products are “dormant beneficial microbes”. I personally have never tried them, instead, I throw a few spades of old compost, soil, or mulch onto the pile to help inoculate it with beneficial microbes at the same time as smothering the bad ones I may have just added to the pile.

Avoiding tomato plant volunteers

In my opinion, this is only a minor problem if you grow hybrid tomatoes, since there is a tiny risk that the seeds germinated from some hybrids won’t produce fruit. But for me, I don’t mind the occasional rogue tomato plant popping up in the garden; it’s a nice surprise!

Under ideal storage conditions, tomato seeds can remain viable for up to 7 years, but your average backyard compost pile is far from providing ideal conditions. I strongly advocate a three-bin compost system, since this gives the pile an adequate amount of time for any seeds to either germinate or decompose within the compost pile itself.

Some people avoid putting any fruit in their compost bins for fear of their seeds germinating and potentially spreading disease if they were inoculated the previous year by disease. But rotten tomatoes tend to have mold growing on them, which is perfect for the compost pile. Mold helps to break down plant matter quicker. Once again, I suspect those that had issues with volunteers popping up throughout their garden simply didn’t give their compost the adequate amount of time (2 years) to decompose thoroughly. 

So… Can you compost tomato plants?

It seems like the easiest and most environmentally friendly solution to these three issues is sectioning off a portion of your garden in order to set up a three-bin compost system that provides enough time, starvation, and heat to decompose even diseased tomato plants. A single, small bin emptied at the end of the year is not going to cut it.

If you only have a small compost barrel instead of a compost system, then I can understand why you might be reluctant to throw anything in it that takes longer than 3 months to break down. With regular usage, these typically fill up within 1 – 2 months and many novice gardeners spread the contents around within about 6 months, thinking it will be a sufficient amount of time to decompose everything. But while it may look decomposed enough, it is definitely NOT ready to be used after such a short period of time. Even compost that is a year old is likely to still have seeds and plant matter for fungus and bacteria to survive and feed on.

For gardeners in cold climates, where the summer is mild, short, and doesn’t provide enough energy to heat up the compost pile enough to kill pathogens and destroy seeds, then a separate, long-term compost bin can be used to break down difficult or diseased plant matter.

Instead of trying unsuccessfully to increase the temperature, you could try decreasing the pH of the compost pile which severely limits many diseases. This could potentially be achieved by not turning the pile which creates anaerobic conditions within the center of the pile. Other options for cold climates are compost barrels which are made of black plastic which means they heat up quickly in the limited sunshine. It takes longer in cold climates to make compost.

Final thoughts

When I read some of the issues people have with disease, volunteers and decomposition of tomato plants I can guess it’s probably because they are using compost that is only half finished, most likely due to not giving it enough time, which is also dependent on the temperature.

So, in answering whether or not you can compost tomato plants, healthy or diseased, I say go ahead! Chop it up and chuck it in. But this is only if you employ the three-bin compost system in order to give the tomato plant time to break down in the first year and then starve the pathogen of any new host material in the second year. Without this system, you severely limit what you can compost effectively, including your tomato plants.





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