Can You Compost Toxic Plants?

By Paul Smart •  Updated: 11/27/21 •  6 min read

Can You Compost Toxic Plants? If you are someone that has a reasonable size garden there is a fairly high chance that at least some of the plants you have in it are toxic in some way shape or form. There are a surprisingly large number of common cottage plants such as foxgloves and delphiniums which are toxic as the list compiled by the University of California shows. Given the degree of toxicity of some of these plants is it safe to compost them? Or will my compost be toxic as a result of adding these plants?

Toxic plants can be successfully composted as the majority of compounds that make up the plant are broken down during the composting process. A study by the University of California showed that toxic compounds are largely undetectable after 50 to 100 days. However, the precise rate of degradation of the toxic compounds would vary depending upon the composition of the heap and how the raw materials were processed prior to being added.

The process can be accelerated by shredding input material in fine pieces, which is what was done in the University of California Study. The reason for this is that it increases the surface area of the plant material accelerating the reaction rate. In addition to this, the size of the compost heap will also affect the rate of breakdown of the material. To maximize the rate of decomposition hot compost heaps need to be at least 3 ft by 3 ft.

However, it is important to note that due to the variability of the decomposition rate it is advisable to always use gloves and long sleeve shirts when turning compost heaps and handling the compost to avoid any chance of an issue.

Identifying Poisionous Plants

Identification of poisonous plants is difficult because there are no hard and fast rules that can be applied to the problem. Website such as the Gardening Know How and the CDC provide indications in a very limited way of some of the possible things to look for such as Milky sap, the presence of naturally shiny leaves, or yellow and white berries.

However, these rules not apply in all cases and there are many poisonous plants that do not fit into these categories, as such it is best to assume that all plants are poisonous unless you know otherwise. This means that you should always use gloves when handling plants.

If you need to know 100% whether a particular plant is toxic or non-toxic the most authoritative list I have been able to find is from the University of California which includes which plants that are completely safe and those that have some degree of toxicity. This list also includes common names as well as scientific names which makes the plant easier to identify.

However, it is important to note that common names can change over time as they are usually used as marketing tools by plant companies. To reduce the chances of mistaken identity visit the creative commons website as it has photographs of most species if you search by the scientific name.

Are All Edible Plants Non-Toxic?

When dealing with edible plants it is also not safe to assume that all fruit and vegetable plants are completely non-toxic. Some quite well-known examples of this include rhubarb which has very poisonous leaves that sit upon the top of an edible stalk.

Another example is the members of the Solanaceae family, which are sometimes referred to as nightshades vegetables. These plants which include potatoes, tomatoes, peppers, and eggplant produce steroidal alkaloids such as solanine and chaconine in the leaves and stem of the plant. This makes the foliage poisonous while the fruit or tubers are edible.

The exception to this is green potato tubers which contain solanine. The solanine forms as a result of the tubers being exposed to sunlight and is one of the reasons that potatoes need to be earthed up.

The consumption of solanine can cause a range of mild symptoms such as diarrhea and headaches, in extreme cases, paralysis and death can occur.

Can You Compost Diseased Plants?

The Compost diseased plants have some risks associated with it because harmful bacteria and spores can be somewhat rampant in diseased plants. There is a therefore a chance that disease spores can survive if the temperature of the compost does not get above 140°F (60°C).

To get a compost heap to reach 140°F (60°C) you need to ensure that the heap is at least 1 cubic metre in volume. It also needs to have the composting material added to the heap over a fairly short period to ensure that the heap does reach these temperatures. However, for most home gardeners this is not possible. I personally find that I usually only get to 122°F (50°C).

However, if you don’t have a compost thermometer, which is available from Amazon, to measure the temperature you can usually tell if you are reaching that temperature by the number of weeds popping up in the areas where you are spreading your compost. Alot of weeds means you are probably reaching between 86 to 122°F (30 to 50°C).

So what do you do? If you can possibly manage it I try to get rid of diseased material through the green waste bin however, it is common for me to have more material than bin space at certain times of the year. In these cases, I prioritize the removal of plant material with serious diseases and I generally take the pragmatic approach of composting the material with a minor disease like powdery mildew which affects my zucchini and pumpkins every year.

Can You Compost Weeds?

The composting of weeds is generally not a huge problem provided that they have not set seed. If they have set seed it is pretty likely that the weeds will be visiting your garden next year. However, the one thing that I would absolutely avoid is adding perennial weeds like oxalis and bindweed to the compost heap as this will definitely increase the rate of spread. I personally had trouble with oxalis weed spreading throughout my vegetable patch.


Most materials can be composted whether they are toxic, diseased or generally contaminated with chemicals. However, when handling these materials always take adequate precautions by covering up exposed skin with gloves and long sleeves. Avoid adding the remains of vigorous growing plants to the compost as they will almost certainly come back to haunt you the following year.

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Paul Smart