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Does Coffee Kill Plants?

Coffee seems to be able to help anything. So, you can’t blame me for thinking maybe my gardening skills need a caffeine kick as well.

So, does coffee help or does coffee kill plants? Here’s what I discovered:

Yay or Nay: Coffee Grounds as Fertilizer

As per many researches, it is seen that most home and commercial gardeners use coffee grounds as fertilizers. It can be added directly to the plant or can be turned into compost. The quality of the soil happens to improve due to the gradual breakdown of the coffee grounds which raises the temperature and releases nitrogen. The coffee grounds contain by volume around 2% of nitrogen which is broken down in the compost.

So, does coffee kill plants? Well, only un-compost coffee grounds will result in stunt growth of your plants. Thus, I found that adding them in compost and allowing the microorganisms to perform the breakdown process was a much better way to incorporate coffee to make my plants grow and thrive.

Even though the grounds cannot add nitrogen to the soil directly, there are still some benefits of using it as a fertilizer. Some of these benefits include:

  • It adds organic materials to the soil. And because of it, it will improve water retention, drainage and aeration in the soil.
  • t can lower or raise the acid level pH of soil. Here I would like to mention that this works only in the case of unwashed coffee grounds as used coffee grounds are neutral while fresh ones are acidic in nature.
  • It will help microorganisms to thrive and attract earthworms which are essential for proper plant growth.

Say Yes to Coffee Grounds Compost

Just throw your used coffee grounds along with those coffee filters in the compost pile. Yes, it is that easy to add nitrogen to a compost pile. But I forgot to consider that they are considered as green compost material. So, you do have to maintain the balance and add some brown compost materials whenever you throw in used coffee grounds to avoid my mistake.

The science behind it

cup of coffee with science formula in a paper

It should be said that too much caffeine is bad for you. This is true in the case of plants as well. The caffeine building enzymes that are found in coffee plants are members of N-methyltransferases. These are found in almost all plants and build a wide range of compounds.

Caffeine is considered as a chemical stimulant that can affect the increase of biological processes in the plant in question. These processes are absorption of nutrients and water from soil, ability to photosynthesize, increasement of pH levels and such others.

In fact, there are numerous studies that show initial cell growth in certain plants to be stable. While continuous increasement of caffeine level resulted in caffeine killing or distorting the cell, meaning stunted or dead plants.

Coffee as Insect Repellent?

They say that caffeine is an effective snail and slug killer. However, I noticed that it can also kill milkweed bugs, mosquito larvae, butterfly larvae and hornworms. According to many studies, caffeine is effective as an insect repellent because in such mentioned insects it causes distorted behaviour by suppressing certain essential enzymes and interferes with their reproduction and consumption process as well.

It is definitely better than many harmful chemical insecticides in my books!

So, as long as you do not overdo it, using coffee grounds even as an insect repellent can help your plants grow and thrive.

Other uses of used coffee grounds in a garden:

  • It is a perfect worm food in case of vermicomposting.
  • It is also used as a mulch.
  • My cat hates it, acts as a great cat repellent and will ensure your flowerbed is not their litter box.

What about fresh coffee grounds?

Fresh coffee grounds, in some situations, can work wonders and can definitely help your plants to grow. Here are some instances where fresh coffee grounds become a must:

  • The allelopathic properties of fresh coffee grounds can suppress weed and some fungal pathogens.
  • It is great for acid-loving plants like carrots, lilies, hydrangeas, azaleas, blueberries and such others.

Can you water plants with Coffee?

old man taking care of the plants with coffee on the side

But what about brewed coffee which is now cold, lonely sitting on your work desk? Does coffee kill plants if you use it instead of water? Well, brewed coffee contains potassium and magnesium which are, as you know, needed for healthy plant growth.

Hold on! Don’t just dump your cold coffee on to the plants just yet. The trick is to dilute our coffee first before using it to water your plants. And of course, you cannot substitute water with diluted coffee. Every once in a while, it is highly recommended even if you are using fresh/used coffee grounds as a fertilizer/compost.

Water Plants with diluted coffee

I add some plain tap water to my cold cup of plain brewed coffee and use this diluted coffee to water acid-loving plants like aloe, impatiens, gladiolus, African violets, amaryllis, bromeliad, roses, ferns, lupine, begonias from time to time. So, now you can see how to use diluted brewed coffee to water both indoor and outdoor plants.

How can you tell if watering your plants with diluted coffee is killing your them or not? If you also grow the habit of dumping your unused, now-cold coffee on your plants, then you need to keep an eye on them. Yellow foliage and/or tip of the foliage turning brown are usual signs for too much acidity in the soil. In such cases, you need to stop immediately and balance out the soil to ensure the plant does not die.

Yet if you are a cream and sugar kind of coffee drinker then the answer to the question- does coffee kill plants will be a big YES undoubtedly. Even if you dilute your cream and sugar filled coffee, fats and sugars (even artificial ones) can harm plants, and not to mention that it will attract pests and fungus gnats creating a foul-smelling mess which you need to avoid at all cost.


Hannah in Spokane Valley, Washington bought just now