My worms are probably addicted to caffeine by now. Since my wife and I brew one, if not two, pots of high octane java each day, it is easily the most common of our household’s food waste that we feed them. Add these grounds to the ones I already collect from the two Starbucks in my neighborhood, and I’m pretty sure my worms would get seriously grumpy if I stopped, what with all the withdrawal headaches and what not.
In moderation, coffee grounds are considered a superb food for earthworms. The good levels of nitrogen coupled with the grit that allow better digestion of other foods make them ideal. They are also convenient to use since the grounds are in a consumable form, i.e., no processing, chopping, or cutting required.
But some home worm composters report mixed results when feeding their worms coffee grounds, not witnessing the expected swarming to the scoops of Arabica deposited in the bins. Worse yet the worms may find the coffee repellant, and a full scale worm jailbreak ensues. If you’ve had no success with getting your worms used to a daily espresso, try sticking to the following guidelines.
While composting worms can eat 50-100% of their weight daily, I would recommend not feeding them any more than 25-50% of their weight in coffee grounds. The high nitrogen content makes for good worm food, but it makes for an excellent conventional composting ingredient, not exactly the characteristic you’re looking for when feeding worms as the thermophyllic (smarty-pants word for “hot”) stage of composting coffee grounds will repel or even kill your worms if they have nowhere to go.
While limiting your feedings will limit the heat produced by the grounds, so will pre-composting them. While this allows for the heat to come and go, it allows for something far more important: microbial growth. Remember, worms don’t really eat food. They eat the microbes growing on decomposing food. So those fresh coffee grounds you’re so eager to feed your wigglers? Any microbial activity has been washed away by the scalding water you just poured over them. Your environmental factors may be different than mine, but I try to let my coffee grounds sit for 3 days before including them in any food.
Coffee is acidic. Starbucks, Peet’s, and other more expensive coffees can strip lacquer off of your furniture. Too much of a good thing can drive your pH to such an acidic level that your worms will seek friendlier climes. Avoid combining coffee with other acidic food waste – like citrus – and test your soil for low pH when coffee is a regular part of your worms’ diet, especially if you find them trying to escape. Low pH can be corrected with a light sprinkling of lime followed by a watering or a generous application of crushed eggshells. Also, a regular turning of your worm bed will eventually help increase pH buffering. Cheap soil testers can be found on Amazon for about 6 bucks. It’s not going to give you extremely accurate results, but it’s going to tell you if you’re way of out whack.
If you have a worm composting operation that requires more coffee grounds than you make in a day, your local coffee shop will be more than willing to give you their spent grounds for free, often rebagging them and setting them out for customers to take, no questions asked. However, if they don’t, the simplest way is to ask for their trash. That’s right. The trash. Unless this is a coffee shop that serves large amounts of food, the trash at a coffee shop will be primarily coffee grounds, mixed with filters (which you can also feed your worms), or the odd cup and other sundry items which can easily be separated from the usable material.
If you have any experiences to share, please post in the comments below!