Hostas will benefit from an application of coffee grounds used as mulch because of their relatively high nitrogen content, but you need to use the grounds judiciously. Too much coffee grounds spread around Hostas can form an impermeable layer that hinders water and air from reaching the roots.
The best practice is to add coffee grounds to your compost pile so you can get the benefit of the Nitrogen content and mix it in with other organic matter for a broader spectrum of nutrients and better soil structure.
You can then use the coffee grounds and compost mix as a mulch around your hostas in the spring for added fertility, moisture retention and improved soil structure.
Why Coffee Grounds are Good for Hostas
Hostas as perennial plants go are very heavy feeders, hence their large size and luxuriant leaves.
In fact the larger the hosta the more nutrients the plants will require to grow to their full potential, in terms of size, shape and health, with the giant varieties such as ‘Sum and Substance’ and ‘Jade Cascade’ requiring the soil to be replenished with nutrients each growing season.
Therefore they make heavy demands from the soil in the spring and summer months, particularly Nitrogen, which is found in a good concentration in coffee grounds.
Coffee grounds are also particularly beneficial for stimulating worm activity in the soil. Worms love to eat coffee grounds and produce ‘casts’ after digestion which is a highly concentrated form of nutrients that is chelated, which means it is easier for the hostas roots to uptake.
As the worms feed on the coffee grounds they create channels as they move through the soil.
These channels benefit the hosta as they aerate the soil, ease compaction and create space for the hostas roots to grow and become established. This will allow the roots to grow deeper and tap into moisture supplies deep in the soil, which means the hosta will be more resistant to drought or dryer conditions.
The channels that worms create also increase the drainage of the soil. Hosta roots like moist soil but they do not like slow draining, saturated ground.
Therefore the increase channels and space in the soil allows excess water to drain away from the roots so that they are not sat in water which would cause fungal disease.
Hostas also require the soil to be within pH 6-7 which is mild acidity to pH neutral. Coffee grounds can vary quite widely in terms of pH when fresh however once the grounds are fully decomposed they will be within the optimal range for hostas with a pH of between 6.5 to 6.8.
How to Spread Coffee Grounds to Benefit hostas
Hostas do best when they receive a mulch at the start of spring. Ideally the mulch should add nutrients, conserve moisture, suppress weeds and improve soil structure.
You can add coffee grounds straight to the soil that surrounds the hosta, but it is important not to add too much in one go. A thin layer of about half an inch will be enough in one go.
Keep a 6 inch gap between the coffee grounds and the crown of the hosta as the crowns do not like to be covered in mulch.
If you add too much coffee grounds in a thick layer, then the grounds can form a hard crust that hinders water from infiltrating and deprives the roots of oxygen.
Hostas are best served if you use a compost and coffee ground mix so that the soil structure is improved by the compost and the hostas get a hit of nitrogen from the coffee grounds at the start of the growing season when it is in high demand.
Compost, leaf mould and manure are all great materials to use in conjunction with coffee grounds as they all have an exceptional capacity to hold onto moisture which is essential for healthy hostas as well as a broad range of nutrients.
These materials also break down to a friable porous structure that will work complimentary with the coffee grounds to ensure good drainage, aerated soil and a good hit of Nitrogen for optimal hosta growing conditions.
Distribute a 2 inch layer of the compost and coffee grounds mix (ideally 50% coffee grounds and 50% compost) around the hostas leaving a 6 inches of soil between the mulch and crown of the hosta. 2 inches is the perfect depth of mulch to help retain water and keep the soil around the hosta roots moist for during the dryer summer months.
Alternatively you can throw the coffee grounds on your compost pile to encourage worm activity which can increase the rate of decomposition of your compost.
Despite being brown in colour, coffee grounds are considered ‘green’ in a composting context because the contain more Nitrogen the Carbon, so it is a good idea to balance coffee grounds with ‘brown’ material (dead leaves, cardboard, shredded paper etc.) in your compost heap for a balanced nutrient profile and to ensure an efficient rate of composting.
Coffee Grounds for Keeping Slugs Away from Hostas
Coffee grounds are sometimes recommended to help repel slugs to prevent them eating hosta leaves. There is speculation that the residual caffeine content in the coffee grounds are perhaps toxic to slugs and therefore they are reluctant to move over the grounds.
There are some anecdotal evidence of this working to some extent, but there is no conclusive proof.
However there have been experiments documented on YouTube which show that slugs will travel over coffee grounds but they do seem to act as a disincentive, as slugs tend not to move over coffee grounds if they can avoid it.
So coffee grounds can potentially contribute to keeping slugs and snails away from hosta leaves but it is by no means a sure fire way to keep hostas safe.
The only technique that has had undeniable success with keeping slugs away from eating hostas in my experience is if you plant hostas in pots that are raised off the ground on ‘feet’. Make a line of Vaseline with salt on the feet or in the rain shadow of the pot. The Vaseline holds the salt in place so it doesn’t dissolve in rain or blow away in the wind.
Slugs and snails will not cross a line of salt as it will dehydrate them via osmosis. Therefore this tactic can be very successful. (Read and learn how to implement it more in my article How to Prevent slugs eating Hostas).
Coffee grounds are good for hostas because of the high nitrogen content.Coffee grounds can be use as a mulch
- Coffee grounds benefit hostas because of their relatively high nitrogen conent. Nitrogen (N) is one of the key nutrients (along with Phosphorous (P) and Potassium (K)) that is required in high demand by hostas for growth and to stay healthy.
- Coffee grounds are good for stimulating worm activity which improves drainage and the fertility of the soil, improving conditions for hostas.
- Do not apply coffee grounds straight to the soil without amendments as it can form a crust that stops water infiltrating the soil.
- Mix coffee grounds with compost and distribute a 2 inch layer around the hostas to ensure good soil structure, preserve moisture and to add nutrients to the soil.
- Coffee grounds may act as a deterrent for slugs and snails, however it is not a full proof way of preventing slugs eating hostas.