Growing food

Can You Plant Seeds Directly in Coffee Grounds?

Some people plant seeds directly in coffee grounds. But science does not support many of the claims made for this material.

You should not plant seeds directly in coffee grounds. Coffee is high in nitrogen, but other plant nutrients are more variable. Coffee grounds compact easily. Caffeine in coffee grounds may also have a negative effect on germination rates. And may suppress the growth of certain plants. 

Instead, add coffee grounds (in moderation) to a compost heap, or add it as a mulch with other materials. Be careful, however. Using coffee grounds in your garden is not always a good idea. Read on to find out more about how where to use them (and where not to use them) in your garden.

You can plant seeds directly in coffee grounds.FALSE
You can add coffee grounds to a composting system.TRUE (But only in moderation)
You can sow directly in composted coffee grounds.TRUE (But only for certain plants)
Coffee grounds make good mulch.FALSE (They should not be used alone)
Coffee grounds acidify the soil.FALSE
You can use coffee grounds to repel pests.FALSE (Claims have no basis in science)

Planting Seeds in Coffee Grounds

Planting seeds in coffee grounds is not a good idea. Coffee grounds have some nitrogen, phosphorus, and potassium, which are important plant nutrients. They also contain multiple micronutrients that plants need to grow. The important fact is that coffee grounds will break down gradually. The result of that is they will not aid plants in a speedy and immediate way. Instead, they will break down slowly to provide slow-release fertilization that will nourish the soil (and plants in it) gradually.

Coffee grounds contain typically around 1-2% nitrogen. However, the level of phosphorus and potassium can vary a lot more and is generally rather low. For leafy, growing plants, nitrogen levels in the growing medium are very important.

When seeds are germinating, however, they do not actually need to obtain nutrients from the medium in which they grow. Instead, they require the right levels of light, oxygen, and water.

You shouldn’t use coffee grounds as a sole growing medium. Over time, they do compact very easily, which reduces airflow to the soil and prevents water from being absorbed by the plant’s roots. Plant seeds directly in coffee grounds, and it is likely that germination rates will be poor. Since the seeds may not get the light, oxygen, and water, they need.

Unfortunately, coffee grounds also contain caffeine (there is a significant amount of caffeine left in spent grounds). Caffeine in plants began as a mutation. It gave the plants with this mutation an edge as the leaves falling down from those plants made it difficult for other plants to grow around them.

Science has found that caffeine can suppress plant growth. This was proved by one study that states: “Applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growth.” Another study also showed that seed germination in certain plants was detrimentally affected by the use of coffee grounds.

Composting Coffee Grounds

The high nitrogen content of coffee grounds means that using coffee grounds as a growing medium for planting seeds can create an imbalance. The result is that plants may grow more leaves instead of flower and fruit – which is not what you would rather want to have in your own garden.

You can use, however, coffee grounds as a very good nitrogen source in your compost. The ratio of carbon to nitrogen in coffee grounds is around 20-to-1. When coffee grounds comprised a quarter of the material in the compost pile by volume, people doing this experiment recorded temperatures of 60-70 degrees Celsius for up to two weeks. So coffee grounds could be useful for use in hot composting systems.

However, in standard cold compost systems, the addition of coffee grounds should only ever be moderate. You should also only use coffee grounds in moderation when composting in place and building up layers in a new lasagna garden or hugelkultur mound.

It is an excellent idea to add some coffee grounds to your composting system. But try to add them only a little at first. Successful composting always involves getting the right ratio of nitrogen-rich (green) and carbon-rich (brown) materials.

You should also be aware that coffee grounds release certain compounds and chemicals that can increase the death-rate of earthworms. When you add large quantities at one time, this can also kill off microbes essential for composting to take place. So this is another reason why you should only add coffee grounds to a composting system in small amounts. 

Planting Seeds Directly in Compost Containing Coffee Grounds

Once you have created a good quality, balanced compost, this can often make an excellent material in which to directly plant a range of seeds. This can be spread over growing areas, used to fill raised beds or containers, or used to make your own potting mix for seed starting.

However, it is important to remember that different seeds and seedlings have different needs. Some need a more free-draining growing medium, and some need a more moisture-retentive one, for example. When deciding on a growing medium, always take the individual needs of the specific plants you want to grow into account.

Mulching Plants With Coffee Grounds

Many gardeners advocate using coffee grounds for organic mulch around existing plants. But this, too, can be problematic.

As mentioned above, coffee grounds may have a growth-suppressing effect on certain crops. They can also become compacted easily when used alone. So spreading them as mulch can stop water from getting to root systems where it is needed. And may create problems with oxygenation in the soil.

As a slow-release nitrogen fertilizer, however, coffee grounds could potentially be beneficial for the growth of certain plants. Leafy, green vegetables, and other nitrogen-hungry plants may benefit from the addition of coffee grounds as part of a sheet mulch. 

If you are going to mulch some of your plants with coffee grounds, you need to mix them well with the rest of the organic materials. For example, you could sprinkle coffee grounds around plants along with a carbon-rich mulch such as straw or fall leaves.

An even better idea would be if you compost them firstly and only after that use the compost to mulch. Or use the compost as a material in filling beds or containers.

Will Coffee Grounds Acidify the Soil?

Some gardeners believe adding coffee grounds will make the soil more ericaceous (lower pH and make it more acid). Those who already have ericaceous soil already believe this is a reason not to use coffee grounds in their garden. On the other hand, those with alkaline soil often hope the coffee ground will help them improve the pH balance and move soil into a more neutral pH zone.

But coffee grounds don’t really have this effect at all. Coffee grounds can vary in their pH level, but on the whole, they are not highly acidic. They are usually only very slightly acidic and have a pH that is between 6.5 and 6.8. The coffee you drink is acidic – but few acidifying properties are left in what you discard.

Will Coffee Grounds Deter Pests in the Garden?

You will also come across many articles that state that you can use coffee grounds in various pest control measures. But any evidence that coffee grounds repel, for example, slugs and snails, is purely anecdotal. Evidence is lacking, and what evidence there is suggests that coffee grounds will neither kill these pests nor deter them to any significant degree.

So, while it cannot be denied that coffee grounds are useful in your garden, they are not quite so versatile and useful as the many articles on the subject imply. Yes, they can be beneficial when added to a composting system in moderation or as a component in the mixed organic mulch. But many of the other claims made for coffee grounds in the garden are either exaggerated or untrue.

When deciding how and where to use coffee grounds in your garden, it is important to follow the science. Science definitely does not have all the answers. But it does tell us that we should not plant seeds directly in coffee grounds. And that we should take care when using them in other ways in our gardens.

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