How to use stinging nettles in the garden

Stinging nettle tea is a fermented plant-based solution used to fertilise plants. Also known as nettle manure or fertiliser, nettle tea is made by grinding down nettles and leaving them to soak in water for a few days to create a nutrient-rich liquid. This type of plant-based liquid fertiliser is one of the easiest natural fertilisers to make which also makes it one of the most popular!

Stinging nettles are entirely safe to use in an organic garden making them a great way to cut down on pesticides. What's more, nettles are abundantly available. It couldn't be more straightforward to make your own nettle fertiliser and, best of all, the process is entirely cost-free!

1. As a nutrient-rich growth booster

High in nitrogen (both mineral and organic), phosphorus, potassium, calcium and magnesium, stinging nettles work as a natural fertiliser to boost leaf and root growth in a range of plants.

2. To enhance the natural defences of plants

Stinging nettles can help to boost the natural defences of plants by stimulating the production of antimicrobial substances such as phytoalexines which are naturally produced by plants when they are attacked by pests or pathogens.

A nettle fertiliser can therefore be used as preventative care to ward off cryptogamatic parasites or to deal with diseases like peach leaf curl and gooseberry mildew.

3. As a powerful pest-repellent

While it won't kill insects, nettle fertiliser can have a repellent effect on certain harmful insects such as aphids or mites.

4. To prevent crop-attacking fungi

Nettles can also work to inhibit fungi spore germination. Nettle tea is most effectively used as a preventative treatment to block the development of diseases like mildew and peach leaf curl.

5. To inject compost with bacteria and enzymes

Nettle fertiliser is rich in nitrogen, bacteria, lactic ferments and enzymes, meaning it is able to break down organic matter more quickly. This makes it a great tool to help activate your compost pile.

There are two main species of nettles: common (Urtica dioica) and dwarf (U. urens). Dwarf nettles are the smallest plants but they also have the most powerful sting. Both types of nettle can be used to make nettle fertiliser. Here are the steps to follow.

1. Gather your materials

Here's what you'll need to make your own stinging nettle fertiliser:

• a container (bucket, bin, tank etc.) that has a larger volume than the amount of nettle fertiliser you want to make (for example, if you want 10 litres of fertiliser, use a 15-litre bucket);

• a cloth to cover the container;

• equipment to filter the solution: e.g. colander, sieve, fabric or a pair of tights;

• a funnel to transfer the liquid;

• plastic or glass bottles to store the liquid (must be airtight and opaque).

2. Collect your nettles at the right time

It is best to pick young nettles that have just started growing as these plants will contain more active compounds. The best time to do so is early April but be careful: young nettles still have a powerful sting! Once cut, the nettles will put out new growth if their roots are left in the ground meaning you can make a new batch of fertiliser in the summer.

3. Follow a nettle fertiliser recipe

1. Collect about a kilo of fresh plants (stems and leaves). Cut them up roughly and add them to a large plastic container along with about 10 litres of rainwater. The plants must be completely submerged.

2. Cover the container with cloth. This will help to prevent insects from getting in while still letting enough oxygen in for the mixture to ferment.

3. Place the container in a sun-free spot (the ideal temperature will be about 15 to 25°C).

4. Leave the solution to ferment making sure to stir the mixture every day. Once the liquid is no longer producing bubbles when stirred, fermentation is complete. This should take around 5 to 12 days, depending on the outdoor temperature; the warmer it is, the quicker the mixture will ferment.

5. Filter the liquid using a sieve then pass it through fabric like cheesecloth or a pair of tights. The solution is now ready to use.

4. Store your nettle fertiliser

If you can't use the nettle tea right away store it in airtight containers (such as bottles or jerry cans) and keep it in a cool, dark place. Be sure to fill your containers up to the neck to prevent oxygen from getting in. Stored in this way, you can keep your nettle fertiliser for up to a year.

There are two main ways to apply nettle fertiliser.

1. With a watering can

Dilute the mixture to about 20% if you plan to use as a watering tonic. Applied in this way, you should use about 2 litres of nettle tea to every 10 litres of water. This ratio should provide your plants with everything they need to grow. Please note that any stronger and the solution may inhibit growth.

2. With a spray bottle

Alternatively, you can dilute the nettle fertiliser to 10% and spray it directly on leaves to deter mites and aphids. A 5% solution is best to protect your plants from cryptogamatic diseases like powdery mildew or grey rot. This type of mixture can be used every two weeks as a preventative measure.

It's worth noting that you can alternate between this type of solution and a Bordeaux mixture. This will help to reduce any negative environmental effects of the latter (as it is high in copper).

• It's best to use rainwater to make nettle fertiliser as it does not contain calcium or chlorine. If this isn't possible, use water from a well. If you have to use tap water, leave it to air out for two days taking care to stir it regularly; this will help the chlorine to evaporate.

• Do not prepare your mixture in a metal container (with the exception of stainless steel) to prevent oxidisation. Similarly, don't use a wooden container as this can alter the end results. Instead, use glass, plastic or terracotta.

• When filtering, work in two or three steps. Start with a rough filter to take out the largest debris (a sieve, for example) and work your way down to an old insect net or pair of tights.

• Do not mix species when making a nettle fertiliser. This can prevent the mixture from fermenting properly. However, you can make several different mixtures by preparing them separately.

• If you're dealing with a sudden infestation and don't have time to wait a few days you can make a quicker solution by following the above recipe and leaving it for at least 24 hours. You can then spray the solution on neat without diluting.

• Finally, if you don't fancy making the mixture yourself (the smell alone can be enough to put you off!) you can always buy a ready-made nettle fertiliser or manure.