Using Horsetail (Equisetum arvense) as a garden spray.

Often dismissed as a stubborn, invasive weed, Horsetail (Equisetum arvense) is actually a "living fossil" that has survived for over 300 million years. For the organic gardener, it is a potent resource—a natural, mineral-rich tonic that acts as both a defensive shield and a growth stimulant. Using horsetail as a garden spray provides a range of benefits that chemical alternatives simply cannot match, primarily due to its unique chemical composition and ancient evolutionary resilience.

The Power of Silica: Structural Reinforcement

The most significant benefit of horsetail is its extraordinary silica content. It contains the highest concentration of silica in the plant kingdom, comprising up to 25% of its dry weight. When brewed into a tea or decoction and sprayed onto plants, this silica is absorbed into the plant’s tissues, where it is deposited in the cell walls.

This process essentially "armors" the plant. By strengthening the cell membranes, horsetail makes the plant physically tougher and more rigid. This structural reinforcement is the first line of defense against environmental stressors like high winds and heavy rain, but its most critical role is in disease prevention.

A Natural, Non-Toxic Fungicide

Horsetail is widely celebrated as one of the most effective organic fungicides. Most fungal pathogens, such as powdery mildew, downy mildew, rust, and black spot, rely on penetrating the plant's outer "skin" (the cuticle) to infect the host. By thickening these cell walls with silica, horsetail spray makes it significantly harder for fungal spores to gain a foothold.

Furthermore, horsetail has a natural drying effect. Fungi thrive in damp, stagnant conditions; the sulfur and flavonoids found in horsetail create an environment that is less hospitable to spore germination. It is particularly effective as a preventive measure for:

• Roses: Protecting against black spot and rust.

  
  

• Tomatoes and Potatoes: Reducing the risk of early and late blight.

• Cucurbits: Keeping powdery mildew at bay on zucchinis and cucumbers.

  
  

• Fruit Trees: Helping prevent peach leaf curl and scab.

  
  

Pest Repellent and Soil Health

Beyond fungi, the "armored" leaves created by horsetail spray are less palatable to sucking insects. Pests like aphids, spider mites, and whiteflies find it much more difficult to pierce the hardened leaf surface to reach the sap. While it isn't a traditional insecticide that kills on contact, it serves as an effective deterrent that disrupts the feeding cycles of these common garden nuisances.

When used as a soil drench rather than a foliar spray, horsetail serves as a "remineralizer." It is rich in potassium, magnesium, and calcium, which are essential for overall plant vigor. In biodynamic farming, horsetail (often referred to as preparation BD 508) is used to balance the "watery" forces in the soil, preventing rot and encouraging healthy root development.

How to Prepare a Horsetail Spray

To harness these benefits, gardeners typically create a decoction or a fermented tea. Unlike many herbs that release their properties with a simple steep, horsetail’s tough structure requires boiling (simmering for about 20–30 minutes) to break down the silica.

1. Fresh or Dried: Use roughly 1kg of fresh horsetail (or 150g dried) per 10 liters of water.

2. Boil: Simmer the mixture to extract the silicic acid.

3. Dilute: Once cooled and strained, dilute the concentrate (usually at a 1:5 or 1:10 ratio with rainwater).

   
   

4. Application: Spray your plants in the early morning on a sunny day. The sunlight helps the silica "set" and bond to the leaves.

   
   

   By incorporating horsetail spray into a regular garden maintenance routine, you shift from a reactive "cure" mindset to a proactive "prevention" strategy. It is a sustainable, cost-free, and highly effective way to bolster your garden’s immunity, turning a "nuisance" weed into your plants' strongest ally.

5. While a quick decoction (boiling) is great for immediate silica release, fermenting horsetail creates a more potent, biologically active "tea." This process uses beneficial bacteria to break down the plant material further, making the minerals more bioavailable and adding a boost of microbial life to your garden.

   Here is the step-by-step guide to making and using Fermented Horsetail Tea.

   Phase 1: Preparation & Materials

   Because fermentation can produce a strong (and sometimes unpleasant) odor, it is best to do this outdoors or in a well-ventilated shed.

   What you will need:

   • Horsetail: 1 kg of fresh plants (chopped) or 150g–200g of dried horsetail.

   • Water: 10 liters of dechlorinated water (rainwater is best; if using tap water, let it sit out for 24 hours so the chlorine can evaporate).

   • Container: A large plastic or ceramic bucket (avoid metal, as the fermentation process can react with it).

   • Breathable Cover: A piece of burlap, cheesecloth, or a loose-fitting lid to keep flies out while allowing gases to escape.

   Phase 2: The Fermentation Process

   1. Chop the Horsetail: Cut the stems and "leaves" into small pieces (about 2-5 cm). This increases the surface area for the bacteria to work on.

   2. Submerge: Place the horsetail in the bucket and pour the water over it. Ensure the plant material is fully submerged. (Tip: You can put the horsetail in a mesh bag and weigh it down with a stone).

   3. The Wait: Store the bucket in a shaded spot.

   4. Daily Stirring: Stir the mixture once a day with a wooden stick. This introduces oxygen and speeds up the breakdown.

      • Observation: After a few days, you will see small bubbles forming. This is a sign that fermentation is active.

   5. Completion: The tea is ready when the bubbling stops and the plant material has mostly disintegrated (usually 10 to 14 days, depending on the temperature). It will have a dark color and a pungent, earthy smell.

   Phase 3: Straining and Storage

   • Strain: Pour the mixture through a fine sieve or cloth to remove all plant solids. This is crucial if you plan to use a pressure sprayer, as tiny particles will clog the nozzle.

   • Storage: Store the liquid in a sealed plastic jug in a cool, dark place. It will remain potent for up to 6 months.

   Phase 4: Application and Dilution

   Never use the fermented tea at full strength, as the high mineral concentration can be too "hot" for delicate foliage.

Application Type	Dilution Ratio	Best For...
Foliar Spray	1:10 (1 part tea to 10 parts water)	Preventing mildew, rust, and aphids.
Soil Drench	1:5 (1 part tea to 5 parts water)	Strengthening roots and remineralizing soil.
Seed Soak	1:20	Helping seeds germinate with built-in fungal protection.

Pro-Tip: For the best results, spray your plants in the early morning during a period of dry weather. The silica needs to dry on the leaf surface to form that protective "glass-like" barrier.

The Biodynamic Association sell Horsetail either dried or as a fermented prep

They have a identification guide here.

Charles Mitchell normally has some at The Croft.