How to Make Insecticidal Soap at Home

The first batch of insecticidal soap I ever made was in 2015, in my cramped apartment kitchen with a borrowed spray bottle and a bus-ride purchase of Dr. Bronner’s from the co-op. I mixed it way too strong (three tablespoons per quart — don’t do this), sprayed my sad little balcony tomato plant, and watched half the leaves turn crispy within a day. But the aphids? Gone. Every single one.

That failure taught me the most important lesson about insecticidal soap: the recipe matters less than the ratio. Get the concentration right and you’ll kill pests without hurting plants. Get it wrong and you’ll damage both.

Here’s the method I’ve refined over 12 years and taught to hundreds of community garden participants.

What You’ll Need

The beauty of homemade insecticidal soap is its simplicity. You only need three ingredients:

1. Pure liquid castile soap — I use Dr. Bronner’s Unscented Baby-Mild. The peppermint version works too, but unscented is safer for sensitive plants. (Why castile? Read our soap selection guide)
2. Water — distilled or rainwater is ideal. Tap works in a pinch, but hard water reduces effectiveness (more on this below).
3. A spray bottle — 1 quart / 32 oz works well for most gardens. For larger gardens, use a 1-gallon pump sprayer.

Optional add-in: 1 teaspoon of vegetable oil (neem oil preferred) for improved leaf adhesion.

The Basic Recipe

Mix 1 tablespoon of pure liquid castile soap per 1 quart (4 cups) of water. That’s a roughly 1% concentration — the sweet spot recommended by the University of California IPM Program.

For heavy infestations, use 2 tablespoons per quart (roughly 2%) — but always patch-test first. Above 2%, you risk phytotoxicity.

Step-by-Step Instructions

Step 1: Choose Your Soap

This is the most important decision. Your soap must be:

• Pure — no degreasers, fragrances, or moisturizers
• Liquid — bar soap can work but requires grating and melting, and it’s harder to get a consistent concentration
• Castile-based — made from plant oils (olive, coconut, hemp)

Avoid anything labeled “antibacterial,” “ultra,” or “with moisturizers.” These contain additives that harm plants. I’ve written a detailed soap comparison guide if you want the full breakdown.

Step 2: Prepare Your Water

This step gets overlooked, but water quality genuinely matters. Hard water contains calcium and magnesium ions that react with soap to form insoluble salts — the same stuff that creates soap scum in your shower. On plants, these salts:

• Reduce pest-killing effectiveness — less active fatty acid available
• Leave white residue on leaves — blocks photosynthesis
• Clog spray nozzles — frustrating during application

My recommendation: Use distilled water or collected rainwater. If you must use tap, add 1 teaspoon of white vinegar per quart to partially neutralize mineral content. I keep a 5-gallon bucket under my downspout specifically for spray mixing.

Step 3: Mix the Solution

1. Fill your spray bottle with water (leave room for soap)
2. Add soap — pour slowly to avoid excessive sudsing
3. Gently swirl to combine — don’t shake vigorously; excessive foam makes even spraying difficult

Step 4: Test First

Always test before full application. Spray a few leaves and wait 24 hours. If you see any yellowing, wilting, or brown spots, dilute the solution further.

Plants particularly sensitive to insecticidal soap include jade, sweet peas, bleeding hearts, hairy-leaved herbs like borage, and some young tomato varieties. See our complete plant sensitivity guide.

Step 5: Apply

• Spray both sides of leaves — pests hide underneath, and that’s often where the densest colonies form
• Apply until leaves are dripping wet — the soap must make direct, thorough contact to work
• Target clusters of visible pests first, then spray surrounding foliage as a preventive buffer
• Spray in early morning or late evening — this protects pollinators and avoids leaf-burn from sun hitting wet foliage

Step 6: Reapply

Insecticidal soap has no residual effect — it only kills on contact while wet. Once it dries (usually within 30-60 minutes), it’s completely inert. New pests that arrive afterward are unaffected.

Plan to reapply every 4-7 days until the infestation is under control. During peak aphid season (late spring in most regions), I spray weekly as a preventive measure.

Why It Works

Insecticidal soap works by dissolving the waxy outer coating (cuticle) of soft-bodied insects. The potassium salts of fatty acids in castile soap are amphiphilic — they break down both water-based and oil-based barriers. Without this protective cuticle layer, the insect rapidly dehydrates and dies, typically within hours of contact.

This mechanism is why insecticidal soap targets:

• 🟢 Aphids
• 🟢 Whiteflies
• 🟢 Spider mites
• 🟢 Mealybugs
• 🟢 Thrips
• 🟢 Scale (crawler stage)

And is ineffective against hard-shelled insects like beetles, which have thick exoskeletons the soap can’t penetrate. For more on the science, read What Is Insecticidal Soap and How Does It Work?

Common Mistakes I See

After years teaching this at community garden workshops, these are the errors I correct most often:

1. Using dish soap instead of castile. Dawn, Ajax, and Palmolive are detergents — they strip plant wax along with insect wax. You’ll kill the pests but stress the plant.
2. Mixing too strong. “If 1 tablespoon works, 3 must work better” — no. You’ll burn leaves. Stick to 1-2 tbsp per quart.
3. Spraying at noon in summer. Water droplets magnify sunlight. Combined with soap stripping some protective wax, midday spraying in hot weather guarantees leaf burn.
4. Only spraying the top of leaves. Flip them over — that’s where the pests are. If you only spray the tops, you’re wasting solution.
5. Giving up after one spray. Insecticidal soap has no residual effect. Eggs hatch, new pests fly in. Plan for 3-4 applications minimum.

Pro Tips

• Use soft water — hard water minerals react with soap and reduce effectiveness
• Add 1 tsp vegetable oil for extra sticking power (neem oil adds pest-fighting properties)
• Don’t use detergent — household dish detergent (especially Dawn Ultra) contains chemicals that damage plants
• Store in a cool, dark place — premixed solution lasts about 2 weeks
• Label your bottle — sounds obvious, but when you have multiple garden sprays, you don’t want to mix them up

Ready for a specific recipe? Start with the Basic Castile Insecticidal Soap Spray — the recipe I use most often in my own garden.