How to Mix Neem Oil for Plants Without Leaf Burn

Two different ingredients are sold as neem. Azadirachtin, extracted from neem seed oil, does most of the killing and repelling by disrupting the hormones that control insect growth and reproduction. What remains after extraction is clarified hydrophobic neem oil, the active ingredient in the ready-to-use sprays sold at garden centers, and it kills insects by suffocation, coating them with oil that blocks their breathing openings.

Neem is most effective against immature insects, while adults are not typically killed and may keep feeding and laying eggs.

The label carries the rate, the protective clothing and the first-aid directions, which is why the EPA's pesticide label page tells users to always read it before mixing.

Find the line on the label that matches your plant and the pest or disease you identified, then copy down four numbers from it.

Scale the product in the same proportion as the water. If the label gives an amount per gallon and your sprayer holds half a gallon, use half the labeled amount. For an odd-size tank, multiply label amount x actual batch volume / label water volume, with every volume in the same unit.

Use a marked syringe, medicine cup, or measuring spoon that will never go back to food preparation.

Oils can burn foliage, and a stronger mix does not help when damage looks severe. For horticultural oils, UNR Extension warns that a mix at dormant-season strength, sprayed after leaves have emerged, may kill the young leaves, so use the rate the label gives for the season.

The amount per gallon comes from the label of your product and your use, because neem products carry one of two different active ingredients and the page gives no rate that fits both.

A label that lists spoons per gallon is also giving a percentage of the finished spray. One US tablespoon is about 14.8 mL and a US gallon is about 3,785 mL, so 1 tablespoon per gallon is about 0.39 percent product by volume, and a 1 percent mix by volume is 10 mL of product in every liter of finished spray.

UConn Extension gives the same pairing for horticultural oils, not for neem. It lists 2 percent, or up to 5 tablespoons of concentrate per gallon of water. Five tablespoons is about 1.95 percent, so a percentage and a spoon count can describe one spray.

Scaling the rate to a smaller sprayer

A US gallon holds 4 quarts or 8 pints, so a smaller fill takes the same fraction of the per-gallon amount printed on your label.

One tablespoon equals 3 teaspoons, and 2 tablespoons equal 1 US fluid ounce. A US gallon is 3.785 liters, so a 4-liter jug holds slightly more, and an Imperial gallon is 4.546 liters.

Oil and water separate unless an emulsifier holds them together, which is why spray oils are formulated with an emulsifying agent and why a label's mixing order and agitation directions matter. Use the label's sequence whenever it differs from the steps below.

Put on the protective clothing the label names before you open the bottle. Neem oil can be slightly irritating to the eyes and skin, and azadirachtin can be very irritating to the skin and stomach. Mix in a clean sprayer that has never held an herbicide, away from food, children, pets, ponds and storm drains.

An emulsifiable concentrate already contains an emulsifier, and household soaps contain chemicals likely to cause foliage burn, according to UConn Extension, so skip the soap unless the label calls for it.

If an oil layer or clumps persist after the agitation the label calls for, stop and recheck the water volume and the label directions instead of adding household detergent.

Neem can burn the foliage of any kind of plant, yet the plant has to be completely covered for the spray to work. The way through is testing a small area first, and if no toxicity symptoms appear in that spot, the whole plant can be treated.

The symptoms to look for are the browning or burning of leaves that horticultural oils can cause. Mark the sprayed leaf and check it for the length of time the label gives before you spray the rest.

Different plants show sensitivity at different times of the year, so a test is worth repeating at another time of year. Colorado State Extension says neem insecticides have shown good plant safety on greenhouse ornamentals but calls for precautions on these plants.

Neem oil works on contact, and the oil has no effect after it has dried, so only the insects wetted during the spray are reached. That is why coverage has to include both the upper and lower surfaces of the leaves.

Results are not immediate and reapplication may be necessary, so the pest's life cycle needs watching when you time a spray. Repeat applications may be needed at seven- to ten-day intervals for fungi and more often for insects, according to UNR Extension, and your label's interval overrides both.

Neem is practically non-toxic to bees when applied late in the evening or early in the morning while bees are inactive, so spray then. Horticultural oils do not affect pollinators once the spray has dried.

Neem can still harm beneficial insects as well as pests, so spot-treat the infested shoots in a bed planted for beneficial insects. For houseplants, the houseplant neem guide covers where to spray, and your label must permit indoor use.

Neem can burn foliage on any plant, and horticultural oil guidance lists the conditions that raise the risk.

These figures come from horticultural oil guidance, so the figure on your neem label governs.

Dry leaves help, but the plant should also be well irrigated, so water it ahead of time and spray after the foliage has dried.

Neem products may be registered for fruits, herbs and vegetables as well as woody plants, but each label lists its own crops and pests. Neem should be applied only when the pest is listed on the product label.

On a food crop such as tomatoes, the crop list and the days before harvest on the label decide whether you can spray at all.

Match what you see to its likely cause before you change the rate.

Results are not immediate, so judge a spray by pest numbers at the next check and by the label's interval before you change the rate. If aphids keep coming back, fix the coverage and the cause first.

Some symptoms that look like pests are diseases. Neem can help limit the spread of powdery mildew but will not cure tissue that is already infected, so powdery mildew on roses needs a label that lists fungal use. For other low-toxicity options, see other low-toxicity controls.

Make only the amount you will spray, and never pour mixed product back into the concentrate bottle. Azadirachtin breaks down quickly, with a half-life in water of 48 minutes to 4 days, so a leftover mix of an azadirachtin product may lose strength while it waits.

Keep rinse water and leftovers out of storm drains, ponds and streams, because neem oil is slightly toxic to fish and other aquatic animals. Follow the label's directions for rinsing the sprayer, storing the concentrate, and disposing of unused product.

Write down the product, the batch rate, the plant and the date. That note lets you respect the label's interval before the next spray and trace the cause if leaves react.