Lime sulfur spray is made by boiling sulfur powder and hydrated lime (calcium hydroxide) in water until the mixture turns a deep reddish-orange color. The process takes about an hour of active cooking outdoors and produces a concentrated liquid you then dilute for use as a dormant-season fungicide and insecticide on fruit trees, berry bushes, and grapevines.
Ingredients and Ratios
The standard recipe uses a roughly 2:1 ratio of sulfur to lime by weight. A USDA technical report lists an optimized concentrate formula as 80 pounds of sulfur, 36 pounds of pure lime (calcium oxide), and 50 gallons of water. For a home gardener, that scales down to something far more manageable.
A practical small-batch recipe:
- Sulfur powder (flowers of sulfur): 1.6 pounds (about 725 grams)
- Hydrated lime (calcium hydroxide): 0.8 pounds (about 360 grams)
- Water: 1 gallon
Use agricultural-grade sulfur powder, sometimes sold as “flowers of sulfur” or “dusting sulfur.” Hydrated lime is widely available at garden centers and farm supply stores. Do not substitute garden lime (calcium carbonate), which won’t react with the sulfur. You need calcium hydroxide specifically.
Step-by-Step Cooking Process
This must be done outdoors. The boiling mixture releases hydrogen sulfide gas, which smells like rotten eggs, irritates your lungs, and can form explosive mixtures with air in enclosed spaces. Never cook lime sulfur in a kitchen, garage, or greenhouse.
Start by bringing your gallon of water to a rolling boil in a stainless steel or enamel pot. Avoid aluminum, as the sulfur solution will corrode it. While the water heats, mix the sulfur powder and hydrated lime together in a dry bucket so they’re evenly blended.
Once the water is at a full boil, slowly add the dry sulfur-lime mixture while stirring constantly. The mixture will foam and bubble aggressively at first. Keep stirring. Reduce the heat to maintain a steady simmer, not a violent boil, and cook for 45 to 60 minutes. Stir every few minutes to prevent the sulfur from settling and clumping on the bottom.
As the reaction progresses, the liquid changes color. It starts out as a murky yellow-green slurry and gradually shifts to a deep reddish-orange or dark amber. This color change tells you the sulfur and lime are combining into calcium polysulfide, the active compound. When the liquid has reached that consistent dark reddish-orange and the undissolved sediment at the bottom is minimal, the cook is done.
Let the pot cool, then strain the liquid through cheesecloth or a fine mesh strainer into glass or plastic containers. The sediment left behind is inert and can be discarded. What you have now is lime sulfur concentrate.
Safety Precautions
Lime sulfur is corrosive and the fumes produced during cooking are genuinely hazardous. Hydrogen sulfide gas is released throughout the boiling process, and continued heating also produces sulfur dioxide. Both irritate the eyes, skin, and respiratory tract.
At minimum, wear chemical-resistant gloves, safety goggles (not just glasses), long sleeves, and old clothes you don’t mind staining. A respirator rated for acid gases is strongly recommended, especially during the most active phase of boiling when the mixture is foaming. Stand upwind of the pot whenever possible. If the wind shifts and you catch a face full of fumes, step away immediately.
The finished concentrate will stain concrete, wood decks, painted surfaces, and fabric a yellowish-brown that’s nearly impossible to remove. Work on a surface you don’t care about, and store the concentrate in clearly labeled containers away from children and pets.
How to Dilute for Spraying
The concentrate is far too strong to spray directly on plants. Dilution rates depend on whether you’re applying during dormancy or during the growing season.
For dormant-season applications on fruit trees and deciduous shrubs, a common dilution is roughly 1 part concentrate to 9 or 10 parts water. University of Arkansas extension guidelines suggest 10 gallons of lime sulfur concentrate per 100 gallons of water for dormant grape sprays. For blueberries, they recommend about 5 gallons of product in 50 to 70 gallons of water, which works out to a similar ratio.
For growing-season use on actively leafed-out plants, you need a much weaker solution, typically 1 part concentrate to 30 or 40 parts water. The active compound works by breaking down into elemental sulfur on leaf and bark surfaces, which disrupts fungal spores on contact. At dormant-season strength, this same action would burn living foliage.
If you’re unsure of your concentrate’s strength (homemade batches vary), start with a weaker dilution and test on a few branches before spraying an entire tree.
What It Controls
Lime sulfur works as both a fungicide and an insecticide. The calcium polysulfide decomposes into elemental sulfur on plant surfaces, creating a protective barrier against fungal infections like apple scab, powdery mildew, brown rot, and black knot. It also softens the waxy coating that protects scale insects, effectively smothering them. This makes it useful against overwintering scale, mites, and their eggs on dormant wood.
It’s most effective as a preventive spray applied before problems start, not as a cure for active infections. The standard timing is late winter to early spring, after the coldest weather has passed but before buds break open.
Plants to Avoid Spraying
Some plants are highly sensitive to sulfur-based sprays. Apricots are notoriously intolerant and can suffer severe damage even from dormant applications. Many Japanese maple varieties, some plum cultivars, and certain ornamental plants also react poorly.
Never apply lime sulfur within two weeks of an oil-based spray (such as horticultural oil or neem oil). The combination causes severe phytotoxicity, essentially chemical burns across every sprayed surface. If you’ve already applied oil, wait at least 14 days before using lime sulfur, and vice versa.
Temperature matters too. Avoid spraying when temperatures are expected to drop below freezing within 24 hours, or when daytime highs will exceed 85°F. Both extremes increase the risk of plant damage.
Storage and Shelf Life
Homemade lime sulfur concentrate keeps for one to two seasons when stored in a tightly sealed, non-metal container in a cool, dark place. Over time, the calcium polysulfide breaks down and the solution loses potency. If your stored concentrate has lost its strong sulfur smell or has turned pale yellow instead of dark amber, it’s likely too degraded to be effective. Make a fresh batch rather than doubling the application rate of old concentrate, which risks burning plants without improving disease control.