Spray Rainfastness & Reapply Decision

It depends on the product. A systemic (absorbed) product is largely rainfast in about an hour, so an hour's gap usually means little loss and no reapplication. A contact product that only sits on the surface needs roughly six hours of dry time, so an hour before rain can wash off a large share and warrant a respray. Enter your product type, the hours before rain and the rainfall amount and the tool gives the wash-off % and the verdict.

The rain-free period is the minimum dry time a freshly applied spray needs before rain so the active ingredient is absorbed into or firmly bound to the leaf and can no longer be washed off. Systemic products need about 1 hour, translaminar/local-penetrant products about 2 hours, and contact/residual products about 6 hours. Beat the rain-free period and rain causes little loss; miss it and a meaningful fraction washes off.

It models how much of the active is still on the leaf SURFACE (and therefore washable) when the rain arrives, using the product's absorption half-life and the hours that passed, then multiplies by how aggressively the rainfall amount strips the surface residue. The worst case — a contact product, rain within minutes, and a downpour — approaches about 90% loss, while a fully absorbed systemic approaches near-zero loss, matching extension guidance that loss is negligible once the rain-free period is met.

Yes. Systemic and translaminar products move into the leaf, so once that uptake has happened rain cannot wash them out — they become rainfast quickly (roughly 1–2 hours). Contact and residual products such as chlorothalonil, mancozeb, copper and sulfur protect only where they were deposited and never move inward, so rain physically washes them off and they need the longest dry period before a shower.

Even a trace or drizzle (under about 2 mm) removes only a small fraction, a light shower (2–6 mm) more, and moderate rain (6–15 mm), heavy rain (15–30 mm) or a downpour (over 30 mm) progressively more of whatever active is still surface-bound. The amount matters most when the rain beats the rain-free period; after the product is rainfast, even heavy rain causes little additional loss.

Yes. A sticker/spreader or oil adjuvant speeds drying and binding and leaves a more rain-resistant film, so it shortens the rain-free period and trims the wash-off. In this tool, ticking the adjuvant box reduces both the dry time needed and the surface wash, which can move a marginal situation back into the safe range. Confirm adjuvant compatibility and rate on your product label.

The tool advises reapplying when the estimated wash-off is high enough that efficacy is compromised (about 30% or more), monitoring when the loss is marginal (roughly 15–30%), and taking no action when only a little washed off. On a marginal call, do not respray blind — scout the field in a few days and reapply only if the pest or disease rebounds, which saves an unnecessary pass and money.

Rainfastness is about a single application surviving rain in the hours just after spraying. The re-treatment interval is the minimum time the label allows between repeated applications of the same product, set for crop safety and residue limits. If you must reapply because rain washed a spray off, check the label's minimum re-treatment interval and maximum number of applications before respraying.

The clock effectively starts when the spray is deposited and begins drying. A spray that has dried onto the leaf is partway to rainfast; one still wet on the surface is the most vulnerable. The hours-before-rain input represents the time from application to the start of rainfall, which is what the model uses to estimate how much active had already been absorbed or bound.

No. The label is the legal authority for the rainfast statement, rate, re-treatment interval and use restrictions, and rainfast periods vary by formulation and adjuvant. This calculator gives a science-based estimate from extension rainfastness research to help you decide, but you must confirm the rain-free period and any re-treatment limits on the actual label you hold before respraying.

Rainfastness is only one cause of failure. Poor coverage, the wrong product or rate for the target, spraying weeds or disease that were already too far advanced, hard water antagonising the active, or resistance can all reduce control regardless of rain. If the rain-free period was clearly met and wash-off was low, look at coverage, timing and product choice rather than reapplying the same way.