Plant Growth Regulator & Which PGR, What Rate, What Stage?

Every registered PGR has a legal label rate band — a minimum and maximum for each crop and target. You choose the product, then a 0–100% intensity dial sets the applied rate inside that band: rate = low + (high − low) × intensity. Trinexapac-ethyl (Moddus) on wheat for lodging, for example, has a 200–400 mL/ha band, so the 50% dial gives 300 mL/ha. The tool then multiplies by your area for the product volume and by the carrier rate for the spray water.

For cereals the main anti-lodging PGRs are chlormequat chloride (Cycocel/5C, ~1.0–2.0 L/ha at GS 30–32), trinexapac-ethyl (Moddus, ~0.2–0.4 L/ha at GS 31–37) and ethephon (Cerone/Terpal, ~0.5–1.0 L/ha at GS 37–45). Trinexapac and ethephon shorten and stiffen the upper internodes; a chlormequat-plus-trinexapac split suits tall, lush, high-risk crops. Timing is critical — too late risks ear damage.

Mepiquat chloride (Pix/Mepex, 4.2%) is usually applied as low-rate split sprays totalling roughly 0.25–1.2 L/ha from early square to first flower, with the higher end reserved for rank, well-watered, vigorous cotton. Drought-stressed cotton does not need a PGR and can be set back by one — only treat a crop that is genuinely too vigorous, and start low.

Paclobutrazol (Bonzi/Piccolo, 0.4%) is applied during active vegetative growth, before bract colour in poinsettias or one to two weeks after pinch in chrysanthemums, at roughly 5–60 ppm. It is highly active — start at the low end (5–15 ppm) and re-apply if needed, because over-dose hard-stunts plants and shrinks bracts or flowers. Spray runoff onto the media gives a stronger, longer effect than a foliar spray.

Gibberellic acid (GA₃, ProGibb) sizes seedless grape berries at roughly 20–60 ppm around bloom to fruit-set and improves citrus set and rind at ~10–25 ppm. NAA (naphthaleneacetic acid) improves tomato fruit set in cool weather at ~25–50 ppm applied to open flowers. These are set/sizing PGRs, distinct from the height and lodging chemistries.

NAA (Fruitone) is a chemical thinner applied at the 8–14 mm fruitlet stage at roughly 5–20 ppm; it causes the tree to abscise surplus fruitlets, improving size and return bloom. Over-thinning is irreversible, so start at the low end, watch the weather window and don't blindly stack thinners. The tool flags the over-dose risk as you push the dial up.

When the dial drives the rate to the top of the label band (≥ 85%), the tool flags an over-dose / phytotoxicity risk — for PGRs, too much is often worse than too little because over-regulation stunts the crop, deforms fruit or scorches foliage and can't be undone. Mid-band rates are the typical effective dose; only go high on tall, lush, high-risk crops.

PGRs act on actively elongating or differentiating tissue, so the same product gives the wanted effect inside a narrow stage window and a damaging or useless effect outside it. Cereal lodging PGRs work on internode elongation around GS 31–37; ethephon ripeners need fruit at the breaker stage; thinners need a specific fruitlet diameter. The on-screen timeline shades the correct window against the crop's stages.

The tool multiplies the chosen rate by your area for the total product (e.g. 300 mL/ha × 10 ha = 3 L of Moddus) and by the label carrier rate for the spray water (150 L/ha × 10 ha = 1500 L). Always calibrate your sprayer to actually deliver that volume per hectare, and use clean water at the label water rate for good coverage.

They are representative label-range planning values compiled from product labels and extension PGR-use guides; registered rates, crops and formulations differ by country and by product concentration. Use the tool to choose the chemistry, the rate band and the stage window, then confirm against the registered label for your specific product before mixing.

No — PGRs are preventive, not curative. They must go on before the problem (before lodging, before the crop gets too tall, before fruit is set) and never on a crop under drought, frost or heat stress, which a PGR will worsen. If a cereal is already leaning, a PGR can't stand it back up; plan the application by stage in advance.

It covers 16 crop-target programs across twelve crops — wheat, barley, rice, cotton, grape, apple, tomato, poinsettia, chrysanthemum, turf, sugarcane and citrus — using seven chemistries: paclobutrazol, chlormequat, trinexapac-ethyl, ethephon, gibberellic acid, mepiquat chloride and NAA, each with its label rate band, stage window and caution in the reference table below.