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A growth regulator is a natural or synthetic compound that regulates plant or insect growth. A growth retardant is a natural or synthetic compound that reduces or stops plant growth. A plant growth regulator (PGR) can be naturally produced by the plant (a hormone), or synthesized and applied to the plant. Poinsettias are often sprayed with PGRs before sale, to keep the plants small enough to be shipped without excessive plant breakage.