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What Does ‘Rainfast’ Mean?

Posted in Garden Advice on December 23, 2019

What is the rainfast period of a spray product? 

You may find on labels and webpage details of products the word ‘rainfast’ as in this product is rainfast after 2 hours. This usually refers to spray products that are applied to exterior surfaces; walls, fences, decks, plants or lawns. And the products are to be effective for extended periods so need to adhere to, and remain on, the surfaces.

Simply, rainfast means not washed off by the rain. So, if a product states that it will be rainfast after two hours this means that in normal conditions the spray will not be washed off by a shower of rain, or in the case of plant applied systemic products the actives have been absorbed by the plants. However, the length of time before a spray becomes truly rainfast on a surface will depend on several variables which determine the drying and curing time of the product on the surface; the weather conditions, the surface texture, the amount of spray applied and the formulation of the product.


  • In warm conditions, the spray is likely to dry and cure in a shorter period, while cool conditions will lengthen the drying/curing time.
  • Humidity will influence drying; humid conditions will extend drying times while surfaces will dry faster in dry, low humidity.
  • Windy conditions will speed up drying, and still, conditions will extend drying times.


  • Porous surfaces will absorb spray liquid and the roughness will increase surface area and speed up drying, plus the spray will adhere to the surfaces more efficiently.
  • Non-porous surfaces may repel sprays and have reduced adherence and the spray will often take longer to dry.

Spray volume

  • Light sprays will dry off more quickly than heavy sprays. Often, two light sprays are better than one heavy spray; take the rain fastness time from the application of the second spray.


  • Depending on the product may be formulated to have shorter or longer drying/curing times; Insecticide sprays such as NO Bugs Super and PLANThealth Spectrum are formulated to dry quickly, and systemic herbicides such as Weed Weapon Extra Strength are formulated for rapid absorption so that rain in the garden is not likely to wash it off. But NO Borer Wood Protection is formulated to dry slowly, giving time for the active ingredients to penetrate deep into the timber where it controls the borer larvae eating the wood.

Note also that the severity of the rain falling after the rainfast period will influence the rain fastness. Products that say rainfast in 2 hours will, in normal conditions, not be washed off by a shower of moderate rain that affects the sprayed surfaces 2 hours after application. However, if the rain is torrential, driving rain this is likely to remove some, or all, of the spray from the surface.

Contact Kiwicare if you need advice on application and rain fastness of a product and/or surface.

David Brittain

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