Pesticides or Fertilizers: Just the Right Dose


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“If a little is good, more must be better!” That seems to be the motto of some gardeners who double or even triple the recommended concentration of pesticides, thinking they’ll be more successful in controlling pests or weeds. But they’re very wrong. Nearly all pesticides, even the safest ones, like insecticidal soap and horticultural oils, are phytotoxic (poisonous to plants) if used at excessively high concentrations. And any excess product is likely to eventually find its way into the water table and the food chain. The general rule is to only use pesticides when they are really needed and, when you do, to follow the application guide to the letter.

And don’t imagine that if you dilute a pesticide more than is recommended, you’re helping the environment. It will simply not be effective, leading you to apply more in the future. Again, apply any pesticide at the recommended rate, no more, no less.

To avoid making mistakes, it is always wise read the label every time you use a pesticide: your memory may not be as good as you think.


The situation is similar with fertilizer, at least, when it concerns applying a greater concentration. Never increase the recommended dose of any fertilizer or you could burn the roots of the plant to which it is applied. Furthermore, minerals applied too generously eventually end up polluting rivers, lakes and underground water sources. Also, as many gardeners know, excess nitrogen (the first number on the label of any fertilizer) stimulates fast but weak growth in plants, leaving them susceptible to insects and other predators.

Unlike pesticides, however, you can reduce the application rate of fertilizer without danger. In fact, the rate recommended by the supplier is often calculated to encourage you to use a maximum amount of their product, forcing you to use it up quickly so you’ll buy more. Experiment if you want, but you can usually get beautiful results by applying any fertilizer, especially a fertilizer in which the percentage of one or more of the 3 minerals listed is greater than 15, at a quarter of the recommended rate.

Article originally published on November 12, 2015.