Should I Fertilize Before Heavy Rain?

[addendum 10/23/21]:

A Facebook friend wrote:

Q: Interesting perspective, Paul – how does this differ for time release fertilizers?

A: Time-release pellets slowly dissolve over a period of weeks or months. In a 5-inch downpour (as is predicted for 10/24/21) the pellets will dissolve more slowly than granular fertilizer. I’d still hold off until the rain relents and the soil drains somewhat, however. Any phosphorus / nitrogen runoff into gutters and storm drains is undesirable.

Water-soluble/granular fertilizers — ones with roughly the consistency of dust — are the ones absolutely to be avoided until we see a 10-day forecast of clear weather.

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Lamorinda is forecast to receive close to four inches of rain between Tuesday afternoon (January 26th) and February 2nd. Heaviest rain will be Tuesday through Thursday morning, but it’ll probably rain every day for 7 days (we might get a brief break on Saturday). We haven’t had a storm like this since the winter of 2017/18. The ground will be saturated by Wednesday night, and water will begin pooling and running off. There may be flooding.

As Carrie Bradshaw used to sagely intone, I got to thinking.

Isn’t it a great idea to fertilize right before a heavy rain? Think of all that plant food soaking down into the root systems.

No. It’s probably not the best idea.

Lawns, especially, use up a lot of fertilizer, and a lot of it runs off into storm drains in heavy rains. Large amounts of nitrogen and phosphorus aren’t good for bodies of water (where most storm drains empty). It causes algae blooms, dead zones, and other bad things.

Besides, you want the fertilizer to collect around the roots and stay there. You don’t want it diluted by nearly four inches of rain.

So hold off on the plant food until things dry out a bit. Container plants, especially, have all the nutrients flushed out by extended, heavy rains. So make sure your potted outdoor plants get some food. 0-10-10 fertilizer is good at this time of year.

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