Why Fall Fertilizing is Important

The science behind why fertilizing in the fall is important.

Agastache x 'Ava'
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  • Topic: Fall Gardening
  • Author: Cindy Bellinger
  • Keywords: fall, fertilize, cold-hardy, roots, Fertilizing
  • Date: October 2003

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Well, here we are. Our gardens are fading and returning to the earth. They’re deep in the heart of fall and preparing for winter. Dieback is hard to take sometimes. It’s distressing. Everything looks dead. But dormant isn’t dead. What’s happening is hidden because it’s all taking place in the stems and twigs, roots and trunks.

When the cooler and shorter days come, they begin sending a signal to plants to slow down on the above ground growth. Now they are to concentrate on what’s underground—the roots. This makes fall an excellent time for fertilizing.

Dirk Muntean, a biology consultant for Soil and Plant Laboratory, Inc. in Bellevue, WA, explains it this way: “Those roots are taking up nutrients in the soil throughout the fall until the soil temperatures drop low enough to inhibit root function.”

He says fall fertilizers improve the cold-hardiness of plants and it all has to do with chemistry.

When temperatures drop to around 40 degrees F, the physiological and biochemical changes in plants become stimulated. They begin producing higher concentrations of dissolved sugars, amino acids and other soluble organic molecules. Then the cells create concentrations of chemically-bound water. This aids the elasticity of the protolasm, which in turn makes a plant resilient during freezing.

Fertilizing in the Fall is Different

Plant needs are different now than in the spring and summer. In warmer months they are busy leafing and blooming. In the fall, with all their energy going to the roots, plants don’t want food that stimulates the upper growth. This time of year their needs require fertilizers low in nitrogen because nitrogen inspires above ground vegetative growth.

What you want now is a fertilizer rich in phosphorus and potassium. Look on the bag or box for the set of three numbers such as 2-3-4. These indicate the ratio of nitrogen, phosphorus and potassium (sometimes called potash) in the mix and are referred to the NPK.

  • N = nitrogen
  • P = phosphorus
  • K = potassium.

Nitrogen is the first number. For a fall fertilizer make sure it’s small, somewhere between 2 and 5.

We suggest using Yum Yum Mix® and Earth Juice, both of which are low in nitrogen but rich in organic and natural nutrients. We also suggest Gro-Power Flower and Bloom with a 3-12-12 ratio. Planters II, a natural trace mineral fertilizer, provides over 30 trace elements and should be applied in the fall. High Country Gardens is a strong advocate of using natural and organic fertilizers to ‘feed the soil’ which in turn converts nutrients into usable forms for root absorption. A healthy, fertile soil is the key to gardening success.

Lightly scratch granular fertilizers into the top of the soil and where appropriate, apply a nice layer of mulch over the top. Water the fertilized areas thoroughly if fall rains are lacking.

Also, don’t forget to fertilize your lawn. October and November are the best months for this. For specific tips on fetilizing lawns, see the High Country Gardens website.

If plants have developed chlorotic foliage (yellow tissue between green veins) over the summer, they are showing signs of diminished iron uptake. Fertilize now with Greensand and water it in with the Saltwater Farms Liquid Seaweed to increase root absorbtion. Trees such as Aspen, Purple Leaf Plums, Peaches and Wisteria commonly exhibit chlorosis. Greensand is an all-natural iron source rich in iron and potassium that helps “green-up” the plants next summer. Note that it may take several years of greensand applications to correct severe chlorosis, but it will happen.

The best time to fertilize in the fall is after a few light frosts or when trees begin dropping their leaves. Though it may sound odd to feed plants now, an ample supply of fertilizer in the fall will give plants a head start when it’s time to grow in the spring.