By Trish Grenfell, UC Master Gardener of Placer County
Q I was told that fertilizer was bad for plants in the late fall, but is that true of all plants? What should I be fertilizing now?
A Yes, some people believe that fall fertilization inhibits a plant’s ability to harden off and survive a cold winter. And remember that trees and shrubs growing in their natural habitat survive very well without any supplemental fertilization. Leaves, needles, and fallen branches decompose on the ground and recycle nutrients. But when landscapes are kept neat, these natural “fertilizers” are removed. The most accurate way to determine if any amount of fertilizer is necessary is to do a soil test so that adequate amounts of nutrients are applied without risking toxicities (i.e., one nutrient over applied can affect the availability of other nutrients). You can purchase soil testing kits at your local nursery which check for nutrients as well as pH status.
Early November, before winter sets in, is a good time to fertilize for most plants. But now you want to strengthen your plants' and lawn's roots—not promote above ground growth. You want to give them a strong base on which to thrive next spring. Even though the tops of plants have gone dormant or have slowed down, fall is an active period for roots. They are still growing and absorbing nutrients for next year’s spring flush until soil temperatures inhibit biological activity. Many studies have shown that if you fertilize your lawn only once a year, fall is the best time to do it.
If your soil test does not show an adequate amount of phosphorus or potassium, you can stimulate root growth by adding them now. If you apply mulch around your plants, it would be a good idea to add low levels of nitrogen also to compensate for the nitrogen used in the mulch decomposition process. Since excess nitrogen (roots don’t need it) is mobile, adding more will only result in the nitrates leaching in the winter rains and contaminating the groundwater.
Since fall is the time to plant bulbs, remember to add phosphorus to promote their root growth, insuring strong flowering in the spring. Bone meal in the planting hole is an excellent organic choice.
If plants have developed chlorotic foliage (yellow tissue between green veins) over the summer, they are showing signs of diminished iron uptake. Now is the time to add iron.
If your soil test shows an inappropriate pH, add lime to increase alkalinity or sulphur to increase acidity.
The last step is to work the fertilizer into the soil with a rake/hoe or apply mulch on top. You don’t want the nutrients to wash away with the rains.