It may not be fall yet, but kids (and teachers) are getting ready to head back to school which means an end to summer, and thus the start of fall. Plants think so, too. For them, the nights are getting longer, the suns rays slant in the sky, plus the ground starting to cool off. Plants that go winter dormant are already starting to store what they need for next year in their roots. Yes, even desert plants go dormant, or at least slow down, in winter.  

To help your plants get ready for fall and winter, now is a good time fertilize. Now while it is warm and any new growth you encourage with fertilizer will have time to toughen up and withstand winter, especially if we have another icy one like 2011.  

Fall fertilizer can also help plants if they got too much water over the summer and are suffering minor nutrient deficiencies. All fertilizer labels display three numbers in a ratio to one another. (It’s not just a good idea, it’s the law.) The three numbers are always in the order of nitrogen, phosphorous and potassium. In chemical symbols, that’s N : P : K.  

Nitrogen is used to make chlorophyll and proteins. Nitrogen stimulates vegetative growth. Deficiency symptoms are pale green or yellow-green leaves and dwarf or stunted plants.  

Phosphorous is key for flower and fruit formation and required for healthy stems and leaves. Excessive fruit drop or leaves and stems that are stunted with purple or red discoloration are signs of deficiency.  

Potassium is needed for general growth and development, including the thickness of plant cell walls. This is important to help plants resist the stresses of heat, cold, wind, drought, insects, and disease. Deficiencies are indicated by weak stems and yellowing or browning of leaves at the tips. 

Armed with this knowledge, look again at the three numbers presented on the fertilizer package. High nitrogen fertilizer is best for plants grown for their leaves, like lawns and palms. High phosphorous fertilizer, also called bloom food, is especially important for flowering and fruiting plants like lantana and citrus. Potassium is useful for all plants, but especially important for newly planted landscape plants.   

Fertilize your landscape plants three times per year for good growth and plant health. You can fertilize more often, but not all my readers are avid gardeners. For most homeowners, you should ideally fertilize in spring, summer and fall. Here in the Old Pueblo, go for St. Patrick’s Day, Memorial Day and Labor Day.  

One major exception to needing fertilizer is the legume or pea family. No need to fertilize legumes, they make their own with the help of bacteria in their roots. Applying fertilizer can actually stunt their growth. This includes mesquite, palo verde, acacia, Mexican bird of paradise and many more.  

Always read and follow label directions for any chemical compound, and this includes fertilizers. Too much fertilizer can kill the plant you want to help. When in doubt, err on the side of caution. You could apply half-strength fertilizer two times, waiting two weeks between doses. Caution is also good with slow growing succulent plants, half-strength fertilizer is often enough for their needs.

Lavish a little fertilizer on your plants this fall, deficient or not. You can think of it as their back-to-school supplies.


(Editor’s Note: Fall means Soule returns to Pima County libraries with a number of free lectures. After each talk she will sell and sign her local gardening books. More information can be found at and on her Facebook page, Gardening With Soule.)

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