Anytime between mid-September and mid-October is prime time to fertilize the lawn, trees, shrubs, vines and groundcovers. Fertilizing while it is still warm is best because plants are still in their active growing season and taking up the nutrients provided.
Here’s a repost of DeAnna’s fertilizing guide from earlier this year:
“Anytime between mid-September and mid-October is prime time to fertilize the lawn, trees, shrubs, vines and groundcovers. Fertilizing while it is still warm is best because plants are still in their active growing season and taking up the nutrients provided. Once the weather starts cooling down plants begin going into dormancy and therefore are not as likely to take up the nutrients needed for the fall and winter. Remember to water in the fertilizer with at least a ½ inch of irrigation. Never depend on the rain to water in your fertilizer, because as you know, it never rains when predicted, and also never gives us that perfect ½ inch. It either rains too much, too little, or not at all. Making sure the timing is right and proper amount of water is applied assures your lawn of getting the full benefit of the nutrients it needs to maintain healthy leaves and roots for the dormant season. We carry our Gill Lawn and Garden and also several organic lawn fertilizers. 
Never use Weed and Feed if you have any trees or shrubs in the yard. Trees are a broadleaf just like a weed, so it will try to kill them as well. Weed killers and weed preventers can be used in October once our temperatures moderate in the 80’s. We will talk about them more as the season approaches.”
-DeAnna
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