Many garden tasks can be done almost any time. I’ve transplanted shrubs in July and in December and had success. I fertilize my St. Augustine lawn three times a year but I don’t do it on exact, specific dates.
On the other hand, there are two tasks that must be done before a specific date to get best results: your spring and fall pre-emergent (click for sources) applications.
A pre-emergent is a chemical that prevents seeds from germinating. If you wait until after they sprout, the pre-emergent doesn’t work.
Therefore, it’s important to know when winter weeds (chickweed, henbit, annual bluegrass) sprout and when summer weeds (crabgrass, etc) germinate.
Here’s all you need to know for the metro Atlanta area:
Apply pre-emergent between Sept. 1 and Sept. 15 to control winter weeds.
Apply pre-emergent between March 1 and March 15 to control summer weeds.
All of the following work well:
benefin (Amaze, Balan, Crabgrass Preventer)
dithiopyr (LESCO 0-0-7, Pike Crabgrass Preventer, Dimension, Crab-Ex)