"Should I be applying pre-emergent weedkiller granules now? With the warm weather, I see lots of weeds everywhere. Some of them have little purple flowers starting to form."
NO! Those weeds that you're seeing now are COOL-season weeds. They germinated back in the fall. Pre-emergent weedkillers do just what their name says: they kill weeds before they ever emerge (which means germinate). If you see weeds now, you should have applied your pre-emergent weedkiller last fall.
Here is the schedule. I'm going to tailor it for DFW and the I-20 corridor as my default. Read carefully.
Are you way south of I-20? Move the spring dates one week earlier for each 100 miles you are south of I-20. Move the fall dates later for each 100 miles you are south of I-20.
Are you way north of I-20? Move the spring dates one week later for each 100 miles you are north of I-20. Move the fall dates one week earlier for each 100 miles you are north of I-20.
Remember that your goal is always to apply the pre-emergent BEFORE you can see the weeds -- before their seeds sprout. If you can see the weeds growing, you've blown it.
Spring application (primarily for crabgrass and grassburs): March 5 -- 15. Halts or Dimension.
Second spring application (due to long growing season and possibility that crabgrass and grassburs could still germinate): June 5 -- 15. There is no point in making this application if you did not make the first one. Halts or Dimension.
Fall application (primarily for cool-season grassy weeds such as annual bluegrass, rescuegrass and rye): September 1 -- 10. Halts or Dimension.
Fall application (primarily for broadleafed weeds such as henbit, dandelions, clover and chickweed): September 1 -- 10. Gallery. This treatment can be made the same day as the application for grassy weed prevention, but do them sequentially. Do not try to mix granules in your spreader.
Any time you intend to apply pre-emergent weedkillers and fertilizers at roughly the same time, apply the fertilizer first. Water it into the soil deeply. Wait a couple of days, then apply the pre-emergent granules and water lightly.