Fertilizer and Weed Control
In general, applying lawn fertilizer is best done near the beginning of the growing season, or mid-spring. May and June are good months for this first application. You should also apply in early fall, in September or October. When applying fertilizers, your lawn should be clear of any heat or cold stresses such as heavy frosts or the hot afternoon sun of midsummer. Of course there are exceptions to this general rule. Different areas and grass species require different applications.
Like fertilizers, not all weed killers are created equal. Pre-emergent herbicides kill the weeds before they emerge from the ground and as such should be applied early in the season before these weeds take over. Post-emergent herbicides kill the weeds once they have come up and should be applied when you see the weeds.