And herbicides have been proven to be effective at killing existing weeds and deterring the growth of new weeds. This is also a good time to apply lawn fertilizer. May-JuneApply lawn fertilizer in mid-summer, if you didn’t do so in the spring. You can also spot-kill individual weeds and weed clumps by using a pump-up sprayer to spray herbicide directly onto the weeds. Spread a layer of bark mulch over areas where grass doesn’t grow, but weeds do, such as around shrubs and beneath trees.