Some weeds thrive in the hot months of summer when our lawns are at their weakest.  Most desirable turfs use seeds that do well in the cooler months of the spring and fall, and when there is excessive heat in the summer, they go dormant and turn brown.  This is when heat loving weeds strike. Weeds like crabgrass, goosegrass, foxtail, and purslane pop up in the dried out turf during the more hostile climate conditions, and can thrive with little water and the sun beating down.  There are ways to prevent this and keep your lawn healthy.  The first thing to do is put down a preventative herbicide in the spring.  This will help keep some of these weeds from growing.  This is not a fail-proof method of prevention though.  Since temperatures fluctuated so much this last spring, this may have stopped herbicides from being effective.  The best defense against these weeds is a dense lawn.  When these grasses are green and growing in the spring, that is a great time to fertilize it and help the grass grow in nice and thick so there is no room for weeds to grow.  Before you fertilize, make sure there is an adequate water supply and the lawn is green; if the lawn is starting to brown it may be too late to fertilize it and help it grow more.  Also, make sure you are mowing your lawn no shorter than 2 ½ - 3 inches.  If you mow your lawn too short, you could send it into a state of shock and it will not grow as quickly. Proper lawn care and maintenance such as aeration and de-thatching will keep your lawn from getting to an unhealthy point where more intervention is necessary.