How many times a year do you need to fertilize/weed control a one acre yard?
The lawn care proposes 5 per season of fertilization/weed control ( organic/synthetic combined) 3/16, 5/11, 7/6 8/31/ 10/26/15? - 6 to 8 weeks apart.
Arapahoe County Colorado lawns and turf
Fertilization depends on what type of lawn. Fine fescue lawns only need once a year fertilization in the spring. Tall fescue lawns require more. Warm season Buffalograss requires twice per season during the summer. Kentucky bluegrass needs 2-4 times per growing season, but not during the heat of summer, with a more heavy application (4-5 lbs/1000 sq ft) in fall (mid Oct) than at any other application during the growing season.
"It depends" is also true for weeds. There are different types of lawn weeds: broadleaf summer annuals (purslane, knotweed, pigweed), grassy summer annuals (crabgrass, goose grass, barnyard grass, foxtail), broadleaf weeds (dandelions, clover, spurge), etc. Different types of weeds require different controls at different times of the year.
Here are helpful CSU Fact Sheets:
Finally, the most effective weed control is to focus on good cultural practices that build a strong, healthy, dense lawn. CSU Fact Sheet on proper lawn care, including fertilization schedule here: http://www.ext.colostate.edu/pubs/garden/07202.html.