September is the perfect time to fertilize and strengthen a summer stressed lawn for fall. (Photo: Shutterstock)
Carol O'Meara, Colorado State University Extension, Boulder County
Carol O’Meara, Colorado State University Extension, Boulder County

Running, leaping, swerving and diving is perfect for a relaxing afternoon in the Colorado sunshine.  Sandlots and dirt yards are fine for pickup games and goofing off. But if you’re going to pirouette before mashing your face into the ground, what you love is nice, cushy turf grass. 

Fall is the time for football and renovating your yard, when temperatures cool and grass gets a chance to plug in holes left by high summer heat. Now that the season wanes, get out and get your grass into gridiron shape. Temperatures in the lower 70’s are ideal for cool-season grass re-growth, allowing thin spots to fill in and crowns to send up new shoots. Many lawns that browned under dry summer conditions may start to fill in again. A bit of water helps them recover, and areas stressed from Ascochyta disease or poor irrigation will make a comeback.

But the most important aspect to lawn rejuvenation is a strong fall feeding during September with a turf-builder fertilizer that has both quick and slow release nutrients. All-purpose mixes have 16 nutrients needed by turf to be healthy – a balanced fertilizer goes beyond offering nitrogen, phosphate and potassium oxide (the N-P-K numbers). 

Apply chelated iron to lawns that are slightly yellow from iron chlorosis.  But be aware, not all chelation formulas work in our alkaline soils, so look for ethylene diamine dihdroxyphenyl acetate (EDDHA).  The more commonly, and cheaply, available EDTA (ethylene diamine tetraacetate) is only active in pH-neutral soils. 

Nitrogen, the most important nutrient for turf in fall, depends on temperature and moisture for release into the soil. When we have warm days plus a few rain showers, lawns get a quick boost of food, and under ideal conditions, turf – an active scavenger of nitrogen – will take up the nitrogen within hours of it being put down.

This is perfect for lawns towards the end of September, because the quickly available fertilizer will increase turf vigor, and the slow release will continue encouraging turf rebuilding well into fall.

Thin areas where the grass has died off completely can be over-seeded now. To get the best results from over-seeding, water the lawn 24 hours before aerating.  Pass the aerator over the turf in two to three directions to open up many holes. Immediately over-seed with the grass of your choice, but in general, tall fescues do not blend well with bluegrass, perennial rye or fine fescue because of its wide grass blades.

Grass seed takes a while to germinate, so keep humidity on the lawn for 14 to 21 days. 

Use starter fertilizer at the time of over-seeding to feed the turf without burning the new shoots.

With close attention to care and a little pampering, your lawn will recover.


The Colorado Master Gardener program is currently taking applications for the spring class. If you are interested in helping others garden, this program is for you! Classes run late January through mid-April and will be held every Friday, 8:30 a.m. to 4 p.m. Please call 303-678-6238 for an application.

By Carol O’Meara. Colorado State University Extension, together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development. For more information contact Extension at the Boulder County Fairgrounds, 9595 Nelson Rd., Box B, Longmont, 303.678.6238, e-mail or visit