Several events in winter can cause a lawn to turn yellow. The most obvious reason that a lawn turns yellow in the winter is dormancy. Warm-season grasses, such as Bermuda grass, naturally turn yellow and brown as they go to sleep for the winter. Homeowners deal with this in a number of ways, including allowing it to remain discoloured or overseeding with a cool-season grass. There are other reasons, though, that a lawn may yellow in the winter.
If higher elevations in your lawn turn yellow in the winter it may be suffering from winter kill. More commonly known as desiccation, this disorder is caused by exposure to wind while the turf is dry, either from lack of irrigation or snow cover. The plant's roots, sitting in frozen or dry soil, can't absorb water which causes ice crystals to build in the plant's cells. Properly winterising the lawn in the fall helps prevent dessication. This includes continuing to water the lawn after summer and into fall. Water every two weeks in the winter if the soil isn't frozen and temperatures are above freezing.
Snow mould is the common name for a fungal disease caused by Microdochium nivalis. It turns the lawn yellow in patches that can grow to 1 foot in diameter. Sometimes the lawn appears pink or off-white when it's wet. Lawns under snow cover on unfrozen earth are susceptible to snow mould. To prevent snow mould, mow the lawn throughout fall to reduce mats on which the mould can grow. Rake leaves before the first snowfall. Consult with your county cooperative extension office about fungicides recommended for your area.
Rust is a fungal disease that, although it strikes grass in the summer, it leaves it susceptible to winter kill. A lawn suffering with rust takes on a yellow to orange cast and, when heavily infected, the grass dies. Control rust by maintaining good lawn care practices -- consistent fertilising, mowing and watering. Remove grass clipping after mowing and apply a fungicide before winter.
Winter brings slippery walkways and ice-covered streets and, thus, salt. Whether the salt is from rock salt applied by municipalities or homeowner-applied de-icing products, salt damages a lawn when it is absorbed through the roots. Generally, the damage isn't apparent after one season; it takes time to build to toxic levels. Use coarse sand or kitty litter instead of salt. If the problem is not of your making, consider switching to a more salt-tolerant turfgrass variety, such as tall fescue.