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Good turf quality requires light. Lawns in heavy shade don't thrive. Some lawn seeds mixes packaged for the maritime Northwest contain grasses such as tall fescues or fine fescues that will take shadier locations. But no grass will grow well in the dark areas under heavy tree cover.
What often happens in a landscape is that as trees grow up, lawns that were getting sufficient sun suffer from increasing shade when the garden matures. Thinning the tree canopy may be necessary to improve light availability. If this isn't appropriate, lawn areas may need to be relocated to the sunniest spots in the garden.
During fall lawn renovation, consider replacing lawns doing badly in the most heavily shaded areas. Fill in with ground covers adapted to shade. For a list of ground covers appropriate in western Washington, send a stamped self-addressed envelope to the King County WSU Extension, Bulletin Department. Request Horticulture Fact Sheet # 68.
If the lawn is only moderately compacted (such as in areas that get foot traffic but weren't affected by heavy machinery), aeration can help by opening the ground to allow better availability of oxygen and better penetration of water to turf roots. Aerate when the ground is damp but not wet and sloppy, a few days after a rain or irrigation.
Overseeding bare spots with new grass seed will help fill bare spots. Use a seed mixture containing primarily perennial ryegrass and fine fescues and a small amount of selected Kentucky bluegrasses for the maritime Northwest. Prepare the ground properly, turning it over and pulverizing the soil to create a good seed bed. Pat seed firmly into the surface of the spot and don't allow the site to dry out during the critical germination period. Overseeding must be completed before early October to give the grass seedlings time to germinate and establish roots before colder, darker weather sets in.
Keeping the lawn well-fertilized will also help in managing moss and weed infestations. Lime, contrary to folk myth, doesn't kill moss. However, it can help turf grasses grow better by raising the pH of the lawn, and by adding calcium that's necessary for good turf growth. Turfgrasses grow well at a pH of between 5.5 and 6.5; if you are in doubt, get the soil pH tested before applying lime randomly. Lime may not be needed on your particular grass site. A regular, sensible fertilization plan may work well without extra lime being added. (And as an aside, WSU turf specialists have noted that annual bluegrass, Poa annua, a common weed grass in lawns, grows better if the pH is higher. Liming wouldn't be advantageous if lots of annual blue grass were present. Do not add lime if you are fighting annual blue grass.
Fertilize the established lawn in early September. Regular fertilization will help the lawn grow thick, filling in bare spots that could be colonized by moss or weeds. Use a fertilizer with a 3-1-2 ratio such as a 21-7-14 or 15-5-10, or one close to this. If the label read 3-1-2, (or is in this proportion) this means that the fertilizer contains 3% available nitrogen, 1% available phosphorous, and 2% available soluble potash. Look for a fertilizer that meets these specifications. Apply enough to put on one pound of available nitrogen per 1000 square feet.
WSU turf specialists do not recommend fertilizer containing weed killers (weed and feed type formulas) because they put a pesticide out across areas where no pest exists, unless the lawn is heavily infested with weeds. A weed and feed product should not be used more than once a year at most. (Fall is the best time.) Don't use it regularly with every fertilizer application, but only if needed.
For good results, fertilize the lawn 4 times a year: a good memory device for the timing is Easter, Memorial Day, Labor Day, and Thanksgiving: April, late May, early September, and mid to late November. If you are grasscycling, returning the clippings to the turf when cutting the lawn, it's possible to cut back on one fertilization if you are on the four-times a year fertility program. This is because the grasscycling of the clippings provides some nitrogen for the turf.
Don't neglect basic lawn care in September.
Thanks to Dr. Gwen Stahnke, Turf Specialist, Washington State University, for material used in this column.
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