Fall is a very important time for lawns in the Pacific Northwest. The shortening days and cooler nights stimulate grass growth. The growth of grass roots and rhizomes (underground stems) in particular is stimulated. Additional side shoots develop, making the lawn thicker. Fertilizer added during September through November just stimulates this process. It will also turn the grass a darker green color, usually within two days.
There are many good brands of lawn fertilizer. Some are specially formulated for fall application. Some are labeled as “winterizers”. These winterizers usually contain more potash or potassium. I still feel that the ideal balance for all lawn fertilizers is a 3-1-2 ratio of nitrogen/phosphorus/potassium. This could be 15-5-10 or 21-10-14 or similar percentages. I like to have at least part of the nitrogen from slow release sources, although this is less important for fall application.
Another benefit of fall fertilization is that some of the nitrogen is taken up and used in making amino acids which are stored in the roots and crowns. These are available for immediate growth early next spring before the soil is warm enough for roots to take up fertilizer.
Fall (September-October) is the natural time for grass seed to germinate and grow. It is the best time to seed a new lawn or reseed existing thin areas. If you decide to plant seed, be sure to scratch or loosen the soil surface so seed can come in direct contact with the soil. If spots are small, a rake, tine cultivator or spading fork will be enough. For larger areas you might want to use a power rake or thatch removal machine. Lower it enough so that the blades or tines scratch into the soil. Fertilize at the same time you plant the seed. Rake the seed into the soil leaving some showing on top. The most critical and important step is to keep the soil surface constantly wet for at least 3 weeks. A thin layer of bark dust (soil pep), fine compost or peat moss will reduce the drying rate. Light sprinkling 3 times per day is needed unless rain occurs.
Fall is also an excellent time to control weeds in lawns and other areas. Weeds are busy sending food manufactured by the leaves down to the roots. Weed killer also goes down to the roots quickly. If you have a lot of weeds in the lawn, a weed and feed fertilizer could be applied. If you have only a scattering of weeds, a liquid lawn weed killer is more efficient. A granular weed and feed product should be applied when the grass is wet and should not be watered for 24 hours. The same dry time is needed after liquid lawn weed killer application.
Fall is a good time to aerate and/or remove thatch from Northwest lawns. You can rent aerators and thatch machines or we can do it for you.