That lawn of yours just loves fall weather. Warm days and cool nights are its idea of what the world is all about. You see, it's not hot days that bother a lawn so much as warm nights.
Lawns (indeed all things green and growing) transpire in much the same way that we humans perspire. In doing so, the lawns dissipate energy. This doesn't much matter during the day, when vast amounts of incoming radiant energy are available from the sun. But when transpiration continues on through a long, hot night, there can be a net loss of energy for your lawn. That's when it can start struggling a little, despite the waterings you give it.
On the other hand, consider what happens in the fall. The earth has been absorbing vast amounts of heat all summer long. This stored-up heat remains for some while after cooler weather arrives, stimulating growth - while bright fall days provide more than enough incoming energy.
At the same time, the cool nights all but eliminate energy loss through transpiration. Under these circumstances even the largely untended lawn responds positively.
Fall, then, is the best of all seasons for fertilizing a lawn, particularly in the north. The excess energy taken up by grass in this favorable season is stored up in its root system for use in the early spring. In the South, a light feeding keeps lawns green longer into the fall, although Dr. Robert W. Schery of the Lawn Institute warns against ''extravagent applications, lest Southern grass species be turned more susceptible to winter kill.''
Seeds also sprout well in the warm earth and - because soil cultivates well at this period (it tends to be soggy in the spring) - fall is roughly the best time to start a new lawn if you live anywhere from Atlanta northward, Dr. Schery says.
In the more northerly states, including much of Canada, begin seeding new lawns anytime after the middle of August. Delay this practice by one week for every 100 miles you live to the south of the Canadian border. In southern Ohio, for example, the first half of September is usually the optimum time to start a new lawn; in Tennessee, it is late September.
On the other hand, sod can be laid from now on to between 3 and 4 weeks after the optimum time for seeding.
Overseeding, the practice of sprinkling seed over an established lawn to help thicken it up, is one way to go. This process is helped by scarring the surface with a sharp-tined rake or with a rented ''scarifying'' machine. A lot of old vegetation will have been loosened by this machine. Remove this before seeding.
Another obvious option is to start afresh and totally renew the area in question.
Start by spreading a phosphate-rich fertilizer (nitrogen-rich fertilizers are for established lawns) over the surface, and dig over or till the soil an inch or two deep. Aim for a chunky, rather than a very fine, surface. This provides the seed with little nooks and crannies in which to settle and take root. The first rain or watering that comes along after seeding will compact the soil around the seeds, and germination will follow.
If you have overcultivated the soil so that it has a powderlike surface, you will need to rake in the seed lightly to get the same level of germination.
Bluegrass, the preferred grass in northern climates, because of its cold tolerance as well as its good looks, or seed mixtures in which bluegrass predominates, is spread two or three pounds to 1,000 square feet. Dr. Perry suggests you spread half the seed in one direction and the other half crosswise to assure good coverage. You can do this by hand, provided there is no wind at the time, but a mechanical spreader will do a better job.
Finally, apply a mulch to keep the soil moist during the critical germination period. This can be a weed-free straw spread about three straws deep. Or use chopped twigs, stalks, excelsior, or even woven netting. Pine boughs, removed once the seedling grasses are established, are also effective.
Water regularly to keep the soil moist as the seedlings become established. By the time the grass is long enough for its first mowing, you can safely assume the lawn will need no further nursing.
One final suggestion, and this applies to an established as well as a new lawn: if the fall has been dry water heavily before freezing weather arrives. A lawn must have moisture it can draw on in winter if it is to survive the desiccating winds of winter.