At this time of the year, during the primary establishment period for cool-season grasses, deciding how to best approach mowing after the planting of new turf can be an anxiety-provoking exercise. Mow too soon, and you risk damage to tender seedlings. On the other hand, delaying cutting means a longer wait to realize the benefits of mowing as well as greater risk of excessive defoliation from that first cut. As far as the timing of post-plant mowing is concerned, the best advice is that it is important to be cautious but not shy. Mowing after planting of new seed is a critical step in the establishment process. Mowing actually stimulates the plants to form new tillers, which is a primary driver of turf density. A main goal of most establishment projects, of course, is to achieve sufficient density in the shortest time possible.
When dealing with the planting of new turf, timing the first mowing is as simple as using the good old '1/3 rule' as a guide. The 1/3 rule states that no more than 1/3 of the total leaf area should be removed by a single mowing event. Accordingly, an appropriate mowing height should be determined in advance, and the turf should be cut when it reaches a height around the 1/3 rule threshold. For example, if the desired mowing height is 2", then the turf should receive a first cut when it reaches 3" tall. The exact number of days for how long this will take depends on specific circumstances, as the duration can vary widely based on a number of factors including the desired mowing height, the turf species present, available fertility, and prevailing weather conditions. As with established turf, if the new turf grows higher than the 1/3 threshold it is best to bring it down gradually instead of all at once.
The 1/3 rule should dictate your first mowing following overseeding as well. In this case, the timing for an initial mowing is most often influenced by the existing grasses on the site. As soon as the overseeded stand reaches the 1/3 threshold, it is time to mow. Allowing the stand to grow higher than this threshold could be detrimental to your overseeded plants as a result of increased shading or competition for space. The amount of time that it takes to reach this point is also variable as a result of the same factors mentioned above.
Some managers prefer to suppress existing turf by dropping the height a notch or two just prior to overseeding to give the new plants a sort of head start. The shorter cut serves to lengthen the time between planting and the first mowing. This is especially appropriate when overseeding with slower-establishing grasses such as Kentucky bluegrass. This technique should only be used in late summer or very early fall when adequate time for recovery exists.
If you are concerned about damage from riding mowers to a new turf stand, perhaps cutting with lighter walking equipment for the first few mowings is the way to go. It is always advisable to avoid turning of equipment on the turf, especially when the stand is young and not yet established. Leaf removal is another concern that soon follows late summer early/fall planting projects. Blowing, power bagging or mulching of leaves are all preferable to vigorous raking that can easily dislodge recently emerged, immature turfgrass seedlings. In the end, it is often useful to trust your judgment.