Mowing height has a significant impact on the overall health of your lawn. Lawn grasses only tolerate mowing; it is an injury that requires recovery.
A golden rule of lawn care is to only remove one-third of the grass blade during each mowing event. This means if you maintain the grass at a mowing height of two inches, the grass should be mowed when it is three inches tall. The blades of the grass are the only parts of the grass plant that photosynthesize. When you cut a third of the blade, you remove half of the plant parts capable of producing energy. If you mow too low and trim more than a third of the grass blades, over half of the plant’s photosynthetic capabilities are removed. This practice, called scalping, reduces the lawn’s ability to recover quickly. Repeated scalping diminishes the turf’s energy. Over time the grass becomes thin and bare patches develop.
Each type of grass has a range of recommended mowing heights. Maintaining the lawn at different ends of the range produces different effects on the grass. Cutting the lawn at the lower end of the range encourages more sideways growth of the turf. The lower height also encourages the grass to send out runners or tillers to increase the number of grass blades produced, which results in a thicker lawn. However, lower mowing produces more stems at the expense of root development. Lawns maintained at lower mowing heights need more water and have higher fertilizer requirements. So, if your lawn is looking thin, I would recommend a lower setting for a month or two.
A higher mowing height allows the grass to photosynthesize more. As a result, the roots grow deeper, and the grass is more drought-tolerant. This is desirable during the hotter, drier months of summer. If the grass looks great now, then I would keep the higher mowing height to encourage greater drought tolerance. A higher mowing height also reduces fertilization needs.
If the lawn is being grown in shady conditions, the grass needs to be kept at the highest recommended height to maximize the photosynthetic area and prevent the lawn from becoming thin. To find the recommended mowing height range for each turf species, please see HGIC 1205, Mowing Lawns.