The 150 Rule

The 150 rule is used by many homeowners and lawn care companies to help properly care for lawns. Depending on who you ask, the 150 rule can have a couple of different meanings.

Summer Months & Diseases

For cool season grasses, the 150 rule is used in the summer to determine when a lawn is more susceptible to disease. This calculation is also important when using products on your lawn such as herbicides (weed controls) as they can be more damaging to the lawn.

The calculation of the 150 rule during the summer months is quite simple. Add the air temperature and the humidity together. If the total is at or above 150, the lawn is considered to be in the danger zone. Close to but under 150 is considered the caution area.

The summer of 2010 our local area had the biggest outbreak in lawn diseases in many years. Contributing factors were the high day temperatures along with humidity. In addition, the majority amounts of rainfall we received from May through August were during the evening hours.

Good turf grass management practices are especially important at this time where the 150 rule applies. If you need to water your lawn consider watering in the morning hours to allow your lawn to dry before evening hours set in. A sharp mower blade is also a must! Dull mower blades injure the grass leaf and cause ragged, torn ends which allow moisture to escape and disease to enter. Sharp blades will not only prevent this but will also give a better overall appearance of your lawn.

Avoiding additional stress to the lawn is the main reason fertilizer is not applied to cool season grasses during the summer months as well as why weed control products may only be applied in a spot spraying, rather than a blanket spray that the lawn may receive in the spring months.

Spring Months & Growth

The 150 rule may also apply on the other end of the spectrum when talking about cool season grasses and determining when to expect the grass to grow and when it is in the prime growing season.

The calculation of the 150 rule during the spring and fall months is just as simple as during the summer months, however during the spring the calculation consist of two air temperatures: daytime high and nighttime low. When you add the high temperature on a warm spring day to the lowest temperature over a cool spring night, the total must be at or above 150 for the grass to grow ideally.

While growth may still happen without the total reaching or exceeding 150, please know that it is not actively growing. Due to the grass responding to the warm daytime temperatures with little growth, it is ideal to apply fertilizer during these months and assist the lawn now as well as when it does reach that active growing period.

Weed control and pre-emergent is equally important during these spring months. Although weeds may not be actively growing, they are germinating and preparing to emerge. A pre-emergent barrier can prevent many weeds from emerging and a weed control application can kill any weeds that may be beginning to emerge.

Sum it up

To sum it up, the 150 rule can be used all year long for cool season grasses. In the spring and fall months, the 150 rule determines the ideal growing period and the summer months it determines when it is safe to apply product and what out for pesky lawn diseases.

It is always a good idea to consult with a lawn care professional and ask any questions you may have. Look around, ask friends or family, and research companies on the Better Business Bureau to find a company that you can trust.

Scroll to Top