Turfgrass Growing Guide

Willamette Valley, Oregon, USDA Zone 8b

A well managed lawn can benefit the ecosystem. It is fantastic at protecting soil from erosion while sponging up copious amounts of rainwater. It also stores atmospheric carbon and provides summer cooling. What about all the toxic stuff? Unnecessary when we help our grass outcompete the weeds. I’ll show you how. 

Understanding Turfgrass

I have a rebellious nature and won’t do something just because someone tells me I should. But, if I understand the why of a thing, then everything changes. First let’s start with a few facts:

• Turfgrass has a shallow, matted root system, usually about one to four inches deep. 
• The depth of the root system directly correlates to the height of the blades of grass: taller grass = deeper roots.
• Energy, in the form of carbohydrates, is stored in the green tops of the grass blade. 
• Nitrogen is the primary nutrient grass needs. Timing your applications is key.

Guidelines for Growing Grass 

• ⅓ Rule: Only mow about  ⅓ the height of the grass. If your grass is 4” tall, take off about an inch. If it’s an unruly 6” mow it down to 4”. Why? Any more and you’ve removed a significant portion of carbohydrates and nutrients storage. Scalping the lawn in this way can shock the plant and cause the roots to die back dramatically. So if you leave on vacation and come back to tall grass, it may take a few mowings to work it back down to the height you want.
• Mow your grass frequently during the growing seasons in early spring and fall to avoid breaking the ⅓ rule. 
• Mow your grass no lower than 3” tall. Why? Taller grass encourages deeper rooting to access water and nutrient uptake. It also shades the soil, cooling it, holding moisture, and preventing weed seeds from sprouting.
• Mow on the mulch setting. Why? The clippings are small and break down fast, returning significant amounts of nitrogen to the soil. Clippings do not become thatch, in fact some lawn experts think that the clippings help thatch decompose while building up humus in the soil.

Aerate & Dethatch Once in Spring

Sometime in Mid-April through Mid-May depending on the weather. Moist soil is fine, but waterlogged is destructive to soil structure and more labor.

1. Dethatch with a thatch rake. Why? Dethatching removes dead chaff that can build up in unfavorable soil conditions. As you follow this guide, it may become a non-issue in following years. 
2. Core aerate. Why? Removing soil cores loosens compacted soil. This allows better water infiltration and root development. Dandelions love compacted soil.
3. Overseed. Why? To fill in any gaps with new grass rather than letting weeds blow in or germinate from the seed bank in the soil.

Fertilization Schedule

Use a lawn specific fertilizer. It will be high in nitrogen with very little or no phosphorus. Why? Grass needs oodles of nitrogen, but phosphorus has been shown to adversely affect the health of the watershed by encouraging algae blooms and other problems. So no “P” in the NPK rating. 

• Memorial Day: Fertilize at full strength recommendation on the label.
• July 4th: ½ of Memorial Day.
• Labor Day: Same ½ rate.
• Thanksgiving: Same ½ rate.

Why? This schedule delivers the right amount of nitrogen, just as the rain slows down (less runoff) and the grass really gets growing. Turfgrass grows less in the heat of summer or goes dormant. Then it will produce a new flush of growth in the fall. So one last light feeding around Thanksgiving is helpful. This schedule assumes you are mulch-mowing your lawn.

Irrigation

Turfgrass has a short rooting depth around 3-4” inches, if you follow the tip above to grow about 3-4” inches tall. If you keep your grass shorter, the rooting depth will be shallower, more like 1” above ground, 1” below ground. 

The goal is for water to reach the rooting depth before it runs off. If your lawn is sloped this adds an extra challenge. It may require some fine tuning and experimentation. A good place to start is to follow the Weekly Watering Number provided by regionalh2o.org. I find it to be a little conservative and usually need a little more.

• The WWN recommends a number of inches per week. Usually around 1” per week. I prefer to break this up into three watering sessions of about ⅓” – 1/2″ each.
• To find out what this means for your irrigation system, set out a few graduated cylinders like rain gauges or perhaps a few tuna cans. Run your system for 15 minutes. Use this measurement to adjust your system.
• Set your system to run for early morning to reduce evaporation. Running in the evening encourages slugs and pathogens that enjoy cool moist conditions.
• Take a peek in your soil now and again to see if the water is going deep enough (or too deep.)
• It’s OK to let your grass go dormant (golden) in the summer, provided it is a healthy lawn. It should bounce back as soon as the rain returns. However, if you are building a lawn or have clayey soil with very little humus, you may want to irrigate during our drought like summers.

Do you have any tips to share? I’d love to hear them and share them with our community.

Resources

• OSU Extension: Practical Lawn Care for Western Oregon
• The Spruce: The 8 Best Lawn Aerators of 2023