When is the best time to reseed your lawn?

One of the most frequently asked questions of anyone in the landscaping industry is the one above. Grass seed is pretty tough because it has everything it needs to support itself in the initial phases of life, minus the water and the correct conditions for germination. As with any seed, the young embryo plant is protected by a hard outer coating and an internal food supply that nurtures the developing plant. When the temperature is warm enough and moisture is present, the process of germination will take place. You have probably helped your kids with their science project by placing seed in between moistened paper towels or a glass of water to demonstrate how seeds grow. 

You also know that the seed's internal resources are only there to supply the embryo for a short period of time. For continued growth, the embryo requires a soil structure and an external food supply as well as the correct temperature and a continuous water supply. As long as the temperature is around 70º F for about 10 days a southern cool season grass seed , like Fescue, will germinate if adequate moisture exists. To answer the main question presented in this post we need to look at each factor in the reseeding process of a turf structure.

Temperature

If your only concern is getting your grass seed to germinate, then you only have to meet the above conditions which means you can sow grass seed just about anytime with success, except when its snowing. In Tennessee's climate, grass will grow generally from March through early December. After the seed has germinated, the seed's root system will expand as long as the soil temperature is above around 45º F. This fact is probably the most important in creating a turf system that will stand up during the long hot summers.

The root system is responsible for grabbing the moisture in the soil and providing hydration to the leaves which uses the water to manufacture food for the plant system, including the roots. A turf system goes into failure when enough water cannot be supplied to sustain the metabolic processes. Summer temperatures affect plants the same way it affects humans; humans sweat and plants transpire. High temperatures will cause plants to loose so much water through transpiration that if the plant does not have a well established root system, the roots cannot supply enough water to the leaves which results in desiccation (death). 

Soil Structure

Since a well established root system is the most important factor in creating a stable turf structure, we need to provide the root system the optimum conditions for growth.  Tennessee is generally not blessed with rich, loamy, open soils. We have tight hardpan clay soils that make it a challenge to grow anything; even grass. Our soils will benefit greatly by the addition of organic matter to the first 6" of the soil structure. As this approach is not always practical or even possible in most cases, proper turf management is our best tool.

It is very important when reseeding to ensure the seed actually comes into contact with the soil, not the thatch (the dead grass clippings that have not been removed at the top of the ground). There are two basic approaches to reseeding your lawn; aeration and verticutting. It is also very important to remove enough of the top growth (cut to 1-1 1/2") to allow even seed dispersal and contact with the ground. 

  • Aeration comes in two forms.  Remove the thatch before applying any of the following techniques:
    • Core aeration removes a plug of soil from the soil structure allowing air and water to penetrate the soil structure more easily and collect any seed for germination. This technique is used on golf courses a lot and, if done properly, requires a top dressing of sand with the reseeding procedure. This creates a porous soil structure that promotes deeper root growth. 
    • Spike aeration is usually done with a weighted spiked roller. The effectiveness of this method depends on the weight of the roller and how the roller is pulled (straight lines or serpentine paths). Effectiveness also depends on how hard or dry the soil is which determines how deep the spikes can penetrate. This technique doesn't do much to change soil structure and simply provides a receptive surface for the seed.
  • Verticutting brings the thatch to the surface and creates enough loose soil to provide an excellent seedbed for the new seed. I usually rake up the thatch to expose the seedbed for maximum contact of the seed with the soil. Some machines will prepare the bed and sow the seed at the same time. When using this type of machinery, make at least two passes in perpendicular paths. There is a good article about this subject here.

External Food Supply

Tennessee soils are acidic due to the parent rock from which they are derived. This fact does not affect turf grass as much as some nursery stock but occasional treatment with limestone will provide better usage of nutrients from applied fertilizers. When sowing new seed, the best practice is to provide a "balanced" fertilizer. Typical blends would include 10-10-10 or 13-13-13. The first number is the percentage of Nitrogen (N) which is primarily for top growth, the second number is the percentage of Phosphorous (P) for root growth, and the third Potassium (K) for disease resistance. If you can find a fertilizer with micro-nutrients it will provide some of nutrients in a readily usable form that can be otherwise tied up by low PH soils.

I would not recommend using the high nitrogen fertilizers when reseeding as they are usually low in phosphorous which promotes root growth. Do not use fertilizers with weed prevention ingredients as that will kill the new grass plants.

Moisture

Moisture is not only critical for seed germination, it is probably the most important factor in establishing a mature lawn. After the seed has been sown, all water should be applied very lightly and very often. The seedbed should be kept damp, not wet. If you have irrigation, let spray heads run less than 5 minutes 3-4 times per day. If you have large turf areas with rotor heads, let them run about 15 minutes 3-4 times per day.

After the seed has germinated and filled in and after the first cut (don't let the grass get over 3 1/2" before cutting) you can increase the time you run your watering devices (start out by doubling) but reduce the frequency to twice per day. Depending on the temperature, you may have to increase your run times more.

The critical point to remember in watering a maturing lawn is that you want your root system to go deep in order to withstand the hot dry conditions summer will bring. To accomplish this end goal, water your grass to the point of runoff at least twice per day and let it dry out between waterings. If you water your grass once per day, the entire root system for the grass will be within the top 2-3" of the soil and when the hot conditions come, you will have to water more frequently to keep your grass green which will cost you a lot more. If you cause your root system to reach down deep for its moisture, it will withstand the dry conditions in the top of the soil structure much better.

So what's the answer?

Given everything we've talked about, the optimum time to resow your lawn is from September 1st to October 15th. This time of year usually gives you enough warm weather to let your seed germinate within a short period of time and have enough time to establish a substantial root system. The additional benefit is that your root system will continue to grow and expand as long as the soil temperature is 45º F. This timeframe will give your lawn about 7-8 months to prepare for the stresses of 90+ temperatures and dry conditions. If you follow the above advice, your root system should reside up to 6" below the surface and you should not need to increase your watering culture by very much.

If you miss the fall time frame, the next best time is early spring. As soon as you get temperatures in the 70's, go ahead and sow your seed, you might not get quick germination but the seed will wait on the right conditions. The critical factor is getting the root system going as soon as possible.