Understanding Grass Seed Germination

Grass seed germination keeps you in suspense! You get one chance, but you wait weeks to see if you did it right!

Are your expectations for the outcome reasonable? Does every little grass seed sprout and grow?
Many factors affect grass seed germination. The success of your lawn project is not completely under your control, but you can do a lot to improve the final result.

Seed germination is simply the growth process that occurs when the seed absorbs sufficient moisture to start sprouting. This is regulated by soil temperature according to the variety of grass. The hull, or hard outer casing, may delay this process until conditions are favorable.

Once germination begins, if the environment changes significantly (like being too hot and dry), the seed or sprout will die. It cannot start to grow, then stop because something is wrong, wait till conditions improve, and then continue the process.

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Requirements For Grass Seed Germination

Adequate moisture must be present throughout the process. Too little and the seed can dry out. Too much moisture and the seed can rot. Read the article Watering New Grass Seed for guidelines.

Soil temperatures should be in the appropriate range for the variety of grass. Cool season grass requires a minimum range of 45 - 55 degrees. Warm season grass requires a minimum 55- 65 degrees.

Note that soil temperatures are usually cooler than air temperature. In addition to the density of the ground that is slower to warm up, moisture evaporating from the surface has a cooling effect.

(Editor's note on soil thermometers: These are extremely useful tools for any active gardener who regularly invests in new seeds and bedding plants, especially vegetable gardeners. There is no way to accurately gauge soil temperatures by rule of thumb.

The Luster Leaf 1618 Rapitest Soil Thermometer is an inexpensive but reliable model that eliminates the guesswork and improves your success rate with lawn grass seed. 
This model is a traditional glass tube that has enough separation of the numbers to easily read within a couple of degrees. Other models I have checked have a dial (like a meat thermometer) that has too large of a range (over 200 degrees-in case you want to baste your soil?) and is not as accurate. 
If you use the #1618 soil thermometer in loose garden soil it will penetrate easily. In compacted soil, realize that it is NOT designed to force it in, so use a screwdriver or some time of poker to make a starter hole. Don't make the hole overly large as you want full contact with the soil.)

When the sprout breaks through the seed, it requires oxygen for its growth process. When soil is over saturated with water, oxygen is not present and the sprout cannot grow. Compacted soil can also contribute to this problem.

A seed has limited resources to support growth while below ground. The sprout must break through the surface in order for the photosynthesis process to function and create food for the plant. Sprouts will not be able to reach the surface if seeds are buried too deep.

Factors That Improve Grass Seed Germination

  • Get the seed in direct contact with the soil to absorb moisture. Rake the seed lightly after sowing to mix it with the dirt and/or go over it with a light roller to press it into the soil. Get more details at Planting Grass Seed. Seeds stuck on existing grass or weeds are doomed to failure.
  • Mulch can help retain moisture. A very thin layer, 1/8” to ¼” is all that is required. Seed covers or compost should be fine screened to avoid clumps that bury the seed. Steer manure is stinky, but inexpensive. Use it only if it is certified weed free. Bedding straw is a good option for large areas, but don’t use hay that has seeds in it.
  • Sow grass seed at the ideal time if possible. Cool season grasses will not do as well in higher temperatures. Warm season grasses will not do as well in cooler temperatures. Investigate options at The Best Time To Plant Grass Seed.
  • Trouble from above. Birds love to eat uncovered seed. Do whatever it takes to prevent them. Restrict activity of kids and pets that can disrupt the grass seed germination process.
  • Wait when specified, the appropriate length of time before sowing, when any chemicals like weed killers, fungicides or preemergents have been used on the lawn area. Product labels should indicate the waiting period.
  • Consistent, light, frequent watering is essential. Avoid over-watering. Avoid under-watering. See Watering Guidelines.

Typical Grass Seed Germination Time Periods

Buffalo Grass14-28
Rye Grass5-10

This chart may indicate the amount of time until grass first appears, or until the majority of seed has sprouted. Growing conditions greatly affect this process.

Grass Seed Germination Rates
and the Effects of Storage

The label on a grass seed package provides important information, including the germination rate. A rate of 90-95% is preferred, but some grass varieties will typically be found in the 80% range. The higher, the better. An 85% germination rate means that if you plant 100 seeds, 85 of them should sprout and grow.

Realize that this rating system comes from the supplier doing a test in a laboratory. Your results may vary, usually lower. All the factors discussed above will decrease the grass seed germination rate, as will how long the seed has been stored and in what conditions.

Grass will usually be fine to use for a year after the test date. The germination rate listed should remain valid if the seed was stored in a cool, dry area. Heat will rapidly decrease the seed viability. Moisture may ruin it.

Each year of storage can result in a decrease of the grass seed germination rate by a range of 10 to 25% per year. It can still be used, but the amount applied to an area should be increased proportionately. Because you have to use more seed, don’t automatically assume that older seed being sold at a discount is a good value.

If you have older seed, take a handful and lightly blow across it (outside). Any lightweight seeds that blow away are just chaff, they have died. If a large portion of seeds are dead, start with a fresh batch.
If a smaller amount appear bad, or you are unsure, consider planting a test sample. This will delay your project for a couple of weeks, but it might prevent a disappointing venture.

Watch It Grow, Hopefully

Grass seed germination always seems to take a long time, especially when you watch it closely. That makes it very disheartening when the results are less than expected. Sometimes people wait two, three, four weeks, but never see any grass.

The seed may have failed to germinate. This is unlikely if it is new seed and you followed all the necessary steps. Grass plants can be picked off by pests as soon as they break ground. Birds, snails, slugs, cutworms, pillbugs, earwigs, rabbits, squirrels, rats and mice are all attracted to this fresh salad bar. They can grab it before you see it, or leave such a small percentage that you believe the seed was bad.

Sow a sample crop of seed so you know when to expect the grass plants. Then you can be on the alert for any problems. Plant a number of seeds in a pot, cup or jar and keep it in a warm, but protected area. These seeds should germinate a few days prior to most of the lawn area, since the potting soil should be warmer.

This sample will demonstrate the time to expect your lawn grass as well as the germination rate under better conditions. You can also put a few seeds on a wet paper towel stored in a sealed plastic baggie. Watch for signs of sprouting. Then guard the garden!

Do your best to improve your grass seed germination rate, and you’ll be ready to mow it before you know it! 

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