Check the Color of Grass Blades
A healthy lawn should have a nice uniform green color. It is more important that the color be uniform, than show a deep, dark shade of green in order to indicate health.
The deep green color that has become the standard and is promoted by advertising photographs is typical only of Kentucky bluegrass recently fertilized with a quick acting nitrogen fertilizer. Then, grass plants are briefly bursting with top growth and are as green as they can get. For this reason this rich color is actually an artificial standard. It is unrealistic to expect grass to look this way all of the time. Grass that is properly cared for and living in healthy soil rather than being hyped with fast acting fertilizer is more typically a medium green color.
The color of grass also varies somewhat according to its variety, the weather over the year, and the condition of the soil. For example, while grass tends to be slightly yellow green in the spring when it newly emerges from dormancy, at other times, the same bright, pale green color indicates that it is receiving excessive amounts of nitrogen fertilizer. A lawn suffering from poor soil may have a splotchy look with deep green spots and light green spots. Over time, previous owners may have spot seeded with different varieties of grass, giving the lawn a mottled look. A healthy lawn has a uniform color of medium green over its entire surface. That is what you are looking for.
Notice the Density of Grass Plants
A healthy turf may have as many as 800 plants in a single square foot of soil. When they are that dense, it is almost impossible to see the soil when you get down on your hands and knees and spread the grass plants apart. If you see soil between the plants in your lawn, then you can assume that something is wrong, because healthy grass normally fills in spaces over time. So, thin grass suggests that either your soil is in poor shape, or your grass is tired and needs replacing. Overseeding with new grass may be all that is necessary.
Check Depth of Grass Roots
The roots of your grass plants will tell you a whole lot about the condition of the soil, especially whether it is compacted. Believe it or not, one single grass plant is capable of generating 375 miles of roots with as many as 14 million individual root strands, having a total surface area of 2,500 square feet. That's just one (healthy) grass plant! Isn't that amazing? Since a square foot of vigorous turf typically has up to 850 of those plants, grass roots are extensive and pervasive in the soil.
The point here is that it takes that many roots to provide a grass plant with sufficient nutrients every day. Only in healthy, active, well drained soil can grass plants develop such extensive root systems. Grass that is cut too closely and/or grown in compacted soil is not able to do this. It struggles to survive with only a small portion of its potential root volume which severely reduces its capacity to get nutrients from the soil. So, the depth of the roots of your grass will indicate how badly your soil is compacted.
The best way to check grass roots is to dig a sample core from the turf. Choose a time when the soil is moist. Using a trowel or sharp knife, dig down as far as the tool allows (preferably 4 to 6 inches) and extract a chunk of sod--turf, roots, and some dirt. Examine how deeply the roots penetrate the soil. If they go down less than 4 inches into the soil, chances are it is because the soil is too compacted. Jeff was shocked to discover that our grass roots were only two inches deep. He later learned that, unfortunately, that is typical of most lawns.
Count Your Weeds
One of the inevitable results of having a thin turf is lots of weeds. Weed seeds need light and space to grow. Thin grass, and grass that is mowed short allow sunlight to penetrate to the soil surface and germinate weed seeds that lie in wait for these ideal conditions. Therefore, when you eyeball your lawn and see lots of weeds, it is time to repair the grass. The percentage of weeds present in the lawn actually determines how much work is necessary.
Jeff decided a few years ago that he could live with some weeds in the lawn. It seemed a reasonable trade-off for the savings in time, energy, money and environmental impact that a more low maintenance lawn--one that tolerates some imperfection-- provides. Having made peace with that decision, he discovered that when we achieved an otherwise healthy, dense turf, as many as 10-15% weeds evenly distributed throughout the grass, were barely noticeable. The cost of trying to attain a perfect lawn, one that is virtually weed free, does not seem to us to be worth it.
However, when the percentage of weeds in a lawn sneaks over %, it is time to re-evaluate. Depending on the type of weed, this larger proportion of weeds is likely to become obvious, even to the casual observer. It may be time to take some remedial measures. There are a couple of strategies.
If the weeds in your lawn are noticeable, but do not comprise more than half the green turf area, solve the problem by killing just the weeds with an appropriate herbicide and overseeding the lawn with new grass seed to fill in the spaces and thicken the turf to discourage new weeds. If weeds comprise more than half of the green turf area, we have found its worth it to kill the entire turf and install an entirely new lawn. Measuring the percentage of weeds does not have to be precise. Just look at a small section of your lawn and guess. You are the final judge. If you think it looks okay, then that is all that matters. If it doesn't look okay, then you know you need to address the problem.