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Grass growth is opposite from most other plants, the old growth of the grass is at the top of the plant while the new growth is at the bottom where the key growth point for grass is the crown. 

The crown sits at the soil line and it is also where the shoot section and the root section of the grass meet. The crown is a highly active growing point that pushes stems (and the leaf blades that form around them) upward. 

A lawn is an area of land covered by many individual turfgrass plants. In fact, a lawn may be considered a garden area comprised of many individual grass plants rather than just a few flower or vegetable plants. It has been reported that in a typical 1,000 square feet of lawn there are about 1 million grass plants.

The part of the plant that the homeowner is most concerned with is the leaf blade. That is the most visible part and the part that tells you how good your lawn looks. Upon closer inspection you can see that the leaf blades grow out from the leaf sheath, which is wrapped around the grass stem.

 The stem and sheath are formed in the crown and are continually pushed upwards as new cells are created.  The point where the sheath turns into the blade is called the collar. Grass blades do grow out from their tips a bit and can continue growing after being cut. However, when you mow the grass blade below the collar point, the blade is no longer capable of new growth. But, inside the sheath there are other immature leaf blades that can emerge to form new sheaths and blades when the grass is mowed below the collar.

Root growth.

The crown is also responsible for initiating the roots, but the roots then grow in a more typical fashion.  New growth cells form at the tips of the roots instead of the crown.  The roots elongate in search of water and nutrients and form a very extensive and fibrous network, most of it being invisible to the naked eye. 

According to a lawn site at Purdue, one researcher, who examined the root system of Kentucky bluegrass, found that in one cubic inch of soil there were 2,000 roots with 1,000,000 roots hairs. Poor root growth, for whatever reason, means a grass plant that suffers from poor water and nutrient absorption. 

How Grass Spreads

Some grass types can spread rapidly while others will hardly thicken at all.  The way your grass spreads actually depends on the particular type of grass you have. Bluegrass is different from ryegrass which is different from bentgrass and so on.  And the ability to spread is also influenced by how well you care for the grass as well.

There are just four methods lawn grass plants normally use to reproduce or extend out from the mother plant. Most grass species are capable of spreading by two or more of these methods.

1. Going to Seed

Dropping seeds is one of the ways all lawn grasses can spread and fill in, but please don't depend on this method.  Because we mow the grass before seed heads develop, you rarely see much in the way of seeds on a lawn. If you did not mow your lawn at all, eventually you would see seed heads developing at the top of the stem, extending above the grass blades, like the bluegrass in this image.

Some grasses types, including a few bluegrass varieties, have adapted to mowing and form seeds closer to the ground, below the mower blade. 

With all the hybrid grasses today, there is also the possibility that the seed may be sterile too. I wouldn't count on the lawn going to seed as a good method of your grass spreading if it is thin.

2. Tillering A tiller is a new shoot (a new stem with more leaf blades) that forms off to the side of the main plant. It grows out from the crown and essentially makes the mother plant into a thicker or wider plant.

Most lawn grasses will send out tillers.  Some grasses use this method exclusively for spreading out or thickening. These grasses are called bunchgrasses or clumpgrasses.

The only way this would fill in the bare area would be to get thicker and thicker over time, and that might take a while. Tall Fescue is also a bunchgrass, and most varieties of this are very wide bladed.  When you get a spot of this in an otherwise fine bladed lawn, it sticks out like a sore thumb.

Rhizomes

Rhizomes are underground runners (or stems) that pop out of the ground a distance from the mother plant, forming completely new plants capable of sending out its own rhizomes. You can see a rhisome in the blue picture above on the left. Rhizomes are a great way for grass to spread, and typically grasses with rhizomes also thicken by tillering as well.  Kentucky bluegrass and fine fescue spread by rhizomes.  The rhizomes can spread quite far to find an open spot in the lawn.  They will also pop up in your garden beds, like it or not, if bluegrass borders the beds. Good soils make it easy for rhizomes to grow, while dense clay soils can make it very difficult.

Stolons

Stolons are somewhat the opposite of rhizomes in that they are above ground runners.(or stems)  that travel on top of the soil. They travel a long or short distance from the mother plant and then form independent grass plants as they go along.  You see a stolon on the right side of the blue image up above.  Creeping bentgrass, St. Augustine and Zoysia grass spread by stolons and they will spread rapidly on top of the soil, especially if the soil is easy to root in and not hard or claylike. They can weave their way through other grass types in a lawn and eventually take over the lawn.  Here is an illustration of Zoysia grass.  

Grass Plant Growth and its Relationship to Lawn Care

The first step in any lawn care plan should be to provide an ideal environment for growth and function of grass plants. A healthy, vigorous lawn is the best defense against attack or invasion of various pests. Also, a healthy lawn is much more capable of quickly recovering from modest damage caused by insects, diseases, or physical wear and tear.

Creating a healthy lawn environment is like any other type of gardening. The same good practices used in vegetable and flower gardens are just as appropriate for growing grass plants. The means to achieve those conditions are somewhat different for lawns than gardens, but are just as important for healthy grass plants as for healthy tomatoes.

As with other garden plants, knowledge about the plant and how it grows is often the key to successful growing. Large juicy tomatoes, fresh crisp lettuce, and colorful marigolds are the result of applying knowledge about those plants to growing them in the garden. Likewise, similar knowledge about the grass plant will be very helpful in successfully growing it and creating a healthy attractive lawn.

How Grass Gets Its Food

The commonly held, but incorrect belief is that grass plants get their food from the soil, and if the soil is fertilized the grass has more food. The fact is, when you buy plant food (fertilizer) for your lawn, you're technically not really buying food at all. What you are buying is some of the raw materials that can be used to make food for the grass.

      A grass plant manufactures its own food in its leaf blades.

Do you remember learning about photosynthesis in grade school?  Well, now is when we are going to finally put that knowledge into practical, lawn improving use.

PHOTOSYNTHESIS  

All green plants use sunlight to make their own food in their leaves. Photo means light; synthesis means putting together.  Here is the simple version of how it works.

Direct sunlight helps the leaves turn carbon dioxide (from the air) and water (inside the leaf) into sugars (food) and exhale oxygen - 0₂. The plant can then use this food to create proteins, fats, cellulose and all the other necessary building blocks everything it needs to make leaves, stems, crowns, roots, seeds, runners, tillers and so on. Unused sugars are stored as starch in the roots and leaf blades.  In the absence of direct sunlight oxygen is inhaled and carbon dioxide - C0₂ is exhaled

So what does the plant get from the soil?   It gets water, and dissolved minerals (nitrogen, magnesium, phosphorus, calcium, potassium etc...) and trace elements.  These are considered the raw materials that plants need to convert, using the energy from the sugars, into the other building mentioned above.  Fertilizers and organic matter supply raw materials, but by themselves are not food.

Once again:

1. In the leaf blades of the grass, sunlight is used to convert carbon dioxide and water into sugar (food).
2. The sugars are then turned into proteins, fats, cellulose and all the other  building blocks needed for plant health. Most of these building blocks are formed by combining the sugar with other minerals or raw materials in the leaf blades.
3. The roots of the grass take up the water and dissolved minerals and send them up to the leaf. 

EVERY TIME YOU MOW YOUR LAWN YOU ARE CUTTING OFF SOME OF THE FOOD FACTORY OF THE GRASS.