Root Hairs

Root hairs are where water absorption occurs. Since that water contains nutrients found in the soil, root hairs are important. And fragile.

​You might expect root hairs to grow along the entire length of a root system, but that’s not what happens. Root hairs only occur in specific areas, or zones, of a root system.

Root zones

Roots start out as undifferentiated cells. The very tip of a root is called the root cap, which protects the growing root as it moves through the soil. The next zone is where cell division takes place. As more cells are produced, the root cap is pushed forward. This growth is a relatively continuous process throughout the life of a plant. As new cells are produced and the root moves forward, the older cells stretch and create storage pockets called vacuoles. This is called the zone of elongation. Finally, growth and elongation are complete and root hairs can begin to emerge. This is called the zone of maturation. 

The reason root hairs do not appear right away in the growth process is because they are so delicate that they would be sheared off as the root moves through the soil. This is also what causes transplant shock. The act of transplanting can shear off a majority of the root hairs as the soil gets jostled about and uninformed gardeners tamp down the soil. Rather than crushing delicate root hairs, mudding in new transplants protects those important root hairs.

Root hair description

If you look very closely at a root, you can see the tiny root hairs. They tend to be long and extend vertically from a root. If you opened up a root hair like a road map, you would see that it has a surprisingly large surface area. Root hairs are a lot like the plant hairs, or trichomes, that you see on the aboveground portion of a plant. Trichomes and root hairs are both appendages that grow out of epidural cells. The average root hair is 1/10 the diameter of a human hair.

If you took a cross-section of a root hair cell, you would see that they are shaped a lot like a house key, with a rectangular base that holds the nucleus and a long tail on the other side. That tail is where the magic happens.

Root hairs extend from single plant cells and they grow very quickly, at least 1μm/minute. Since a root hair can live for up to 3 weeks, it might grow to nearly 1-1/4” in length, but most root hairs only live for a day or two. But don’t worry, new root hairs form right away, which is why your transplant is (usually) able to recover.

​Did you know that the reason root hairs are so evenly spaced along a root is because each hair secretes a poison that prevents nearby cells from producing their root hair? I didn’t either.

Root tip showing root hairs (Public Domain)

How root hairs absorb water and nutrients

Nutrient-rich water is pulled into the cytoplasm of root hair cells by osmosis. Root hairs also secrete malic acid, which helps convert minerals into ionic forms that are easier to absorb. Organic molecules in the soil, called chelates, also help root hairs absorb nutrients.

Root hairs as defense mechanism

Because root hairs are so small, they make it very difficult for harmful bacteria to enter the plant through the xylem. When beneficial bacteria, such as those which help legumes fix atmospheric nitrogen, appear, root hairs curl around the welcome visitor. This allows an infection thread to connect the two for everyone’s benefit. Helpful soil microorganisms, called mycorrhizae, are small enough to enter a plant’s root system through the root hairs. Root maggot larvae feed on root hairs.

Plants use phosphorus to grow healthy roots. Before you add more phosphorus to your soil, be sure to send out a sample for a soil test. Too much phosphorus can be just as bad, or worse, than not enough.