Roots

It took plants 50 million years to develop roots as we know them today. Everyone knows that roots provide anchorage and access to water and minerals in the soil. But they do far more than that!

• Fine root hairs use chemical messaging to communicate a plant's needs to nearby soil microbes
• Root plant cells synthesize hormones
• Roots secrete sugars, acids, and amino acids that protect against pathogens and stabilize nearby soil aggregates
• Root systems reduce and prevent erosion
• Some types of roots help aerate plants growing in wet conditions

First root

The first structure to emerge from a sprouting seed is called the radicle. The radicle becomes the primary root. If the primary root continues to grow and develop, the plant will have a taproot system. Carrots are taproots. If several smaller roots develop, the plant will have a fibrous root system. 

Cassandra gonzales (CC BY-SA 4.0)

Adventitious roots may arise from stems, branches, or leaves. These opportunist roots come in a variety of growths, such as buttress roots, aerial roots, and stilt roots. ​

Aerial roots (C1815) Public Domain

Stilt roots of corn plant (Krish Dulal) CC BY-SA 3.0

Buttress roots of ficus tree (Wikipedia) CC BY-SA 3.0

Storage roots

There are also storage roots. Beets, carrots, ginger, horseradish, licorice, radishes, rutabagas, sassafras, sweet potatoes, and turmeric are all storage roots. You may think that regular potatoes are a root crop, but they are not. Potatoes are tubers, which means they are the starch storage structures for rhizomes, or underground stems.

​Root structure

Unlike twigs and stems, which have regular branching patterns, roots start out large and then divide, repeatedly and randomly, getting progressively smaller, in their search for food and water. At the end of each root is a root cap. The root cap is like a hard hat, worn to protect the growing meristem tissue, as the root moves through the soil, looking for water and mineral nutrients. Behind the root cap is where cell division takes place. As these new cells are produced, the root cap is pushed forward. As more new cells are produced and the root moves forward, these 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.

Root hairs

Root hairs are not actually roots, as we think of them. Root hairs are actually cells used by plants to increase surface area. Root hairs absorb nutrient-rich water through osmosis. They also excrete chemicals that make mineral nutrients easier to absorb. These threadlike cells are very delicate and early broken off. Root hairs can live for up to 3 weeks, but they generally die within a day or two. That’s okay because new root hairs are constantly being formed.

In legumes, this is also where root nodules used in nitrogen fixation develop. The root hairs curl themselves around the bacteria responsible for nitrogen fixation and allow themselves to be ‘infected’ with these beneficial bacteria. New root hairs are constantly being formed at the root tip, behind the root cap.

Root tip (CNX OpenStax) CC BY-SA 4.0

​Root damage

Transplanting and repotting plants often shears off a large number of these root hairs. This is what causes the initial wilting. Rather than tamping down your transplants, try mudding them in to protect those important root hairs.

​Tree roots can damage sidewalks, patios, and your home’s foundation, so plan ahead before installing trees and large shrubs. Poor health or branch die off on one side of a tree or large shrub can often be directly linked to damage to the root system from construction, digging, or heat islands.

Tree roots

We used to think of tree roots as going down, down, down, into the Earth. In some cases, that is accurate. One tree in the Kalahari desert has a root system believed to be over 220 feet deep, but, in most cases, tree roots don’t look anything like that. Tree roots are most easily pictured as a goblet set on top of a dinner plate. The goblet represents the aboveground portion of the tree while the shallow, far-reaching dinner plate represents the root system. Very often, a tree’s root system is 4 to 7 times the diameter of the aboveground portion. Trees are often classified using a ‘root-to-shoot’ ratio. This ratio refers to the weight of the aboveground portion of the tree to its below ground growth. Normally, this ratio is 1:5 to 1:6. This means that the 5,000 lb. tree you see probably has a root system that weighs over 1200 lbs.

Mango tree root system (Aaron Escobar) CC BY 2.0

The majority of a tree’s root system is found in the top 18 inches of soil. This is why trees fall over when the soil gets waterlogged. Trees growing in heavy clay soil, such as we have in San Jose, California, tend to have smaller root systems. This is because clay soil holds far more water and nutrients than other soil types.

Root depth

Most garden plant root systems are relatively shallow. Rooting depth of garden vegetables can generally be classified as:

• Shallow (12” to 18” deep) - garlic, lettuce, onions, peas, radishes, spinach, zucchini
• Medium (24” to 36”) - beans, beets, cantaloupes, carrots, cole crops, cucumber, eggplant, peppers, potatoes,
• Deep (> 36”) - chard, sweet corn, lima beans, melons, parsnip, pumpkins, rhubarb, winter squash, tomatoes, and asparagus​

What does all this mean?

This means that it can get crowded down there, if you are not careful. If you have deep raised beds, save them for the medium-sized root systems. Most deep-rooted perennials are best planted directly in the ground. The most shallow-rooted plants can often be grown in containers.  Root depth also plays a big role in how drought tolerant a plant can be, and how deeply they need to be watered.

Root pests and diseases

Root systems are subject to specific pests and diseases. Common root diseases in California include:

• armillaria root rot
• aster yellows phytoplasma
• damping-off
• fusarium crown and root disease
• phytophthora root and crown rot
• root rot

Peach root knot nematodes, crown and root aphids, root weevils, flea beetle larvae, and root borers are just a few of the garden pests that attack root systems.

When removing plants from your yard or garden, it is best, whenever possible, to cut the plant at ground level and leave the roots in the ground. As the roots decompose, they will feed the local soil microorganisms, which will migrate, over time, to help another plant thrive.

Whatever type of root system your plants have, good soil health and soil structure are critical to plant health.