We all know what roots are—they’re the part of the plant that’s usually underground. If we have a mental image, it’s probably a mass of wiggly, white strings poking their way through the soil. We should pay more attention to roots. After all, they’re an essential part of a plant (as well as the only part remaining after some hail storms!). Knowing a little about how roots work will make us more successful gardeners.

Before I get any further, I should point out that I’ll be talking about your average, every day root. Life is an amazing phenomena, so diverse that there are always exceptions. So let’s skip the orchids (left) and other epiphytes, and the mangroves and other plants with roots growing in water, and focus on our garden flowers, shrubs, and trees.

Roots start to grow when a seed germinates. The embryo inside the seed coat grows in two directions. One part grows upward and turns into the stem, branches, leaves, flowers, and seeds. The other grows downward and becomes the roots. In general, the above-ground portions of the plant contain chlorophyll while the roots do not.

Roots have a number of functions. First, they provide physical support, stabilizing the plant in the soil. These are closest to the trunk or main stem, and are called anchoring roots. That’s all these roots do.

Further out from the plant, are the feeder roots. They absorb things, taking up water and other chemicals. While a plant gets carbon, oxygen, and hydrogen from the air (used to make starches and sugars), all other nutrients the plant needs must be absorbed by the roots. Some of these essentials include nitrogen, potassium, phosphates, and micronutrients such as sulfur, chromium, and iron.

Finally, some plant species have storage roots. These roots are filled with water and extra starches and sugars, hoarded until they’re needed. We often eat storage roots—think of jicama, carrots, and beets. A plant might use these resources to regrow leaves in spring, or after we’ve chopped them off with a hoe. This is why annoying weeds such as dandelions and bindweed resprout so readily.

I had always imagined a plant’s roots as being the same size and shape as the above-ground portion, only pointed in the other direction. I imagined wrongly.

Some plants have tap roots, which reach for water in an arid climate. Others have shallow roots, which avoid the oxygen-depleted zones deep in the soil. In general, plants adapted to areas with high rainfall (or riparian areas with saturated soil) have shallow roots, while xeric plants’ roots run deep.

The surprise (to me, at least) is that for most of the plants in our yards, the vast majority of their roots are within the top two feet of soil! It makes sense when you think about it. Plant roots are alive, and thus need oxygen and water. Most of the oxygen and water in soil is in the top two feet. The more compacted the soil, the shallower the roots will stay.

This is why it’s important to amend soil with some humus. The organic matter acts as a sponge, with spaces for both air and water. Plants will drown in saturated soil, and wilt when the water runs out. The water also acts as a carrier for nutrients, which are absorbed as ions.

Knowing where a plant’s roots are helps us gardeners when it comes time to pull weeds, adjust drip hoses, or renovate our landscapes. We can now see why it’s a bad idea to change the depth of soil around established plants, or to cultivate too deeply.

So if roots don’t grow downward very far, where do they go? Outward. This diagram shows the dripline, and where trees keep most of their feeder roots.

But roots can reach much farther. Tree roots often extend outward to five times the dripline! Think of the implications—large trees don’t transplant well; with most tree spades, you’d have to cut off all the feeder roots. Large trees don’t stay in our small city lots, either; your neighbor has as much influence on the health of your tree as you do! And where should you apply water? It’s important to add concentric circles of drip hose as trees grow.

This is a very basic overview. Roots can be fascinating if you get to know them.
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Tree drawing from http://freecoloringpages.co.uk/