It can seem like plants have an endless supply of tangled roots, especially when you are trying to dig or divide them. Most plants have adapted the root structure to their growing conditions. For example, the roots of fast-growing annuals, such as lettuce and coleus, are shallow and tend to remain near the soil surface to take advantage of summer rains. Many plants will send their roots running down a garden bed in all directions. These rampant growers, such as obedient plant (Physostegia virginiana) and several asters, can quickly become weedy this way. And other plants have developed a surefire way to survive drought by sending their roots deep below the soil surface. These are said to have taproots. If you have ever tried to pull a dandelion out of the ground with your hand, you encountered a taproot holding its ground.
What is a Taproot?
As the name implies, a taproot is typically a long and somewhat thick root that goes deep down into the soil. It is the first root to appear from the seed and remain the largest, central root of the plant. Lateral roots will branch off from the taproots and then more lateral roots will form from the initial lateral roots, but the central taproot will remain the largest and will burrow down into the soil the deepest. A good example is a common carrot. The part we eat is the taproot, but you will also notice smaller roots all along the central root.
Carrots are an example of conical taproots, but taproots do not have to be straight or even tapered. Radishes are also taproots, but theirs are wide in the middle and taper at the bottom and often at the top. They are a "fusiform" taproot. Then there are the "napiform" taproots, like beets, that are wide across the top, becoming very thin at the bottom. The shape may change, but the function remains the same, to keep the root deep enough in the soil to access water.
Benefits of Plants with Taproots
Plants with taproots tend to be very drought tolerant. Many desert plants can send roots down more than 75 ft., allowing them to find water, even in dry climates or conditions.
Taproots can also serve to store food reserves, making them even more self-sufficient and resilient.
Drawbacks of Tap Roots - Transplanting and Dividing
Because the taproot goes so deeply into the soil, it can be very hard to dig and lift a tap-rooted plant. Think of the dandelions in the yard.
Dividing tap-rooted plants is another challenge. You can't simply break off parts of the crown, with roots attached, as you would with something like daylilies or coreopsis because you need to get a piece of that tap root with each division. But again, think back to the dandelion and you'll understand that it's not impossible.
Taproots often form offshoots near the crown, called necks. If your plant has these, you can cut off each neck that has some smaller roots attached to it and replants with good success. Buttery bush (Buddleia davidii) can often be divided this way.
If no offshoots have formed, you can still try taking a small piece of the actual taproot, with at least one eye and some smaller roots attached, and replant that. Much like your beheaded dandelion, it will send up a new shoot.
Young seedlings of taproot plants are much easier to transplant. Plants such as butterfly weed (Asclepias tuberosa) will set many seedlings, so you shouldn't need to disturb the original plant. Don't wait too long, are the additional stress will make moving them harder.
Plants with Taproots
It's hard to give a definitive list of tap-rooted plants because of many plants, like most trees, start out with taproots, but will switch to sending out lateral roots closer to the soil surface, once they are established. Tomato plants grown from seed tend to send down a tap root, but those grown from cuttings will not.
Some common garden flowers and herbs with taproots include:
- Balloon flower (Platycodon Grandi floras and CVS)
- Bugbane (Cimicifuga racemosa)
- Butterfly weed (Asclepias tuberosa and CVS.)
- Cushion spurge (Euphorbia polychrome and CVS.)
- False blue indigo (Baptisia and CVS.)
- Lupine (Lupinus polyphyllous)
- Oriental poppy (Papaver orientale and CVS.)
- Sea holly (Eryngium)
And several weeds survive with the help of tap roots, like plantain and kudzu.