Bumps on tomato stems – are they a serious problem?

Bumps on tomato stems start as hundreds of tiny hairs up and down the stalk. Hairs can turn into roots when buried underground. Above ground, they form tiny nubs or nodules, also called root initials, adventitious roots, or tomato stem primordial. They are the earliest stage of development of a tomato’s roots.

Most of the time, bumps are not harmful to tomato plants and are considered normal.

When do bumps on tomato stems develop?

Root initials emerge on a tomato stem as a result of stress – most often when there’s a limit or blockage in the stem’s vascular flow.

Specifically, a hormone named auxin in branch tips sends a signal to the plant’s roots when it’s blocked in the branch.

If there’s a blockage and the hormone can’t flow freely, it accumulates in the stem.

Nodules or bumps emerge. The blockage can occur anywhere on the stem.

Blockages are caused by a number of things:

internal injury
irregular cell growth
high humidity
overly wet weather
over-watering
poor drainage
in very occasional cases, disease (fusarium wilt or verticillium wilt, for example)
other root damage

Often the blockage is water-related. A tomato plant’s roots need air as much as they need water. Over-watering or excess rain (including poor drainage) chokes them. The plant compensates by trying to develop more roots out through the stem – hence the root initials.

Root initials can be the same color as the stem. They may also be white or turn to brown.

When bumps on tomato stems grow excessively

If you see overgrown root initials on your plants, also check the leaves at the tops and the branch ends. Are they curled or stunted? That can indicate exposure to an herbicide.

Herbicide effects mimic the plant hormone auxin and can also cause abnormal stem growth and leaf distortion.

If your tomato patch is near your yard or a neighbor’s, perhaps a weed-and-feed product, weed control, or other herbicide was applied and drifted. Mist often affects tomato plants.

How do you control and treat bumps on tomato stems?

Root initials may not be attractive, but they do not harm plants.

For root initials on lower stems: When root initials make contact the soil, they will grow. Mound compost around root initials on the lower part of the plant. They will develop into full-blown roots, strengthening the plant’s root system.
For unstaked plants: If you have not yet staked plants and bumps appear on stems, you can continue to let the tomato vines sprawl on the ground instead of staking them. Root initials will grow into roots firmly attached to the soil.
When wilt accompanies root initials: If you have a section of the garden with many root initials on plants that also have wilt problems, then the area is likely too wet there. If possible, alter watering habits.