Legginess is one of the most—if not the most—common problem for people who start their own tomato plants inside.
Legginess is when seedlings grow tall, thin, weak stems. It is characterized by young tomato plants that are stretched, usually pale in color, and that bend or twist as they grow. Frequently, the stems will break from the top-heavy nature and unsupportable growth. Once broken, the plants will die.
Almost anyone who has ever started seeds indoors has had this happen to them. Fortunately, at least for tomato seedlings, legginess is relatively simple to prevent and can be corrected when it does happen.
Causes of Legginess in Tomato Seedlings
The causes of legginess in tomato seedlings are the same as the causes of legginess in all seedlings. Understanding those causes is key to preventing legginess, to begin with.
There are three main causes of legginess in tomato seedlings:
- Inadequate light—including not enough hours of light, light too far away, and the wrong type of light
- Heat—high soil and air temperatures can contribute to legginess in seedlings because they cause rapid growth, and in the early stages, upward growth may be their only option
- Water—overwatering means that seedlings cannot access oxygen and nutrients in the soil; underwatering can cause them to react to water inputs in bursts of growth
Of these, problems related to lighting are almost always the cause of leggy tomato seedlings, so start by addressing that. Tomatoes can grow upwards quite quickly if they are reaching for the light. But don’t ignore the other possible causes because if your lights seem right, something else, like heat or water, is what’s probably causing your legginess issue.
Preventing Legginess in Tomato Seedlings
- Use grow lights. Most homes don’t have enough direct light for enough hours of the day to keep seedlings from growing leggy.
- Make sure your grow lights provide a full spectrum of light. If tomato plants don’t get the right kind of light (I.e., the full spectrum of cool and warm waves), they will still stretch to look for the light.
- Keep lights close to the plants’ tops. Artificial light needs to be kept within a few inches of the tops of your tomato plants. Some high-intensity commercial grow lights may be able to be further away, but in general, those that are being used in a home setting need to be closer than you would think.
- Keep grow lights on for a minimum of 14 hours per day. Light is not just about the right type and proximity; it is also about making sure your tomatoes get enough daylight hours, which for tomatoes is 14 to 16 hours under artificial lighting.
- Remove seedlings from heat mats after germinating. If you use heat mats to keep the soil warm while germinating, move the seedlings off the mat once most of the tomato seeds have germinated.
- Remove domes as soon as tomatoes have sprouted. If you have a dome covering your tomato seedlings—even if it is a clear dome—remove it when the seedlings have sprouted to keep the tray from overheating and to increase air circulation.
- Water consistently as needed. In general, seedlings need watering about every other day, but household conditions and plant growth and stage can change this. Do not ever let your tomatoes dry out completely. When the tomatoes’ pots or cell packs start to become dry at the top of the soil, water them by bottom watering (set pots into a tray of water to soak up water). Water only until the top of the soil at the base of the plant turns dark with moisture. Then remove the pot from the tray of water. Do not let the soil become soggy, muddy, or flooded.
- Add a fan for air movement. Outdoors, stems respond to wind and moving air by growing thicker and stronger. Indoors, they don’t need to because the air is not moving. So they don’t. A small fan, even a small clip-on fan or small stand fan, can make a big difference and encourage the tomatoes to put their growth into thicker, not taller, stems. You can also run a stand fan in the room on oscillate to mimic wind movement.
- “Pet” your tomato plants daily. Once every day or so, when you check your tomato seedlings, brush your hand lightly over the top of the plant’s leaves. This helps to stimulate and strengthen stem growth.
Correcting Legginess in Tomato Seedlings
Leggy, weak seedlings happen to the best of us. Fortunately, with tomatoes, it can often be corrected, and you can usually save your leggy tomato seedlings in a few steps.
Take action as soon as you notice or even suspect your tomato seedlings are growing leggy.
To correct legginess in tomato seedlings, first remove or correct the problem that is causing your seedlings to grow leggy:
- Move grow lights closer to the tops of the seedlings—keep lights about two inches above the plant tops. You can do this by fitting the lights onto chains and ‘S’ hooks so they can be raised and lowered, or by using something to raise the seedlings up closer to the light—for example, a wooden box, blocks of wood, bricks, a small step stool or shelf.
- Water properly and consistently via bottom-watering.
- Remove from high heat sources, whether those are germination heat mats or close-by home heating units.
Next, repot the tomato plant to fix the problem. The sooner you up-pot the tomato seedlings to correct the legginess, the better.
- Pot up tomato plants and sink them deeply into the soil. Tomatoes can grow new roots from their stems when the stems are in contact with soil, so it works to repot the seedlings into fresh pots and plant them deeply in the pot or cell pack.
- Make sure there is at least some soil at the base of the pot or cell pack for the roots to rest on, then fill up almost to the top of the pot (leaving a lip for water retention, etc.).
- If your tomato seedlings do not need the entire pot depth to correct the legginess, fill in with more soil at the bottom of the pot before replanting the leggy tomato seedling.
- Do not cover the plants’ leaves. You should have some stem and all leaves exposed for the up-potted tomatoes to continue growing.
Once you have up-potted the tomato seedlings to correct the legginess, continue with good light and water management. It is a very good idea to run a fan (as suggested above) after you have repotted leggy tomatoes. The air circulation will help prevent fungal diseases like damping off disease—which weak-stemmed and leggy seedlings will be more prone to, and the movement will make those stems stronger. Use the hand-brushing technique, too, to encourage even stronger stems and help the plants catch up.
Tomatoes are one of the easier seedlings to correct when they become leggy. Prevention is the best medicine, but when legginess does happen, correction and cure can save your budding crop, too.