Purple leaves & stems on tomato seedlings #787201

Asked April 18, 2022, 5:04 PM EDT

I started 37 tomato seedlings in the house which are now in 4" pots w/potting soil. I have two grow-lights, therefore, for all the plants to receive sunlight, I have been putting the plants outside in the sun when the weather got warmer. Some of those days the temperature may have been too cool. They were brought into the house at night. Approx. the past week I have noticed that the underside of the leaves, plus, the stems are a purple color & the plants don't look good. What causes this? Could it be the soil getting too cool which has caused the potassium to be locked up & not reaching the plants? Any suggestions of a remedy & will the plants recover? Thank you

Dorchester County Maryland

Expert Response

Some purpling of stems and leaf undersides is very common in tomato transplants grown under supplemental lighting. It is a plant stress symptom probably caused by several factors including nutrient levels, changes in temperature, light and humidity levels, crowding or poor airflow, etc. Cool conditions preventing proper phosphorus absorption is a possibility while the plants are outdoors on cool days. Plants typically grow out of these symptoms after being planted in garden.

Despite the apparent nutrient deficiency symptoms, don't fertilize unless the plants are yellowing and have been growing for at least 8 weeks in the same potting mix. (Even though tomatoes have a high nutrient requirement when maturing outdoors, we don't want to encourage more growth that they can't support in their current growing conditions.) In future years, we suggest you wait to start germinating the seeds until only about 5-6 weeks ahead of planting time outdoors. (So, sowing in early April.) This helps to avoid the stresses of being indoors too long and minimizes legginess as the plants stretch. If their stress is just caused by environmental conditions, there is no remedy except to plant them outdoors once we hit the frost-free period around mid-May.

When moving indoor-grown plants outdoors, they need time to acclimate to the brighter light levels. Although it may not look this way to our eyes, shade outdoors is usually brighter than a bright-light location indoors, so moving plants directly into sun is such a large jump in light intensity that it can cause damage to leaves. (These leaf "burns" won't heal, but subsequent growth should be better-adapted to the light levels and not as prone to damage.) Instead, transition plants more gradually by setting them outside into a mostly-shaded location first, slowly introducing them to more direct sun over the coarse of a couple of weeks.

If their growth has been weak or spindly, perhaps the seedlings weren't getting enough light while indoors. (Grow lights come in a wide range of intensities, though you can adjust for this somewhat by growing plants closer to the tubes/LEDs.) You can send us photos of the plants if you'd like help in determining why they don't look good at the moment.

Mira Talabac Replied April 19, 2022, 4:59 PM EDT

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