Tomato and Pepper plants indoor dieback


Last year when I was starting peppers and tomatoes in an indoor electrically lighted/heated area when I experienced a big yellowing phenomena in primarily tomatoes and peppers. My watermellons also suffered, but I think I started them too early, so I’m concerned about preventing this for the other plants. I used continuous lighting.

I assumed it might be a fertility issue, as the plants were growing nicely until then, maybe 3 inches tall, with a full set of proper leaves when they yellowed off. The peppers responded very well to the fertilizer I gave, a liquid, but the tomatoes did not respond, and most died. I had reseeded, and removed the tomatoes before they did the same thing again!

Any ideas? I suspect the continuous lighting may be at issue, but I’m not expert.

Thank you for your time!


Dear Gardener – thank you for writing about your experiences with indoor seed propagation, because we can be sure there are many growers like yourself, who have had to deal with similar issues, and so this might be valuable reading for many.

For successful tomato and pepper (and watermelon) growing – and numerous other edible annuals – care should be taken to prepare materials and supplies, including: good seeds, clean containers, sterile planting mix, warm, adjustable, air temperatures, good air circulation (a fan), and a good light source. You have a heated space, and lighting available, which is very fortunate. By process of elimination, perhaps your problem can be solved.

You didn’t mention your seed sources: reputable seed houses clean, disinfect, test and sterile-package their seed. If you’re unsure of your seed source, or have swapped seeds, test your seeds for purity (no evidence of foreign matter) and viability (healthy enough to germinate).

Purchase cell packs. If re-using cells, protect new germinating seeds by eliminating any residual fungus and bacteria. Sterilize 10 minutes in a 1 to 9, household bleach-to-water bath. Rinse well with water. Using larger cell packs allows early emerging cotyledons, and then roots, to grow deep and strong.

Planting medium: tomato, pepper and most edibles seeds will only germinate when moist. Use a light and fluffy ‘soil-less mix’ of peat moss, vermiculite and Perlite. This blend retains moisture, yet drains well, provides higher oxygen levels and discourages ‘Damping Off’ disease. And now here’s very critical advice: do not use any mix material that was used before, to avoid spreading shared diseases.

Next, when you talk about fertilizer, alarm bells go off: only very lightly fertilize seedlings with a germinating P (phosphorus) solution. Avoid N (nitrogen), which could burn the shoots and roots. If the roots are burned, they cannot send nutrients and moisture up to the first sets of leaves, which will then fail to thrive.

Regarding your question re continuous lighting, your hunch may be correct, and this may be contributing to your seedlings’ failure: after seeds emerge, they need 14-16 hours of light daily, then 8 hours of darkness to process their food and grow. To help you with this regime, you can set a 24-hr timer, at your light power source, accordingly. Fluorescent grow-lights will promote the best growth: a cool white bulb provides light in the blue/green range. Adjust the lights to 2 – 4 inches above the seedlings, moving up as the seedlings grow.

And regarding heating, could it be that your air temperature needs adjusting? Many gardeners, before seeding, aim for warm mix, cooler air. They place their cell trays on heating mats to warm the mix, and then test for 15 – 25º C.  Cool growing medium slows germination, putting seeds a risk of rot. Air temperatures can be cooler. Once germination is visible, turn off the bottom heat, and now increase the air temperature for the next 4 weeks. From this point on, aim to keep day and night temps equal, for shorter, sturdier plant growth.

You haven’t mentioned watering: after germination, maintain plants on the dry side, to manage plant height. Check every day: do not let the potting mix dry out, but do not let it get too wet. Apply water from the bottom tray up. For good air circulation, to discourage mildew or fungus, set up a small fan, at low speed, to move air horizontally across the tops of the plants.This will promote sturdy stems, and help prepare seedlings for outdoors.

And finally, hardening off: if you live in south-central Ontario, about a week before May 20, start carefully placing trays outdoors in a very shaded, sheltered location — only for a few hours each day — to develop stem strength and acclimatization to air temperatures, and sunlight.

We hope this helps you, and other gardeners, with awareness of many aspects of optimum plant culture, including selection of high quality seed, best growing media, and careful, patient germination, employing regulation and timing. Speaking of which there is still plenty of time this spring for you to take another go at seed germination!

For your further research, you may wish to read another post on Toronto Master Gardeners about growing from seed.