By Carrie Knutson, Horticulture agent

NDSU Extension – Grand Forks County

Seed starting time is just around the corner. I would like to share a cautionary tale about seedlings while there is still time to order or purchase your seed-starting supplies. This way, you won’t have to live in fear of your precious tomato seedlings dying, as I did last year.

Two years ago, I found a tomato variety I really liked by accident. Last year, I was lucky enough to find the seed. It was expensive seed, and I was really watching over my seedlings. To my dismay, after the seedlings were transplanted, the leaves began to curl, and the lower leaves started to die. There was always new growth, but the older leaves just kept dying.

At that point, there wasn’t much I could do. I transplanted the tomato seedlings into new containers with new potting soil. They seemed to hold their own but only improved when it was time to harden them off outside and plant them in the garden. I planted the tomato plants deeply and had the best tomato crop ever.

What was happening to my tomato plants? I am still not sure what happened. All my other seeds germinated, transplanted and grew well. With that in mind, let’s go through a list of things to consider to make sure your seedlings survive indoors and can be planted in your garden.

First, clean and disinfect last year’s seedling containers and tools. After washing them in hot, soapy water, soak them in a 10% bleach solution for 30 minutes. Damping off is a common seedling disease that can kill the seedlings of many different plants and vegetables. The disease-causing organisms can survive in containers used the previous year. Cleaning and disinfecting containers and tools will help prevent the disease.

Second, seed heat mats can help improve plant growth by keeping the soil temperatures warmer. In addition, grow lights are a great investment for seed starting. The grow lights should be on for 12-16 hours every day.

Third, keep your seedlings and transplants healthy by using potting soil or a seed-starting mix. The mix will allow excess water to drain, keeping the mix moist but not overly wet. Always use new potting or seed-starting mixes to help with any disease issues.

Fourth, water seedlings with room-temperature water. Keep the soil moist, but not overly wet. Seedling containers should have drainage holes. An easy way to avoid overwatering is to water the seedlings from the bottom by pouring water into the catch tray. Wait about an hour and drain the excess water from the catch tray.

Finally, fertilizer should be applied only after the first several true leaves have formed. Then use a quarter-strength mix to avoid burning the seedlings. Additionally, a small fan can be used to create air movement and strengthen stems.

I’m still not sure what caused the issue last year. I thought I did everything right. Perhaps the tomato variety is a bit picky when grown indoors. I might give a seed heat mat a try. I will let you know how it goes. Happy gardening!

NDSU Agriculture Communication – Feb. 17, 2026

Source: Carrie Knutson, 701-780-8229, carrie.knutson@ndsu.edu

Editor: Dominic Erickson, 701-231-5546, dominic.erickson@ndsu.edu