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When do I plant tomatoes in the garden?

Peggy Singlemann
Monitoring the soil temperature is critical to knowing when to plant specific crops in the garden.

Waiting is so difficult, particularly when the weather is so warm and beautiful. While the daytime temperatures are rising the night temperatures are influencing the gardening calendar, too. Many flowers and vegetables thrive in the cool shoulder seasons while others thrive during the heat of the summer. Cool season flowering plants such as pansies, sweet peas, coral bells, lobelia, alyssum, and snapdragons decline as the temperatures warm. The same can be said for cool season vegetables such as leafy greens, celery, leek, peas, broccoli and other brassicas, plus many more. When we think of temperature defining these 2 different growing seasons, we must think past the air temperature and consider the soil temperature, too. Cool season plants grow in soil temperatures ranging from 40-70 degrees while warm season plants typically thrive in soil temperatures above 70 degrees.

Monitoring the soil temperature is done with a soil thermometer fully inserted into a garden bed. The entire length of the 8” shaft of the soil thermometer measures the temperature so any exposure to air will influence what the thermometer registers. Every February I insert a soil thermometer into a garden bed and once the thermometer records 40 degrees I know it is time to sow snow pea seeds. I plant potatoes when the soil temperature is 59 degrees but I wait for the soil temperatures to rise even higher before planting my summer garden.

Putting a plant in the ground when the soil and air temperatures are too cool affects the plant immediately. The growth of the plant and its roots become stunted, the leaves often die back, wilting is common due to compromised roots and the weakened plant becomes more susceptible to disease. The seeds of warm season plants sown into cool soil could either fail to germinate, rot, or could have delayed germination as the seeds wait for the soil temperature to rise.

Sowing warm season crop seeds indoors provides extra time to bring that crop to harvest. I typically sow my pepper and eggplant seeds 8 weeks before the last frost date. I then sow my tomato seeds 6 weeks before the last frost knowing I typically have another 2 weeks after that date before the soil temperature is warm enough to plant them all in the garden. I use those 2 weeks to slowly acclimate the plants to the UV rays of the sunny outdoors. Please keep in mind, full sun indoors is equal to full shade outdoors. Plants grown indoors form a thin cuticle or waxy covering on the leaves and stems, this cuticle acts similar to sunscreen to protect the plant from UV-B rays per recent research by the Scientists of the Institute for Mediterranean and Subtropical Horticulture "La Mayora", members of the University of Malaga and the Spanish National Research Council. Thickening the cuticle prepares the plant for the intense rays of the sun and prevents sunscald of the leaves and stems, which is evident when a plant part turns papery white or tan. To thicken the protective cuticles of indoor plants they need to be exposed to the sun in stages. The first stage is to move the plants outside to a shaded area for a few days, then move the plants to part shade with morning sun for a few days. From there slowly lengthen the periods of sunlight over the course of a week until the plant is acclimated to 6 hours of full sun. This entire process should take around 2 weeks to complete. During this period, the plants will require additional water due to exposure to the outdoor elements. I also fertilize my plants at half the recommended rate on the product label during this acclimation period.

Shana Williams
Young plants grown indoors are too tender to plant directly into the garden without a period of acclimation.

Once in the ground I continue with the organic based fertilizer program I use in my vegetable garden, which relies a lot on healthy soil amended over time with compost. I use every opportunity to add compost to garden soil. In the spring I add compost to the depth of ¼” to each planting bed. I do this by scattering the compost like one would toss chicken feed over the soil surface. When I plant, I add a little compost to every hole or row and during the season I occasionally side dress with a little compost. Remember, compost is teaming with millions of microorganisms which will enhance the soil microbiome enabling plants to grow stronger and more resilient to pest and disease issues. The secret to ecological success is healthy living soil and compost is a primary building block of soil rich in organic matter, nutrients, and beneficial microorganisms.

biodegradable kitchen waste on soil. composting organic food leftovers. copy space
ronstik/ronstik -
Compost looks like rich chocolate cake and is filled with microbes which enriches the soil.

Happy Gardening,


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