Growing of indoor crops starts soon

A group of hands holding plants

By Anthony Reardon

Though 2025 has just begun, avid vegetable gardens know that cool-season crops can begin to be grown indoors in just a handful of weeks.

Cool-season crops are typically sown in early February and transplanted in mid-March. Soon, spring showers and extended daylight (yes, the sun will come back again) will welcome new verdant life in our vegetable gardens. But first, we need to know how to start those seeds successfully!

Let’s look at some of the more pertinent factors.

  • Success depends on timing. Before sowing seeds, knowing exactly how long it takes a particular crop variety to grow to transplant size and the recommended timeframe for transplanting will assist in having transplants that are sized appropriately and not over- or under- grown. With these two factors, you can subtract the growth period from the planting time to give you your ideal sowing date for a perfectly timed plug.
  • Moisture is key. Consistently moist soil is critical for successful seed germination. Gardeners should remember that seedlings have minimal root systems. Therefore, these plants are supported by the sustenance in their immediate vicinity. If that vicinity is allowed to dry out, so does the seedling. Always aim for “wet but not sopping” soil to achieve the correct amount of water. No moisture should be able to be squeezed from the soil. Avoid overwatering, leading to root rot and “damping off” seedlings.
  • Light and temperature. Too little light for seedlings can result in wilting or disease issues that ultimately arise. Oppositely, too much light can instigate sunburn or overheating of the plants. Know the light requirements of the specific plants being worked with at their full-grown size, as this will indicate how to treat them as seedlings. Soil temperature kept to the 60s (Fahrenheit) will help prevent plants from over-elongating. Gardeners may also try brushing their hands over the seedlings a few times daily to instigate sturdier-growing stems.
  • “Harden Off” the plants. An often-forgotten factor in transplanting seedlings, “hardening off,” is the process of “toughening up” a plant before leaving it to fend for itself in the elements. Their indoor life has been cushy! Your new job as a gardener extraordinaire is to gradually lessen that cushiness for the plants so that the harshness of the outdoors is ultimately less of a shock for plugs at transplant. Doing so, place the plants outdoors in a protected area for a few hours daily, beginning around two weeks before transplanting. Gradually, the amount of exposure the plants have can be extended.

Finally, don’t forget that early spring is the time of year for growing cool-season vegetables specifically. While I know many are eager to get their annual tomatoes in the ground, the recommended time for transplanting (as a warm-season crop) isn’t until mid-May. This makes mid-March their indoor sow time.

Cool season crops that can be started indoors sooner for a mid-March transplant include cabbage, broccoli, endive, cauliflower, kale, kohlrabi, leeks, lettuce and onions.

Happy gardening!

Anthony Reardon is the horticulture small farms agent at the Johnson County K-State Research and Extension Office.