March 08, 2021

Kreith: Start herb seeds indoors

Herbs are a favorite in most gardens, but transplants can be expensive. As an economical alternative, try starting seeds indoors in early spring.

Start herb seed in early March to be ready for transplanting into the garden in May depending on your region of Illinois. Refer to the Illinois State Water Survey at https://www.isws.illinois.edu for your region’s average frost-free dates.

Thyme, rosemary, basil, sage, chives, and tarragon are great choices to start indoors. The seeds will take about 14 days to germinate. Oregano started by seed may not be true to the type of seed planted, and flavor will vary. Experts recommend propagating strong-flavored plants by root division or cuttings to ensure best flavor.

To start herb seeds indoors, use a soil-less seed-starting mix with a container that provides drainage. Pre-moisten the mix with water until it reaches the consistency of a wrung-out sponge. Fill a container or seed-starting flat with the moist mix leaving about 1/4 inch of space at the top.

Prepare the seeding area by sorting various seeds and labeling containers with the herb name and planting date.

Plant a pinch (about 5 to 10 seeds) of one herb variety per container or cell and lightly cover it with moist mix. As a general rule of thumb, plant a seed two times its thickness under the soil.

After planting, keep seeds moist during the germination period. Cover the flat or container with a clear plastic bag to hold in moisture. However, be sure to monitor the growing media for mold growth. If you see mold, make a hole or open a corner of the bag or remove it completely to improve air circulation. Supplemental moisture can also be provided by using a spray bottle.

The sown containers or flats need about six hours of sunlight per day. A west or south facing window will work well at the beginning, but over time, the herb seedlings will require more direct and intense lighting. Using supplemental grow lights (10 hours per day) has proven to work better than natural sunlight. Place lights as close to the seedlings as possible, adjusting height as seedlings grow taller.

As plants become overgrown, seedlings can be thinned out to one plant per pot. Select the strongest and most compact seedlings.

Monitor the seeds and seedlings daily as the transplants mature. Be on the lookout for insects, rot, and extremely dry soil.

The seeds and seedlings should only need a light sprinkle of water about twice per week, depending on the size of your pot along with temperature and humidity levels. A good practice is to allow the soil media to dry out a little before watering again. Overwatering can lead to a fungal disease or damping-off and constant moisture can attract fungus gnats.

Most herb seedlings should be ready to transplant outdoors in about 10 weeks. Help plants harden off or become acclimated to outside conditions by placing them outdoors on mild days and bringing them back inside every night. Keep plants out direct sunlight initially and continue the process for five to seven days. Once plants are hardened off, they can be transplanted safely into the garden.

While transplanting the seedlings, you may also sow some seeds directly in the ground. Herbs that do well by direct sowing include cilantro, arugula, and basil. Keep in mind that cilantro and arugula are cool season plants and basil likes the warm weather.

For best flavor, harvest herbs just before they flower. Visit web.extension.illinois.edu/herbs to learn more details about specific herbs, their growing requirements, and harvesting and storing methods.

Nancy Kreith is a University of Illinois Extension horticulture educator.