The Importance of Sterility in Your Greenhouse: Tips for Starting Seeds (2024)

FAQs

Does seed starting mix need to be sterile? ›

The first thing to think about getting for seed starting is “soil.” Seed starting mix is, in fact, what's known as “soilless mix.” It's not like garden soil or dirt — it's sterile. That means it doesn't have the microbial life in it that garden soil does.

Put the soil in big flat pans with sides (recyclable aluminum roasting pans work well) to about 3" deep, and cover tightly with aluminum foil. Insert a thermometer through the foil, into the soil. Cook at 190 degrees for about 30 - 40 minutes, ensuring the temperature gets to about 180 (but not above 200 degrees).

Potting media mixed by home gardeners should be pasteurized to kill weed seeds, soil-borne insects, and pathogens. Pasteurization destroys most organisms and is done by applying heat until the soil reaches 180oF (82oC) for 30 minutes.

Put seed trays on a heating mat to encourage faster germination. Ideally, use a mat with a thermostat so the soil does not get too hot. Many seeds prefer a soil temperature between 70 and 85 degrees Fahrenheit. Provide water as necessary to keep the soil moist before germination and during the seedling stage.

Seeds harvested from plants grown in a greenhouse or growth chamber are occasionally contaminated with microorganisms and dust. Growth of Arabidopsis seedlings on different types of sterile media requires prior seed sterilization to remove microbial contaminants such as fungi and bacteria present on the seed surface.

Two chlorine-based chemical disinfectants—dilute household bleach (0.6% sodium hypochlorite) and freshly generated hypochlorous acid (800 ppm chlorine)—were the most effective disinfection agents tested (up to a 5-log reduction in bacteria) that also did not harm seed germination.

By eliminating harmful organisms, weed seeds, and pathogens, soil sterilization provides a clean and uncontaminated substrate for plants to thrive.

Seeds are surface sterilized with bleach solution (30% commercial bleach + 0.02% Triton X-100) for 15 min, washed 3 times with sterile water, and placed at 4C overnight prior to plating.

In agrarian settings, soil steam sterilization is typically achieved by exposing soil to superheated steam either in an open field or a greenhouse; the temperature of the steam kills off viruses, pathogens, bacteria, fungi, pests and more.

Starting seeds in a cold frame or greenhouse is a smart and efficient way for gardeners to extend their growing season and ensure healthier, more robust plants.

Why are my seeds not germinating in my greenhouse? ›

The soil was too wet or too dry:

Too dry, and your seeds won't germinate. Too wet, and they're liable to rot. Ensure that seeds are kept evenly moist by thoroughly wetting and draining the compost before you begin.

Ideally, anywhere between , 80-85°F (26-29°C) is the golden standard throughout the spring and summer months. Anything higher than 90°F (32°C) is cause for alarm. We suggest monitoring your greenhouse temperatures with a good thermometer.

Hybrid seeds from conventional produce may be sterile (will not grow) and if they do grow, will not give you an exact copy of the parent plant, but rather something closer to one of the varieties used to create the hybrid. For more predictable produce from harvested seeds, purchase nonmodified, heirloom varieties.

Potting mix is a sterile, soilless potting medium that holds water and stays fluffy so that roots can grow easily, and excess water can drain away quickly. In general, you need to use potting mix when planting any containerized plantings. Being sterile, it doesn't contain pesky weed seeds, insects or diseases.

Old mix can, however, be sterilized for reuse. It will not be as fresh, but the sterilization process will ensure it is pest and disease free. In fact, I sometimes sterilize fresh seedling mix when I am starting prized seeds.