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Spring into summer by starting your seeds indoors - MessAge Media: Outdoors

Spring into summer by starting your seeds indoors

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Posted: Friday, April 5, 2013 1:24 pm

The U of M Extension has a wonderful educational brief on seed starting. Here is an abbreviated version, but you should really go online and check out the complete brief. It is very non-techy so anyone can follow through. For complete informational brief: www.extension. umn.edu/distribution/horticulture/m1245.html


Start seeds in small, individual containers. It’s best to use divided containers with a single seedling per container, rather than filling a larger container with potting mix and sowing many seeds, because the seedlings’ roots will grow into each other and are likely to be injured later during transplanting.

Plastic sheets of small containers, called “cell flats,” fit into standard solid trays. Small individual plastic pots are also suitable. All seed starting containers must have drainage holes at the bottom.

Most plastic seed-starting containers are reusable, but may harbor plant pathogens once used. Sterilize used containers by soaking the cleaned cups in a solution of bleach or other disinfectant for 30 minutes, then rinse and use.

Mix the solution to the strength recommended on the label for disinfecting surfaces.

Soil-less seed starting mixtures

Commercial seed-starting mixes, usually composed of vermiculite and peat, without any true soil, are recommended for starting seeds. They’re sterile, lightweight and free from weed seeds, with a texture and porosity especially suited to the needs of germinating seeds and tiny seedlings.

Set the cell flats or containers into a solid tray, fill them with potting mix, and water the mix before sowing seeds. The potting mix will settle down into the containers, sometimes dramatically so. Add more potting mix and water again, until the containers or cells are nearly full.


Important: Follow seed packet or catalog instructions, as each species has its own requirements. In Minnesota, annual flowers and heat-loving vegetables such as tomato, pepper and eggplant are usually started in early spring.

Cabbage and broccoli intended for fall crops may be started indoors in June or July.

Sowing seeds

Sow fresh seeds individually into each container according to package directions. If you are unsure about seeding depth, a rule of thumb is to plant a seed four times as deep as its width. Mark each pack with a tag.

Read your seed packet carefully. Do your seeds need light to germinate? Or does it require darkness to germinate? Seeds that require light to germinate should be covered with a thin layer of fine vermiculite, porous enough to permit light to penetrate yet keep the medium moist enough to encourage seed germination. Place cell packs containing seeds that need darkness for germination in dark plastic bags or cover them with several layers of newspaper until seeds sprout.


A windowsill is not a good location for starting seeds. If you’re starting only a few plants and have roomy window sills, a south-facing window may be all the growing space you need. But window sills can be the coldest place in the house, especially at night, and then the hottest during the day. Although sunlight in Minnesota gains strength through April and May, the months when seeds are usually started and seedlings kept indoors, sunlight through a window is relatively weak, compared to artificial light sources kept close to the plants. There are also many cloudy days of very low light levels during a Minnesota spring.

More from Hasselius about starting seeds indoors in Part 2 next week.

Janice Hasselius, originally from Aitkin, has been a University of Minnesota Master Gardener since 2000. She regularly volunteers for writing, teaching classes, and demonstrations on gardening subjects through the University of Minnesota Extension Service.

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