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Extension > Yard and Garden News > Fellow Gardeners—Start Your Seeds

Fellow Gardeners—Start Your Seeds

Meleah Maynard, University of Minnesota Master Gardener

In February, as winter seems to drag endlessly on, gardeners who just can’t wait to feel some soil between their fingers finally have something to do: start sowing seeds. While many seeds need to be planted just four to six weeks before being moved outdoors, others that are slower to mature should be started 10 to12 weeks before being transplanted. Timing is important because you want your seedlings to be strong enough to manage on their own, but you don’t want them getting so big that they crowd each other and compete for light, water and nutrients.

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Photo 1: Seed racks can be found this time of year at most garden centers. David Zlesak

If you haven’t already got a planting area set up in your basement or other cool, out-of-the-way place, you can easily do that now. Bright windows of course will do, but with the short days this time of year and often limited window space, lights are a great alternative. Be sure to find a spot near a power source so you can plug in lights and any heating elements you may need. Garden catalogs are full of seed-starting systems, but you can save money by building your own. I start my seeds on metal shelving I found at one of the big box stores. Shelving units are nice because multiple layers of plants can be grown in a particular space. art3-2_600px.jpg

Photo 2: Shelving systems allow for efficient use of space. Aluminum foil or other reflective material can be used against walls to help reflect light onto seedlings. David Zlesak

Fluorescent shop lights work great (you usually don’t need spendy grow lights) and can be suspended above each shelf from chains. The chains allow one to adjust the height of the lights as the seedlings grow. Generally, you want to keep them within a few inches of the tops of the plants once they have germinated. Florescent light is much less intense that sunlight and having the lights relatively close to the plants increases the light intensity. Keep lights on at least 14 to 16 hours each day to fuel strong growth. I find it’s easiest to just put them on a timer. Although you can keep lights on 24 hours a day, a dark period is useful to encourage plants to open and close their stomates to help them better acclimate when it comes time to transfer them outdoors. art3-3_600px.jpg

Photo 3: Regular florescent shop lights work great for young seedlings and can be put on automatic timers. David Zlesak

Plants need, in particular, the red and blue portion of the light spectrum in order to fuel photosynthesis. Cool white florescent bulbs tend to be rich in blue and borderline in red light. So some people like to mix warm and cool white florescent bulbs in order to get additional red light. This rich mix of red and blue light is what makes grow lights look purple. Bulbs differ based on wattage and lumens. The more lumens, the more light. The highest lumens you can typically find for four-foot T12 florescent 40-watt bulbs is about 3,200. Remember that because florescent bulbs contain a small amount of mercury, they are considered hazardous waste. Many cities have designated drop off sites and free disposal of florescent bulbs of all types. Several drop off sites are located in the metro (www.rethinkrecycling.com/residents/throw-buy/household-hazardous-waste-collection-sites)

People recommend all kinds of different containers for starting seeds, including trays and flats, plastic takeout containers, milk jugs, egg cartons. It really depends on personal preference and what works best with your set up. I like to use 12-ounce, clear-plastic cups (the kind you get for beer at a keg party—not that I go to those anymore). These take more growing medium than I would use in a shallow tray, but the advantage is that I don’t have to transplant the seedlings into larger, individual containers once they really start to take off. I just thin them out in their cups, leaving only one or two plants in each. art3-4_600px.jpg

Photo 4: Various sized and shaped containers can be used and a good germination mix to promote germination and avoid pathogen attack is recommended. David Zlesak

To start seeds, you want a growing medium that is light (to allow seeds to push their way up through it), porous (to allow proper drainage and aeration) and sterile (to preclude disease). You can buy soilless seed mixes or you can make your own using equal parts peat moss, fine perlite and vermiculite.

Seeds usually germinate best when the soil is warm and moist. Some seedlings need light to germinate while others don’t. Many can be put on top of your refrigerator or another warm spot until they germinate and then moved under lights to continue growing. (Windowsills aren’t a good choice, usually, because they can be cold and drafty this time of year.) Or you can purchase heating mats or cables to place under your plants in your prepared growing area. For most species, after the seeds germinate, it is good to reduce the temperature to help promote sturdy, compact plants. art3-5_600px.jpg

Photo 5: Garden centers often have a wide selection of seed starting supplies including heat mats, germination mix, and containers. David Zlesak

Directions on the back of your seed packets will usually tell you what you need to know, including how deeply seeds should be planted, how long until germination and whether the seeds need to be treated in any way before planting. These treatments, such as soaking seeds in water, scuffing the seed coat or placing them in a cold environment for several weeks before sowing, are often necessary for successful germination. If there is no recommendation for planting depth, the general rule is to plant seeds two or three times as deep as the seed’s diameter. Small seeds, such as poppy, snapdragon, and flowering tobacco, should be thinly scattered on top of the soil and very lightly covered, if at all. (Germination tips on a wide range of species can be found on this website: http://www.backyardgardener.com/tm.html.) art3-6_600px.jpg

Photo 6: Plastic domes or baggies help to keep to retain high humidity during the germination process. David Zlesak

You’ll want to moisten your potting mix if it is dry before starting to plant. That way, when you water after planting the mix will easily soak up the moisture. As you sow your seeds, be sure to label each container. Trust me, if you think you’ll remember, you won’t. I’ve finally learned my lesson on that one. Once everything is planted, lightly mist the top of the soil with warm water and cover each container with plastic wrap or domes to help keep the seedbed humid and moist. Continue to mist plants, or water very gently, as needed. art3-7_600px.jpg

Photo 7: Labeling right away helps to keep track of what was planted. David Zlesak

Once your plants have emerged from the soil, you can begin to remove the plastic wrap (or plastic cover if you’re using the trays). Keeping seedlings covered will help to keep them from drying out so fast, but be careful the humidity isn’t kept too high. A lot of condensation can put developing seedlings at risk for attack by pathogens. I cover my cups with plastic wrap, leaving the sides somewhat open so air can get in. As plants grow, I use toothpicks or tongue depressors to keep the plastic away from the seedlings. Heating units can also be turned off at this point after germination is complete. If you germinated your seed without much light, remember to turn the lights on and keep them within a few inches of the plants to promote growth and prevent them from stretching. art3-8_600px.jpg

Photo 8: Higher light levels helps to prevent seedlings from stretching as in the case of this young petunia. David Zlesak

Check your seedlings regularly so they don’t dry out. Once your seedlings get their first few leaves (their first “true” leaves after the cotyledons), begin to feed them with a water-soluble fertilizer mixed at quarter strength once a week to help fuel strong growth. I like to use fish emulsion, though it is quite smelly. Soon the dreaded time of thinning will come. (I dread it, anyway) Though it feels like killing your darlings, you must thin the crowd in each seed container, leaving only one plant, maybe two, in some cases. Extra seedlings may be transplanted to new containers.

Finally, spring comes and your seedlings are almost ready for the garden. But, first, it’s best to help them get accustomed to outdoor conditions by helping them “harden off.” To do this, you’ll be taking the plants outside to a sheltered spot daily, a little longer each day. After a week or two, they’ll be ready to move to their new homes and you can take a few minutes to write down what worked and what didn’t so you’ll be ready for next year. The hardening off process allows the plants to adjust to higher light levels, wind, and temperature variations. If planted directly outdoors without hardening off, they can become sunburned or otherwise damaged.

Plant Type

Weeks needed between seed
sowing and frost-free planting date

Alyssum

8

Begonia

12 or more

Broccoli

8

Cauliflower

8

Columbine

8

Cosmos

4 or less

Dahlia

8

Eggplant

8

Geranium

12 or more

Impatiens

10

Lettuce

8

Marigold

6

Nicotiana

8

Pansy (Viola)

12 or more

Petunia

10

Snapdragon

10

Squash

4 or less

Tomato

6

Watermelon

4 or less

Zinnia

6

The Right Time to Sow Seeds

When starting seeds, you want to sow them so they’ll be ready to plant after the threat of frost has passed, which is usually around Mother’s Day in Southern Minnesota and Memorial Day in Northern Minnesota. (Unless they are cold-tolerant plants, like pansies, which can go out earlier.) Typically very small seeded plants, such as begonias and snapdragons, need additional time for seedlings to develop in size because seeds do not contain much in energy reserves. Other plants just have a slower initial growth rate than others. Whether your seed packets lack the information you need to determine when to start your seeds indoors, or you’ve collected a bunch of seeds from plants you like and don’t know when to plant them, this chart offers sowing times for several plants gardeners often start at home.

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