The Ins and Outs of Seed Germination

By Susan Littlefield, February 5, 2015

Remember when you were a kid and your mother told you not to swallow watermelon seeds or they might grow in your tummy? You probably thought she was joking, but it turns out, she may not have been that far off. About five years ago doctors in Boston removed a pea that had sprouted inside the lung of a 75 year old man! At first they thought he was suffering from lung cancer but eventually discovered that his symptoms stemmed from a 1/2 inch long, sprouted pea seedling that grew from a pea he had accidentally inhaled. While aspirating foreign objects into the lungs is a familiar, if not frequent, problem encountered by doctors, finding sprouting plants is much more unusual; although one doctor pointed out that the warm, moist interior of a lung affords perfect conditions for germination. Even trees have been known to exploit this environment. In 2009, Russian doctors removed a 5-centimeter long little fir tree seedling, needles and all, from the lung of a 28 year old man who they think must have inhaled a seed or bud by mistake. (Note to Mom: the acids in our stomachs keep any seeds reaching there from growing.)

Seed Conditioning
But for those of you who are planning on doing your seed sprouting ″out of body″ this spring, let's take a look at what more conventionally planted seeds need to begin the process of germination. All seeds are in a state of suspended animation, waiting for the right set of conditions to awaken and begin to sprout. Seeds contain chemical and physical inhibitors that keep them from germinating when the environment isn't suitable. Once the seed receives the proper ″conditioning″ that destroys these inhibitors, it ″knows″ that it's safe to start on the journey to becoming a plant. Understanding what conditioning particular seeds need sets us on the path to seed starting success. Check the seed packet for information on specific conditioning needs of the seeds you're planting.

All seeds need moisture to germinate. So why don't mature seeds on a marigold begin to sprout while still attached to the seed head? The seeds of many of our vegetable garden plants, such as tomatoes, zucchini, and cucumbers, and warm season annual flowers like zinnias and marigolds, need to be exposed to a certain amount of drying before they are ready to absorb water and germinate. This drying conditioning keeps seeds from sprouting prematurely while still attached to the flower head or even inside the fruit itself. By the time we purchase commercially sold seeds of these plants, this conditioning has been met.

Helping Seeds Along
Some seeds have a very hard seed coat that in nature is broken in a variety of ways that assure that the seed germinates under the proper conditions. Alternate freezing and thawing temperatures, extreme heat from a fire, or passing through the digestive system of an animal are all ways in which a hard seed coat can be breached to allow moisture in. We can reproduce this conditioning with a procedure called scarification, which is simply nicking, scraping or cutting through the seed coat. For example, to get a hard-shelled morning glory seed ready for sprouting, you can cut off the pointed end of the seed with a sharp razor blade, nick it with a file, or scrape the seed across a piece of sand paper. To scarify a lot of seeds at a time, put them in a jar with some coarse sand and shake vigorously.

Sometimes soaking is enough to soften the seed coat to speed germination. Soaking can also speed germination by removing chemical inhibitors from the seed coat. Soaking parsley seeds for 24 to 48 hours before planting, pouring off the water and replacing it with fresh several times as the seeds soak, will leach out inhibitors and speed germination.

When we think of planting seeds, what usually springs to mind is tucking seeds into the soil. And while some seeds do need the darkness of a soil covering for germination, most will germinate in light or dark, though the covering of soil helps to keep them moist. But some do require exposure to light to break down inhibitors in the seed coat. Lobelia, impatiens, and ageratum are seeds that need light for germination; simply press them on to the surface of the germinating medium, rather than burying them.

Certain seeds, usually of perennials, trees, and shrubs from temperature climates, need to be exposed to cool temperatures before they're ready to germinate. This prevents them from sprouting prematurely when the weather is too cold for growth. In nature, these conditions are provided by normal seasonal changes. When gardeners mimic this process, it's called stratification. Seeds are given a period of moist cold (40-45 degrees F) for about 6 weeks to duplicate going through a cold winter; a refrigerator easily provides the appropriate ″winter″ chill.

Warmth and Water
Once seeds have been conditioned, most need warmth in addition to moisture to germinate well. The majority of the seeds we start early inside (our houses, that is -- not our lungs!) appreciate bottom heat from a seedling heat mat or the top the refrigerator to keep the germinating mix between 70 to 80 degrees F. But be sure to check the instructions on the seed packet for the specific requirements of plants you're starting. And for goodness' sake -- while you're planting, don't inhale!

Seed Starting Indoors and Out
Seeds of many vegetable can be sown early indoors to produce seedlings that will be transplanted to the outdoor garden when weather conditions are suitable. These include tomatoes, eggplant, peppers, broccoli, kale, cabbage, onions, leeks, and parsley, as well as many annual flowers such as zinnias, sunflowers, and marigolds. Pumpkins, melons, cucumbers, and squash are touchy to transplant and usually grow best when sown directly in the garden. But they can be started early indoors if they are grown in individual plantable (biodegradable) pots, like peat pots, to minimize root disturbance at transplanting time. Beans, peas, corn, and root crops like beets, radishes, and carrots don't tolerate transplanting well and their seeds are usually sown directly in the garden. Lettuce, basil, spinach, and chard can be started early indoors and transplanted and/or the seeds can be sown later directly out in the garden.

Consult the chart below to figure out the best time for indoor seed starting of some popular vegetables and flowers. Begin by figuring out the average last spring frost date for your area. Your local Extension Service office or Master Gardener program can provide this information. Next, use the table below to figure out the number of weeks before the last frost date to sow various kinds of seeds, as well as the time relative to the frost date for planting seedlings in the outdoor garden.

Type of plant

Number of weeks before last spring frost date to start seeds indoor

Earliest date to transplant hardened off seedlings outside relative to last spring frost date

Broccoli

6-8

2 weeks before

Cabbage

8-10

4 weeks before

Cauliflower

6-8

2 weeks before

Cucumber

2-3

1-2 weeks after

Eggplant

6-8

2-3 weeks after

Kale

8-10

4 weeks before

Lettuce

7-8

3-4 weeks before

Melon

1-2

2 weeks after

Onion

10-12

4 weeks before

Pepper

6

2 weeks after

Pumpkin

1-2

2 weeks after

Spinach

8-10

4 weeks before

Squash

1-2

2 weeks after

Swiss Chard

6-8

2 weeks before

Tomato

6-8

1 week after

Marigold

6-8

1 week after

Sunflower

3-4

On last frost date

Zinnia

3

1 week after

 


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