A primer on how to start seeds for your garden.
Gather your seeds and supplies: It’s nearly prime time to start seeds for this year’s garden. Most seeds are best planted somewhere around six to eight weeks before the last spring frost.
Nothing is more disappointing than beginning the garden season by starting seeds with high hopes and dreams of a wonderful garden, only to have just a few seedlings emerge and even fewer plants survive into May. Follow these steps to grow healthy seedlings indoors:
• Use only soilless mix, not potting soil, garden soil or previously used potting mix.
Seeds and young seedlings are easily killed by a plant disease known as damping-off, which is caused by soil-borne pathogens.
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These pathogens occur in mineral soils, such as the black soil in that bag of potting soil or soil from the garden.
Damping-off kills seeds before they emerge from the ground, but it can also cause small seedlings to rot and die. These seedlings often collapse at the ground level as if they need water, but in fact the damping-off pathogens have rotted the roots and stems.
Avoid damping-off by using only sterile, soil-less mix, such as seed-starting mix.
Hardware stores and garden centers are bursting with a variety of high-quality seed-starting mixes this time of year. Reconsider trying to save a few dollars by skimping on the soil mix; this is a sure route to seedling death.
• Don’t start seedlings too early.
Some seeds need to be started very early in the season, such as geraniums, which should be started 10-12 weeks before the last frost. Seeds of heliotrope, one of my all-time favorite purple flowers with a magical vanilla scent, need to be started an amazing four months before the last frost, a possible reason this plant is sometimes expensive or hard to find.
Most seeds, however, perform best when planted six to eight weeks before the last frost.
And some seeds, like cosmos or marigolds, perform just as well when planted directly in the garden in mid-May, after danger of frost has passed.
Seeds planted too early tend to get weak, and don’t outperform younger, sturdier plants started with ideal timing.
Not sure when to start those tomato or lupine seeds? Check the back of the seed packet for all the details. Also ask local experts at garden centers and garden extensions.
• Use clean containers.
Containers previously used to grow plants are most likely contaminated with the pathogens that cause damping-off. Planting in dirty containers is a perfect way to introduce pathogens to clean soil.
To keep these pathogens at bay, wash all containers with soap and hot water, then rinse them in a solution of one part bleach to 10 parts water.
• Clean all other items used in seed starting.
This includes small watering cans, planting tools or reused labels. While you’re at it, wash the seed-starting area, including trays and shelves, with the bleach/water solution.
• All containers need drain holes.
As cute as it may look to plant seeds in a coffee cup or cereal bowl, it’s almost guaranteed failure by water-logging plant roots. Roots need air spaces as much as they need water, so be sure all containers have drain holes. Add holes to yogurt containers or other recycled containers.
• Avoid overwatering seedlings, and never let containers share drained water.
The pathogens that cause damping-off are water-loving organisms. They are favored by overwatered soil and stressed plants, so avoid overwatering. The pathogens can move from one pot to an adjacent pot if the two containers share drained water. Make sure all containers drain freely, and make sure containers don’t share drained water. Consider propping containers on a wire mesh to make sure water can drain free and isn’t shared between pots.
• Keep seedlings under fluorescent lights.
Although it might seem bright on the kitchen windowsill, this location rarely provides enough light to encourage healthy, even plant growth. Seedlings perform best when grown under fluorescent lights, such as shop lights. These fixtures can be suspended from a shelving unit to be about three inches above emerging seedlings. As plants grow, raise the lights to maintain the intensity. Special grow bulbs aren’t necessary to start seeds; basic shop lights will suffice.
• Keep seedlings warm with good air circulation.
Tired of reading about damping-off yet? Trust me, it’s devastating to lose whole flats of young seedlings to disease. Prevent problems from the start by keeping plants as healthy as possible. Many gardeners use heat mats under seedling trays to give plants an extra boost. Cool, wet soils favor disease. Heat mats go a long way to helping seedlings escape disease. Once seedlings have their first set of true leaves (after the seed leaves have formed), they are more disease-resistant. A fan can also help circulate air to ward off disease.
• Harden seedlings before planting them outdoors.
The warm, sheltered life seedlings lead inside does little to prepare them for the harsh realities of garden life. To help seedlings make this transition, harden them gradually over seven to 10 days. Cut back on water and fertilizer, and expose plants to increasing amounts of sunlight, rain and wind each day. Tender plants need time to adjust to real-world conditions; the hardening off helps them toughen up and get ready for the move to the garden.
• Don’t plant warm season plants in the garden too early.
Until soils have drained and warmed, plants will do little but “sit there” in the garden if planted too early. This is especially true for plants that thrive in warm temperatures, like tomatoes, peppers and petunias. Better to keep these plants in their containers a bit longer, where they will benefit from warmer soil temperatures and better drainage.
For information about starting seeds, including lots of timing information for individual plants, see “The New Seed-Starter’s Handbook” by Nancy Bubel (Rodale Books).
Denise Ellsworth directs the honeybee and native pollinator education program for Ohio State University.