Today’s plant clinic concern comes from a resident who is new to the Olympic Peninsula: “My seedlings sprouted and were looking great, then, almost overnight, they wilted and died.”

Watching a seedling that you have carefully planted go through the germination process then fail and die is a disappointment we have all experienced. Why does this happen and how can we prevent it?

The major factors that govern good germination are also important as seedlings start to peak above the soil level. The growing medium used, soil temperature and moisture are all key factors in successful germination but they also can create problems if they are not properly controlled as the seed grows. Light and good air-flow are added challenges once a seed sprouts.

A good growing medium is light and fluffy and contains no disease-causing organisms.

Using garden soil as part of any seed- starting mixture potentially exposes seedlings to many soil-borne diseases. The best growing medium for starting seeds indoors is a commercial, sterilized, seed-starting mix.

A constant, warm temperature is essential for fast and consistent germination. Large temperature fluctuations stress young plants, leaving them more susceptible to disease.

Colder, moist conditions favor the development of fungal diseases that can kill seedlings. When directly sowing in the garden, many people new to the Olympic Peninsula may not recognize that a moderately cold spring season keeps the soil from warming up fast. Many warm season crops need to be started indoors or later in the season.

For starting seeds indoors, using a waterproof heat mat under the pots can largely control the temperature issue and maintain a steady amount of heat, especially if a thermostat is used.

Moisture, too little or too much, can be a real killer. Too dry potting mix results in poor germination and uneven or stunted growth, which stresses plants. The combination of too much moisture from over-watering and a warm atmosphere creates ideal conditions for fungal diseases. This is worsened if a non-sterile growing medium is used and/or plastic domes are left over the plants for too long after germination.

To avoid over-watering, check the soil with your finger and water only if it feels dry. Do not wet the foliage; water from below by allowing it to absorb into the pot.

Light is critical for plant growth after germination. Very often in the Northwest, even a south-facing window will not generate enough light for development and we need to resort to supplemental lighting. Supplemental lights need to be as close to the plants as possible but not touching, or bleaching and burning may occur.

In addition, lights need to be raised as the plants grow. If you do not use supplemental lighting or if the lights are too far away, you will often get long, spindly seedlings as they reach for more light. This produces weakened, disease-susceptible plants.

Good air circulation is also important. Remove plastic domes or open their vents after seeds have germinated to improve air flow and reduce humidity.

The most common cause of early seedling death is damping-off, a fungal disease that affects the new plant stem at soil level. The stem becomes brown, rots, and the little plant topples over and dies. Damping-off fungi are more of a problem in cold soil with poor drainage in conjunction with over-watering.

So remember, using sterile planting media, maintaining a constant temperature, keeping the media moist but not too wet, ensuring good air flow and providing adequate light are the keys to successful seed germination and seedling growth.

Bob Cain is a certified WSU Clallam County Master Gardener.