Seed Rot

Warm winter days and glossy seed catalogs lure many novice gardeners into starting seeds too soon.

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Okay, it’s true. Veteran gardeners are vulnerable to the same overly idealistic behavior. Most of us have learned that starting seeds too early nearly always leads to damping off and seed rot. And it is a waste of good garden seeds since both conditions are nearly always fatal. Even if they live, they will never thrive.

White mold on rotting seed (SHIRAS WC) CC BY SA 4.0

Seed needs

Locked inside each healthy seed, safe under a hard shell (seed coat), an embryo and stored starches (endosperm) wait for conditions to be Just Right. Once specific levels of heat, light, air, and moisture are present, enzymes within the seed are activated. These enzymes convert the endosperm into usable sugars, and the embryo can begin to germinate. If conditions are not correct enough, the young plant will be at a disadvantage that can be fatal.


What causes seed rot?

Cold and wet are a recipe for disaster for most seeds. Seed rot occurs when soil temperatures are too low and moisture levels are too high. Moisture softens the protective outer seed coat, allowing fungi, such as Achlya plebsiana, Fusarium, Pythium, and Rhizoctonia, to enter. These are not the beneficial mycorrhizae that help your plants find food. These fungi are out for blood. Okay, not blood, but they will suck the life out of your seeds and seedlings.


Seed rot symptoms

Planted too early in the season, seeds often plump up and become mushy. Fungi form white, fuzzy halos around these vulnerable seeds. Then, several algae species will colonize these fungal growths, turning them green. In both cases, fungi and algae absorb the nutrients seeds need to grow.


How warm is warm enough?

The minimum soil temperature for many common garden plants is listed below. Soil temperature refers to two inches below the surface, where most plant roots occur. Keep in mind that the optimum temperature for germination is often 20°F to 40°F higher:

• 35°F - lettuce, onion, parsnip, spinach
• 40°F - beets, cabbage, carrots, cauliflower, chard, parsley, peas, radish, turnips
• 50°F - corn, tomatoes
• 60°F - cucumber, eggplant, melons, peppers, pumpkin, squash

Until your last frost date has passed, the only way to safely start seeds any earlier is indoors or in a greenhouse, under grow lights, and on a seed mat that provides steady, controllable heat. [Do not use the same heating pad you use on your aching back - it is not designed for moisture, will get too hot, and can become a fire hazard.]


Preventing seed rot

While fungicides are frequently used in commercial farms to prevent seed rot, the following good cultural practices in the home garden can be enough to protect your seeds:

• Wait until soil and air temperatures are consistently high enough for the particular species.
• Install seeds at the recommended depth.
• Keep the soil moist but not soggy.

As tempting as it may be to start earlier, waiting until conditions are suitable is one of the simplest ways to ensure a good harvest.