Stunted Seedlings

Are your seedling stunted? The seed starting mix may be the culprit, as many seed starting mixes do not contain added nutrients that are essential to plant growth. Why?

Have you ever started seeds in a plastic bag with a ball of moist cotton? They sprout! Seeds contain all of the nutrients and energy needed to produce the first set or two of true leaves of the plant. After that, nutrients are needed. Seed starting mixes act much in the same way as the moist cotton ball – they provide a sterile, soilless growing medium (soil can sometimes harbor pathogens or pest eggs/larvae) that retains moisture, and provides a good amount of air space for root development. Some brands of seed starting mixes do contain a small amount of nutrients. While too much can actually hinder seed germination, a small amount is just enough to help the seed grow to a larger size before transplanting, without added inputs. You can determine if your seed starting mix has nutrients by looking at the label – usually the amounts of nitrogen, potassium and phosphorous are represented by numbers (2-2-2) or by percentages.

In the case of these lettuce seedlings, growth stopped about 2 weeks after planting in a nutrient-less seed starting mix. Despite being under strong light and staying moist, they did not grow much after two more weeks. We’re looking at four-week old lettuce that’s at least a tenth the size it should be. What to do?

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At this point, we can:

-Add a balanced liquid fertilizer at half-strength (the plants are really tiny, after all!).
-Transplant to a larger container (“pot up”), and fill the remaining space in the pot with seed starting mix or potting soil mixed with granular fertilizer.
-Transplant outdoors to a prepared garden bed after hardening off. To add nutrients, either add 1 teaspoon of balanced granular fertilizer to each planting hole, or rake in an appropriate amount for the transplanting area before planting.

Before transplanting, the seedlings should be separated from each other. Imagine a full-size head of romaine. Now imagine a full-size head of romaine fitting in the space of each seedling pictured above – it can’t. When plants are too close together, their growth can be held back as they compete for light, root space, water and nutrients. While we tend to overseed a little when starting seeds, after germination it’s important to either snip seedlings until one remains in the appropriate amount of space, or carefully separate each plant and pot up or transplant.