Our preferred method to thin most types of seedlings is to trim out the unwanted ones. Not only might the roots of the pulled seedling get damaged, thwarting your efforts to save it, but the seedling you are hoping to save and leave behind in the container may get damaged or completely pulled up along with it. We do not use or encourage this practice since it can be a bit risky. Like many aspects of gardening, this is sort of up to personal preference… but here I will share the two methods we primarily use: trimming, or gently separating apart.Ī third option is to pull up on the unwanted seedling, manually plucking them out of their container – sometimes with the hopes of keeping the pulled seedling to replant. I have experimented and experienced this first hand, numerous times! After thinning, the chosen keepers will take off and thrive! Seedlings can quickly become four times the size of the ones that were left unthinned, in just a matter of weeks! Leaving them un-thinned stunts their development immensely. When it comes time to thin, you get to choose the strongest, healthiest, best-looking seedling to keep around. Once a few of those pop, it helps you scope out the best looking seedlings. The true leaves more closely resemble what the mature plant leaves will look like, but miniature. Those very first ones are the embryonic leaves, called cotyledon, and are often times heart-shaped and indistinguishable between different types of plants. The “true leaves” are the ones that emerge after the first set of sprouting leaves. It is best to wait until a couple weeks after sprouting, once the plants have developed a set of true leaves or two, but to not wait too much longer after that. All of this applies to seedlings (or plants) that are started indoors in containers, and outside in the garden. Bad guys like powdery mildew love cramped conditions, and also spread spores when leaves rub against one another. This is primarily due to the reduced airflow between the plants.

Those are not things you want to deprive your seedlings of! In addition to concerns about competition, crowding seedlings also increases the risk for disease. When left un-thinned, seedlings that are in tight quarters will compete with one another for nutrients, water, air, and root space. One bench in the greenhouse, just three weeks after we sowed all of our seeds! Look how crowded it is in here! Definitely time to thin.

Zinnia seedlings