Thinning plants is a necessary evil we must all face in the gardening realm. Knowing when and how to thin plants is important for their overall health and success.
Why You Should be Thinning Seedlings?
The practice of thinning plants is done to allow them plenty of growing room so that they can receive all the proper growth requirements (moisture, nutrients, light, etc.) without having to compete with other seedlings. When you thin seedlings, you're also helping to improve the air circulation around them. Crowded plants limit air movement, which can lead to fungal diseases, especially if the foliage remains wet for extended periods.
When to Thin Seedlings
Knowing when to thin seedlings is also important. If you do it too late, the overdeveloped roots may cause damage to the remaining seedlings during the thinning process. Depending on what you are growing, you'll want to thin plants out enough so that each seedling has a couple of inches (5 cm.) of space (or two finger widths) on either side. Make certain the soil is reasonably damp beforehand, which simply makes it easier to pull the plants out intact and with less damage—similar to weeding young sprouts. You can soak the area with water to soften the soil if it's too dry. Seedlings should have at least two pairs of true leaves and be about 3 to 4 inches (8-10 cm.) tall before thinning. Evening hours are a good time to thin seedlings as the cooler temps and darker conditions make it easier for the remaining seedlings to bounce back from any stress they may have received. Of course, I have found cloudy days to be just as effective.
How to Thin Seedlings
Learning how to thin plants isn't difficult. However, not all plants handle thinning the same way. Those having fragile roots, like beans and cucurbits (melons, squash, cucumbers), should be thinned as soon as possible, before their roots have a chance to become intertwined with one another. Otherwise, the remaining seedlings may suffer from root disturbance. Gently pull out the unwanted seedlings, leaving the healthiest in place. Many flowers and leafy vegetables can also be thinned this way. They can be gently raked to remove excess seedlings as well, though I prefer pulling them one by one to limit any damage. Root crops are a bit more sensitive to thinning and should be pulled out with extra care or even cut at the soil line. Again, depending on the plants and their mature size, spacing may vary. While most people prefer a finger width between seedlings and on either side of them, I like to use two-- it's always better to be safe.