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the Passionate Gardener for March 25, 2010

by Karyn Ames

Master Gardener

New Mexico Certified Nursery Professional

    By now your seedlings probably have their true leaves.  The first set of two leaves that emerged were the cotyledons, which stored food for the emerging seedling.  When the true leaves appear, the cotyledons drop off.  Now the true leaves begin the photosynthesis of your little plants and it is time for you to do your part to ensure robust plants by thinning and fertilizing your seedlings.

    As I mentioned in previous articles, I plant my seeds directly, at a rate of two to three seeds, into cell packs, so that after thinning, each plant will have its own “territory.”  When your seedlings have four true leaves, thinning is essential because if a seedling cannot have room to spread out roots and leaves, it will be spindly and useless in the garden.  If you are nimble with your fingers, you can pluck out seedlings and transplant them.  But the drawback is that you might disturb the roots of the seedling you have chosen as the one you want to keep.  Keep only the healthiest seedling in the group you are thinning (see photo).  If the roots from two or more seedlings have grown together, cut the unwanted plants off at soil level.

    The major macro nutrients in plant fertilizers are nitrogen, phosphorus, and potassium, the elements N-P-K you see on fertilizer bags.  Additionally, plants need micro nutrients such as calcium, sulfur, magnesium, boron, and others.  No single nutrient is more important than another, although some nutrients are needed in greater or lesser quantities during various stages of the plant’s life.

    Traditionally a high phosphorus fertilizer is recommended for seedlings.  However, the low light and moderate temperatures for indoor-grown plants encourage plant stretching if high levels of phosphorus are used.  This results in plants that are weak and difficult to handle.  Ideally, I recommend a balanced N-P-K fertilizer, such as a 6-6-6, and that the fertilizer package also state that it contains all essential trace elements.  Because the fertilizer industry usually insists on high phosphorus fertilizers for seedlings, you will probably have more luck finding a 6-12-6 mix.

    Dilute application of the fertilizer is critical.  Most fertilizers are dry and are mixed with water at a rate of 1 tablespoon per one gallon of warm water for a 6-12-6 fertilizer, for instance.  Read the fertilizer package carefully and err on the side of a weak mix instead of a stronger mix, which could burn and kill the tender seedlings.  You can apply diluted fertilizer biweekly.  Be very careful not to over-water and over-fertilize your plants.  Strong, healthy plants need some stress to be compact and ready for the garden.