Keeping them healthy

So let’s see where we are. Planting materials sanitized and seeds planted? Check. Damping off avoided? Double check. So the next step in our quest for the perfect vegetable is keeping those seedlings healthy. There are many factors that go into keeping seedlings healthy. Fertilization and proper light are the likely most important, and easiest to remedy.

Fertilization

Your average seed starting mix or potting soil has no fertilizer in it. When seeds first germinate they have nutrient reserves in their cotyledons (see ‘What happens while you wait’ post). Those nutrient reserves are enough to help the seedling germinate and develop roots, but not much more. To help those seedlings grow, you should plan on adding a dilute (1/2 or less of the recommended rate) fertilizer as soon as the cotyledons are opened. I would plan on fertilizing them with the dilute mix every 1 to 2 weeks. Once the first true leaves are fully developed, fertilize with a regular dose of fertilizer every two weeks until you transplant. Foliage should be a nice rich green color, not a lime green or yellowish color (check the varieties that you’re growing though. Some cultivars have been bred to have a lime green foliage). Sickly seedlings will be a pale green/yellow color and are more likely to succumb to disease or environmental stress. The objective is to keep your seedlings a healthy, but manageable size, until they can be transplanted. Healthy foliage is good, but a healthy root system is better.  Consistent watering, and fertilization will help promote the development of a nice vigorous root system. Even if the upper part of your plant is a little small, once they are transplanted they can catch up in growth if their roots are well developed.

Notice the healthy white roots on the plant on the left compared to that on the right

 Light

The two most common symptoms of light deficiency (not enough light) are pale leaves and leggy seedlings. Leggy seedlings are skinny and tall and look like they are reaching for any stray sunshine they can find. If you don’t have a nice sunny south facing window with 8-10 hours of sun to keep your seeds in then you probably need artificial light. Even if you have a window, artificial light may still be needed. Standard fluorescent bulbs work just fine. You can get a 4’ two-light fixture (T12 or T8) from Home Depot for about $15 and two fluorescent bulbs for another $10. Using 1 cool white and 1 warm white is best. Bulbs should be placed about 2-3 inches from seedlings. If the seedlings are still getting leggy try replacing the bulbs – bulb life is typically two years and light quality goes down as they age. As the plants grow, you will need to raise the fixture to keep from burning the leaves. When using artificial light sources, you’ll need to provide them with 14-16 hours of light each day. For ease of use, consider putting the lights on a timer. If you decided to turn them on and off manually, be sure to turn the lights off for 8-10 hours each day. Plants, like people, have internal clocks and the dark helps plants set their clock.

Nice dark vigorous green foliage of tomatoes after 3 weeks of growth

To give credit, where credit is due, these sources contributed information to this post:

http://extension.umass.edu/floriculture/fact-sheets/fertilizing-bedding-plant-seedlings

http://extension.missouri.edu/p/G6570

http://www.hort.purdue.edu/ext/ho-14.pdf

http://www.extension.umn.edu/distribution/horticulture/M1245.html