Starting Vegetable Transplants under lights

Starting and growing transplants under lights
Curtis Swift, Swift Horticultural Enterprises, LLC

Introduction:

Transplanting is the process of shifting of plants from one place or growing medium to another. If done properly plant stress will be minimized and the production of flowers, fruits, and foliage can be enhanced.

Starting your own transplants (seedlings) indoors under lights or in a greenhouse where adequate light is available avoids problems with direct seeding into garden soil where seedlings often have to battle their way through soil crusts. Starting your own transplants can result in higher quality plants and cultivars that are not available at local greenhouses or garden centers

Starting plants indoors under lights the seeds germinate under ideal conditions. Starting them at the proper time can extend the growing season and create earlier harvest when compared with direct seeding in the field. Transplants you grow are typically not exposed to the myriad of virus, fungal or insect problems commercially-grown transplants may be exposed to.  While it is often difficult to pinpoint when a transplant was exposed to a problem, the timing of symptom development often leads one to suspect the origin of the infection was the commercial greenhouse. Your goal is to produce vigorous and healthy plants ready to be planted in the garden when conditions are appropriate.

Not all vegetables are worth the effort to start as transplants. Even though you can grow them from seed in the home or greenhouse, it is difficult to successfully transplant these to the garden. These include carrots and beets, leafy biennial herbs (dill), heading types of Chinese cabbage, and cucurbits (cucumbers, pumpkins, squash); they don’t like their root systems disturbed.

Radish, leaf lettuce, and spinach develop so quickly when seeded in the garden starting them as transplants is not worth the effort. Radishes are often ready to eat in fourteen days from the time of seeding.

Vegetables such as tomatoes, peppers, eggplant, okra, head lettuce, and broccoli are typically started as transplants and that is where I would suggest you focus your efforts. Sweet potato grown from rooted cuttings is another vegetable that would benefit from being started in the home.

Age affects transplant quality:
 

Smaller, stocky plants that have not started to bloom and/or set fruit adapt to the garden more easily than leggy transplants or plants with small fruits. Tomato plants four to five weeks old grow and produce a higher yield than older transplants. The younger transplants often develop healthy root systems which helps avoid problems with the splitting of the fruit and blossom-end rot common when older transplants are used.  Using younger plants does not guarantee these problems will be eliminated but helps control these problems. The ideal plant is short and stocky, which for many transplants will be about six inches tall and six inches wide. The table at the end of this article provides the recommended age for vegetable transplants when planted into your garden. Since transplants are more susceptible to cold than plants direct-seeded to the garden, transplants are usually planted after the average last spring freeze unless some form of frost protection is provided. If you know that date, count back the number of weeks indicated on the table, add the number of days needed for germination, and plant the seed accordingly.

 

Problems you need to avoid:
 

Low light, excessive levels of nitrogen, and high temperatures during transplant development causes excessive stem elongated.  Such stems are more susceptible to diseases and breakage. In most cases the diseases that attach young seedlings are the result of using soils (planting media) that have not been properly pasteurized.  If you insist on mixing your own planting media for seedlings, it should be placed in the oven and cooked to eliminate all the water-molds and other plant pathogenic organisms that live in that soil. Pasteurization occurs when the soil is moist allowing heat to reach every bit of soil. Forty-five minutes in a two-hundred degree oven usually does the trick. If you have a temperature probe insert that into the center of the soil mass and when the temperature reads one-hundred and eighty degrees for thirty minutes the soil has been properly pasteurized.  Exceeding that temperature for longer than thirty minutes is not recommended.

 

Damping-off is caused by fungi or fungal-like organisms which attack the base of the stem next to the planting medium. Non-pasteurized planting medium, the use of dirty flats, trays, or pots, and dirty tools are often the cause. Fungus gnats can also carry some of these plant pathogens from contaminated house plants to your seedlings.

The best planting medium for starting seedlings are the soil-less mixes.  They are usually free of pests, have good water-holding capacity, are well aerated and drain properly, and are low in salts.  Some even have fertilizers added to help keep your young seedlings perking along in a healthy condition.  This doesn’t mean however, you don’t have to fertilize.

 Transplants can be started in individual pots, cubes or open flats. When started in open flats, the young seedlings will need to be pricked out (transplanted) and repotted in individual pots when the seedlings have their first true leaves. Some gardeners will do this by pulling the tiny plant out of the flat of planting media by their seed leaves. This is a fast and easy way to do transplant a large number of seedlings but can be damaging to the plant. Some gardeners remove the tiny seedlings with sharpened Popsicle stick.  

 Lights:

 Plants grown without sufficient light are yellow or lighter green than they should be.  These plants are said to be chlorotic. They stretch for the light, have thin stems, and in general are not as healthy as plants grown with adequate light. Gardeners who depend on natural light for the proper development of their seedlings cannot produce quality transplants.

The light used by plants comes in different frequencies and the packets of energy carried by the different frequencies energize different responses in the plant. Exposure to red light increases seed germination while far-red light increases legginess. Since regular incandescent bulbs produce more far-red light than fluorescent lights, the latter are preferred for seedlings.  To ensure adequate light reaches all parts of the seedling, fluorescent lights should be placed no more than four inches above the top of the plant. The plants need to be spaced far enough apart so shade is not a problem. Full spectrum grow lights are recommended if you have fixtures with only one tube but if you have a two-tube fixture one cool-white and one warm-white tube provides the same spectrum of light.

If you decide to use incandescent lights for your seedlings, they need to be placed between one foot and three feet from the top of the plants even then you might burn your seedlings unless you use bulbs specifically designed for plants. The Spot Grow type incandescent bulbs are said to provide the proper plant growth enhancing light characteristics. Plastic sheeting is available designed to reduce the amount of far-red light and is an option for those growing transplants with incandescent bulbs.

Temperature:

Plants placed on the window sill, while not providing adequate light, also are subjected to large temperature fluctuations between day and night and sunny and cloudy days. Temperatures cooler than optimum often increase disease problems and can cause the development of rough fruit in tomatoes and bolting (i.e. development of a seed stalk) in onions and other biennial vegetables. Warmer than optimum temperatures can cause weak and spindly seedlings.

Fertilizing your transplants:

Nitrogen (N) is what your seedlings require to develop into healthy transplants and different types of vegetable seedlings require different rates and frequencies of applying nitrogen. Tomato seedlings need to watered with fifty to one-hundred parts per million (ppm) of N or once a week with water supplemented with two hundred and fifty to three hundred ppm of N. Peppers prefer a daily watering containing one-hundred ppm of N, while cole crops (cabbage, broccoli, cauliflower, etc.) prefer one-hundred to one-hundred and fifty ppm of N per week. The vine crops (squash, pumpkin, etc.) need two to four applications of one-hundred to one-hundred and fifty ppm of N per week.

Ok, now that the amount of N these transplants require has been provided, how do you mix up a solution containing one-hundred ppm of N?

When you purchase a granular or liquid fertilizer at a garden center it will most like give the percentage of N along with the percentage of phosphorus (P), potassium (K) and other nutrients.  The container may also give the parts per million but this is seldom the case. The percentage of N in the product is easy to convert to ppm as follows:

10% = 100,000 ppm
12% = 120,000 ppm
15% = 150,000 ppm
20% = 200,000 ppm
and so forth.

Now that you know the ppm of N in the product you purchased, you need to dilute with water to obtain the ppm of N needed by your seedlings. This is quite easy to do with a liquid fertilizer as you are adding a liquid (the fertilizer) to a liquid (the water). With a granular product you have to turn the fertilizer into a liquid.  This is done by adding it to water. Measure out a measured amount of water and add sufficient dry fertilizer to double the level of the water in the container.  This will reduce the percentage of the fertilizer by half; a 20% N dry fertilizer will then contain 10% N.

You will need a pipette, eye dropper, or small measuring cup like those that come with cough syrup to dilute the fertilizer product to what is needed by your seedlings. These are usually marked in milliliters and that is what you need.

The formula you need to use is C_i x V_i = C_f x V_f where

C = ppm
V = volume in milliliters
i = initial
f = final

If you have a product that contains 12% N (120,000 ppm) and need a final solution of 100 ppm of N, plug 120,000 into the formula at C_i. Let’s make one gallon of this mix. One gallon is 3,800 ml so the formula C_i x V_i = C_f x V_f will look like this:

120,000 ppm x Vi = 100 ppm x 3,800 ml. When you do the computations you receive the number of milliliters (3.2) that need to be added to one gallon of water to create a N solution of 100 ppm.

If you want to make less than a gallon, plug the following into V_f:

1 quart = 950 ml
1 pint = 475 ml
1 cup = 240 ml

To make one gallon of a 100 ppm N fertilizer solution it will take:

3.8 ml of a 10% N fertilizer product
3.2 ml of a 12% N fertilizer product
2.5 ml of a 15% N fertilizer product
1.9 ml of a 20% N fertilizer product

Hopefully this article helps you grow healthy vegetable transplants.

 

Table 1

Vegetable	Soil Temperature for seed Germination 1			Time Required for growing Plants for Field Transplanting 4
	Optimum Range (oF)	Day 2 (oF)	Night (oF)	Time (weeks)
Asparagus	60 - 85	70 - 80	65 - 70	8 - 10
Broccoli	68 - 86	60 - 70	50 - 60	5 - 7
Brussel Sprouts	75	60 - 70	50 - 60	5 - 7
Cabbage	45 - 95	60 - 70	50 - 60	5 - 7
Cauliflower	45 - 85	60 - 70	50 - 60	5 - 7
Celery	60 - 70	65 - 75	60 - 65	10 - 12
Corn, Sweet	60 - 95	70 - 75	60 - 65	3 - 4
Cucumber	60 - 95	70 - 75	60 - 65	3 - 4
Eggplant	75 - 90	70 - 80	65 - 70	6 - 8
Lettuce	40 - 80	55 - 65	50 - 55	5 - 7
Muskmelon	75 - 95	70 - 75	60 - 65	3 - 4
Okra	85 - 95 3	70 - 75	65 - 70	4 - 6
Onion	50 - 95	60 - 65	55 - 60	10 - 12
Pepper	65 - 95	65 - 75	60 - 65	6 - 8
Squash / Pumpkin	70 - 95	70 - 75	60 - 65	3 - 4
Tomato	60 - 85	65 - 75	60 - 65	5 - 7
Watermelon	70 - 95	70 - 80	65 - 70	3 - 4

 

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1. Adjust temperatures slightly to alter growth rates.
2. Select the lower temperature on cloudy days. Daily fluctuations to 60^o F. or lower at night is essential for some vegetables.
3. A hard seed coat prevents good germination. Treatment of seeds with concentrated sulfuric acid for 2-3 hours or soaking seeds in hot water (113 ^o F) for 1 1/2 hours improves germination.
4. Transplants older than recommended suffer more shock when transplanted to the garden and often produce less of a crop over the course of the growing season than transplants of the proper size and age.

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References used:

Lorenz, O.A., and D.N. Maynard. 1988. Knott's Handbook for Vegetable Growers, Third Edition. Wiley-Interscience Publications
Splittstoesser, W.E. 1984. Vegetable Growing Handbook, Second Edition. AVI Publishing Company, Inc.
Yamaguchi, M. 1983. World Vegetables; Principles, Production and Nutritive Values. Van Nostrand Reinhold Company.