University of Maryland Extension

Tips for Growing Vegetable Transplants

Author: Jon Traunfeld, Extension Specialist, Fruits and Vegetables,and State Master Gardener Coordinator
This article originally appeared in the Winter 2012/2013 HGIC eNewsletter


New Year’s Day is the first day of the 2013 growing season! It will soon be time to begin searching seed catalogs for those favorite vegetable, herb, and flower cultivars. Many gardeners are also busy wiping the dust off their light stand, and collecting trays and containers in preparation for growing transplants indoors. The questions and answers below contain tips that I hope will guide new gardeners and serve as reminders for the experienced hands.

Can I grow tomato plants on a windowsill?

The natural light from a window is seldom enough for good, strong seedling growth. Plant stems usually stretch and lean towards the light and will not produce sturdy plants. Acceptable plant growth usually only occurs in south-facing bay windows or solariums. Light is measured by its intensity (closeness and brightness), duration (length of time the light is available), and quality (includes blue and red wavelengths). Fluorescent light fixtures and tubes are the best choice for growing transplants indoors.

I inherited my dad’s old shop lights. Should I switch to the new, thinner fluorescent tubes?

Up until recently, most fluorescent fixtures accepted T-12 tubes. The number after the “T” refers to the diameter of the lamp in eighths of an inch. Therefore, a T-12 lamp is 12/8 or 1.5 inches in diameter. T-8 and T-5 are two newer types of fluorescent tubes that are increasing in popularity for indoor gardening. T-8 is 1-in. diameter, and T-5 is 5/8-in. diameter. Compared to T-12 tubes, the T-8 tubes are 20-25% more efficient, have a longer life, and are slightly more expensive. But if you already have and use T-12 fixtures, and are happy with the results, there is no reason to switch. Focus on the rated lumens. There is not that much difference between T-12 and T-8. T-5 fixtures and tubes are pricey and use more energy (54 watt) but produce more lumens per watt than T-12 or T-8 fixtures.

Do I need to buy special grow light bulbs?

Chlorophyll absorbs most of its energy from the Violet-Blue and Orange-Red wavelengths. Cool, white tubes (40 watts) produce light in the blue and yellow-green segments of the light spectrum. They are the least expensive and the mostly blue light can produce healthy, stocky salad greens and vegetable transplants. More expensive full-spectrum fluorescent tubes (“grow lights”) are available that produce a balance of warm (red) and cool (blue) light. “Grow lights” enhance foliar growth and produce thicker stems than cool white tubes, and are needed for producing flowers on indoor plants. Some gardeners insert one warm and one cool tube into a fixture to gain the same affect.

                                Comparing Fluorescent Lights

Diameter of tubes
Cost of 4ft.-long fixture
(2 tubes)
Cost per
4-ft. tube
Light output
(initial lumens*
 per tube)
Tube Life (hrs.)**
T-12 (40w)1 1/2 inches  $15$2 - 41500 - 320010,000 -20,000Until recently, the industry standard
T-8(32w)1 inch$20$3 - 5280030,000 -40,00020-25% more efficient thanT-12
T-5 (54w)5/8 inch$10$13 - 15500030,0009% more efficient than T-8
SylvaniaGro-Lux Wide Spectrum (T-12)1 1/2 inches$15$16170020,000

Type of “Gro-Light” (adds red and far-red to blue; mimics cool white with incandescent)

*Lumens measures light intensity over the visual spectrum- the brightness that we see. 
** Over time the bulbs begin to lose their intensity. T-8 bulbs have a slower period of decrease,losing about  10% of initial brightness after 7,000 hours. T-12 bulbs can lose 20% after the same number of hours.

Why do my transplants sometimes topple over and die? I water them every day and fertilize weekly.

You may be killing them with kindness. Wait until the top of your growing media is nearly dry before watering. The idea is to keep the root system supplied with water and oxygen. If your containers are saturated with water the roots will stop growing and pathogenic fungi that like wet soil will infect and kill your babies. And slow down on the fertilizing. Most potting mixes contain enough fertilizer to grow a seedling for 5-6 weeks.

Do I need special potting soil or can I use soil from my garden?

Never use garden soil- it’s too dense (about 75 lbs. /cubic foot) to grow healthy transplants and probably contains weed seeds and plant pathogens. Soilless growing media is the stuff to look for (contains no mineral soil). It’s light (10 lbs. /cubic foot), porous, and drains well. Typical ingredients are peat moss, vermiculite, perlite, a little lime and fertilizer. Read product labels for ingredients and reject products that feel heavy (these are usually labeled as “planting soil”). Also, don’t be tempted to buy a separate bag of “germination” media for starting vegetable seeds. This is only useful for sowing extremely small seeds. If you grow lots of transplants and also do container gardening you might want to invest in a large bag or compressed bale of commercial soilless growing media. It’s cheaper than buying the same amount in small quantities.

I’ve heard that peat moss harvesting harms the environment? What are the alternatives?

Peat is an organic substance formed from plants (principally sphagnum moss) that decompose very slowly in waterlogged soils (bogs). Peat is valuable in horticulture because its fibrous structure helps it retain a lot of water and air. There is concern over the ecological effects of excavating peat moss. Rising fuel prices have increased the cost and caused professional growers to look for alternative ingredients for growing media.  Ground up coconut husk fibers (coir) are a popular alternative, though the sustainability of harvesting and shipping this material from the tropics is widely debated. Some gardeners and small farmers have learned to blend finished, screened compost with commercial growing media. Compost is heavier than soilless media (about 25 lbs. /cubic foot) but is less expensive (especially if homemade!) and supplies nutrients and other compound that promote plant growth and health. Try mixing compost 1:2 with soilless media.

My transplants are very slow to germinate and grow. What am I doing wrong?

Grow transplants at 70-75 degrees F. during the day and slightly cooler at night. You can offset cool basement temperatures by covering your plant stand with clear plastic. This will trap some heat from the light ballast and raise the humidity level. Cover newly seeded flats or pots with clear plastic to hasten germination. Remove the covering when plants pop up. Keep fluorescent light tubes 1-2 inches above plants at all times. This will increase light intensity and keep plants stocky and strong. Lights should be run on a timer for 14-16 hours each day (vegetable plants need 6 hours of rest in the dark).

How much does container size matter?

There has been much research done in this area. Tomato, eggplant, and pepper transplants establish more quickly in the garden and produce earlier and higher yields when grown in large containers (> 9 square inches). Most other vegetable transplants will grow fine in 3-5 square inches.

Does it pay to grow bean and lettuce seedlings indoors?

Some vegetable farmers grow bean transplants to catch the early market. This rarely makes sense for backyard gardeners. Pre-germinating corn and bean seed, on paper towels or in growing media, does make sense if you are planting early in cool soil (less than 55 degrees F). If you hate to thin crops like radish, beet, arugula, lettuce, spinach, and broccoli raab, then by all means start them indoors. You’ll be able to grow and harvest more per square foot with transplants.

My tomato plants usually get too tall and hit the lights or flop over. It's hard to plant them in the garden.

I can empathize with you here. Try sowing tomato seed 6-7 weeks before you expect to plant. You'll end up with stocky 8-10 in. tall plants. If they do get too tall you can lay them down in a trench when planting and turn the growing tip up so only the top 2-3 sets of leaves is above the soil.

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