Vegetable Transplant Growing Secrets of a Greenhouse Professional

Reader Contribution by The Natural Gardening Company

After planting a vegetable seed (let’s say it is a tomato seed), there is an extended holding period while the seed germinates and grows into a plug that is large enough to transplant. Typically, tomato seeds germinate in 7-14 days. Following germination it takes another two weeks for the seedling to size up so it is ready to move into a larger container, where it may grow for another 2-3 weeks. During this early growth period you need to provide your immature plants with healthy growing conditions so they develop into sturdy seedlings. The critical elements are light, water, air circulation and nutrients. You need the role each plays in the development of a healthy seedling.

Saturate Your Seedlings with Light

Vegetable seedlings need lots of light. They are outdoor plants that grow their best when they are surrounded by light. It is challenging to match the intensity of outside light in an inside growing area. While a south facing window may appear to be full of light, the light is entering through one plane only. Your plants need 360 degrees of light surrounding them to grow their best. The ideal growing environment is found in a greenhouse or cold frame that has a good orientation to the sun so your seedlings are bathed in light from sunrise until sunset.

Photo (left): Thin, pale stems without enough light.

If you are unable to buy, build or borrow space in one of these structures, you may elect to set up an indoor growing area using artificial light. Fluorescent bulbs are commonly used in these setups. While these can be effective, their energy dissipates rapidly the farther away they are from your plug trays. Place your fluorescent bulbs a mere 2” above your vegetable plugs for best results.

There are easily recognizable signs that your plugs are not receiving enough light. The stems become awkwardly elongated and look pale, thin and frail as your plants stretch to find light. If you don’t take immediate action, your seedlings will collapse under their own weight or their tissues will be so weak that they cannot survive the transition to the out of doors.

Healthy plugs have dense cell structures and compact stems. A healthy tomato plug has a purple stem covered with tiny hairs – adventitious roots.

Water Regularly

Small vegetable transplants need steady water to grow their best. Plugs can’t hold much water. They saturate quickly and dry quickly. But there is subtlety to watering your seedlings. If they are over-saturated they will not grow their best because the roots need air which is not available in waterlogged soil. The plants essentially suffocate. If the soil dries out, the roots have no water to absorb and the growth of the plant is comprised, sometimes severely, depending on how long they are left dry. You need to find the middle ground, providing your vegetable plugs with soil conditions that are always moist but not saturated. The soil should feel slightly damp to the touch, like a wrung-out sponge.

Photo (above): Stocky stems with enough light.

In our greenhouses we meet these requirements with an automated watering system that waters for short intervals multiple times per day. The length of these intervals is determined by outside weather conditions: temperature, and the length and quality of the daylight hours. In January, when we start our first seedlings, the cool, somewhat dark conditions require that we water only two or three minutes per day total, usually in one minute intervals up to four hours apart (8:00 AM, noon, 4:00 PM). By April we may water eight minutes or more per day in the course of 4-6 watering cycles, depending on weather conditions.

If you are a working person who is gone for most of the day, watering at regular intervals may not be possible without investing in an automatic timer and other specialized watering accessories. One alternative to frequent automated watering cycles are propagation tray kits with a solid holding tray at the base. The holding tray can be used as a reservoir to provide water for absorption from below. You need enough water in the tray so that the openings at the base of the cells in your plug tray touch the surface of the water, but not so much that the roots are sitting in water 24/7. As mentioned before, that will drown them.

Another low-tech tool that can be very effective is capillary matting, available from some gardening catalogs. This matting is placed beneath your plug trays with one end sitting in water. The water is absorbed from the reservoir and travels through the matting where it is absorbed through the holes in the base of your plug trays.

You can also try the old-fashioned approach that relies on self-discipline: water before you go to work and when you return home. Check closely when you return home to see if the soil in your plugs got too dry during the day. If so, you need to consider one of the solutions above.

Air Circulation and Temperature

Vegetable plugs grow their best when they grow in gently circulating air. Fresh air is ideal. Gentle air movement strengthens the stems and other tissues and disperses excess humidity which can make your plants grow too rapidly, leading to rank growth. Fresh air also contributes to a growing environment that is less likely to encourage colonization by unwanted insects and disease. If the air feels damp in your growing area, your plants aren’t in optimal conditions.

Photo (right): Floating row cover protects seedlings.

We generate airflow in our greenhouses through vents along the base and top. If you have a cold frame, keep the door open during the day if conditions allow. Vent openers that automatically open and close are ideal for this purpose, and require very little tending.

Regarding temperature, 70 degrees F is ideal, but most seedlings will tolerate a range with the optimum temperatures between 60 and 80 degrees F. Our greenhouses are unheated and nighttime temperatures sometimes dip into the 40’s or even 30’s. When they descend to the 30’s plants must be monitored closely. We keep our smallest, least mature plug trays on heat pads set at 60 degrees F and cover them with floating row cover to create an insulated pocket of warm air.

Add Nutrients

Once your seedlings are out of the cotyledon stage and developing true leaves (the second tier of leaves is the first set of true leaves) they need nutrients. Water passing through the plugs leaches nutrients; they must be replenished regularly. The easiest way to feed vegetable plugs is with a concentrated liquid fertilizer that is mixed in water. The best liquid fertilizer is a hydrolyzed fish or fish/kelp combination fertilizer. Hydrolyzed fish fertilizers are composed of 100% fish that has been liquefied by the additional of enzymes to make the fish solids decompose. They provide everything a vegetable transplant needs in a balanced formulation. This type of fertilizer is the organic gardener and grower’s best bet to feed their plants during the seedling stage and after they are planted outside.

We feed once a week using an injection system that mixes the fish fertilizer in our greenhouse watering system. Injectors automatically proportion the mix of fertilizer to water; ours are calibrated at 200:1, water to fertilizer. This degree of precision isn’t essential. You can mix your fertilizer concentrate in a watering can and dispense that way (follow the directions for the proper dosage, more isn’t better). You can also obtain a simplified fertilizer injector (Syphonject is one brand) that attaches to your faucet and runs fertilizer while you water through your hose, a great time-saving device if you do a lot of fertilizing during the growing season.

Photo (above): All-purpose, hydrolyzed fish emulsion.

Next time we will take a look at how and when we transplant our vegetable plugs. See you in two weeks!