Our world has changed thanks to tissue culture. From exotic orchids at Trader Joe’s to the Autumn Glory maples along our city streets, consumers and landscapers have both found the superior quality, health and vigor of these plants to be amazing.
Even before a plant enters the lab, it undergoes rigid viral testing by companies like Agdia, which runs as many as 21 different tests to make sure plants are as virus free as possible. These plants are called elite stock.
Under the sterile conditions of the lab, numbers of plants begin to increase after being placed into an agar gel with necessary hormones, sugars and salts. Multiplying these plants in geometrically increasing numbers, at specific intervals, can quickly bring a starter culture of a few plants to a hundred thousand in a year’s time. At some point, when the target goal has been reached, roots are needed. Plants are then transferred to a different medium that contains root hormone in the agar. Tissue culture plants are incredibly vigorous, but the agar-grown roots are extremely tender and must be replaced by stronger roots after being placed in the soil media.
With increasing shipping costs of full-sized plugs being a deterrent factor, more labs are sending huge numbers of plants out in Stage 3. These are rooted plantlets that come in agar or are placed on paper towels and gently rolled. Thousands of plants can then be sent in a standard cardboard shipping case. Temperature extremes must be avoided at this tender stage. Some of the world’s more primitive tissue culture-weaning facilities are in South Africa. Some of the world’s best are in Gensingen, Germany. All facilities produce plants, but losses can be high unless clean work and growing areas can be provided.
Know that plants have been in a very stable growing environment under very high humidity (in the growing vessels). Terra Nova uses Delta T bench heat tubes under a steel bench mesh with a capillary mat and a woven poly top. The bottom heat is maintained at an even 72°F. The mesh prevents cold spots that can cause uneven results in rooting. Humidity needs to be monitored by a humidistat to maintain a relative humidity (RH) of 65 to 80 percent.
Some crops do require 80 to 90 percent RH and a responsible lab will help you with this information. Tents or tunnels on the benches covered with white poly can provide this high level of humidity. The sides of these tents are slowly raised over time as the plants are acclimated.
Additionally, Terra Nova uses 400-watt HPS lights for day length extension and night interruption. Lights are activated by a sensor that turns the light fixtures on if enough light is not received during the day. Cooler night temperatures of around 66°F are maintained. A nighttime heat retention blanket saves energy losses and a shade cloth is pulled over to shade the crops at a sensor-driven light point. High air-flow fans (HAF) keep a healthy flow of circulation around the plants and are not turned off.
Media & Planting
A sterile mix of 80 percent peat to 20 percent perlite is used in the plug trays. The soil is lime adjusted, taking into account the water pH (adjusted to 5.8 to 6.3). The soil is also given a weak starter charge of fertilizer. The trays are prefilled and watered via a water tunnel before planting. Plantlets are carefully pulled from the agar by crews working on stationary tables. Planters will pre-dibble holes with their forceps before placing the plantlet in the hole. All agar must be removed from the plants as it can be a hotbed for Botrytis.
The planting crew is responsible for grading the tissue culture plantlets by root size, crown and top growth. Burying a plant too deep will kill it by drowning. Planting it too high will kill it, too, as it would then desiccate the plant. Knowing which planters have low success rates can flash a red light for retraining on technique. Again, a responsible lab would show a customer the proper depth and planting technique. The trays are labeled and lot numbers are assigned. After planting, the flats are run through the water tunnel to be settled in.
Every greenhouse has its own microclimates, and you will find certain crops like ferns prefer cooler spots while others prefer hot spots. Flats are placed on benches by variety and preferences.
For the first five days, misting by hand is the best procedure. A gentle, 50 ppm calcium-based fertilizer (CLF) is applied five to seven days after planting when regular watering can start. As small as the plantlets are, they have great vigor, almost seedling-like, and roots can be to the side of the cell in seven to 10 days. Terra Nova uses both biological and chemical control for shore flies or fungus gnats, which can be monitored by yellow cards. These pests can be devastating to a crop, so it’s important to start immediately with control.
Again, the more clean and sterile the growing areas are, the fewer problems you’ll have. It’s also important to sterilize benches between crops and watch for algae, a fungus gnat Motel 6. Botrytis and root diseases and molds like Thalaviopsis, Phytophthora and Rhizoctonia can be kept in check by fungicidal measures. Those on a more organic scheme can use nematodes for gnat control, and Mycorrhizal fungi inoculants and BT products like Serenade for fungal control.
Plants will be in the weaning process for four to 12 weeks depending on the variety. Once out, it can go into less humid greenhouses or shade houses. Always keep your eyes open for signs of desiccation. After four to six weeks of outside weaning, plants can be shipped or potted up into larger containers. Always remember to not bury plugs too deep–it’s the number one killer of tissue cultured material.
With the benefits of superior plant material and much-reduced shipping, it makes sense to bring in plants in Stage 3. As with all other crops, there is a formula–perhaps you could call it a recipe for success. Don’t underestimate the need for the proper soil medium, planting depth, high humidity–even light and temperatures and keeping it clean.