Tissue cultured plantlets: just from the lab
Today it is possible to purchase from a number of laboratories stage III tissue culture plantlets that are grown through stage IV in greenhouses. The improvements in this process result in good uniform crops that are essentially disease free when arriving at the greenhouse operation. The plantlets can come from anywhere on the globe and pass through phytosanitary inspections readily due to their being cultured in a sterile medium. This also gives a grower a choice of source (or sources) to improve year round product availability, and provide some degree of competition in costs. A wide range of plants are available from this type of propagation today, and tissue culture (tc), has made new plant selections and hybrids more rapidly available to greenhouse growers than in the past.
If you have already been involved in finishing tc plantlets, you are likely to aware of the considerations required to succeed in this type of production. If you have not tried tc finishing in your operation you may wish to consider the differences from unrooted cuttings and seed propagation before investing in tc plantlets. Stage III plantlets come to the greenhouse grower with roots, and leaves established upon the plantlet but the plantlet is not acclimated to the 'real world'.
Some environmental and managerial considerations for tc finishing are paramount for success. A mist or fog system is required, good temperature control, light control (both day length & intensity), and good technical skill. These plantlets are not the same as unrooted cuttings in that they were grown in a sealed vessel, and do not operate as would an unrooted cutting from a stock plant grown in a greenhouse. They are not photosynthesizing properly, their stomata are not operating as would an acclimatized plant, the leaf cuticle is missing or poorly formed, and the roots are also not functioning as would an acclimatized plant.
Further the tc plantlets typically were in an environment of 75-80 degrees F, with a 16-hour day length, before being shipped to the grower. To meet the needs of these plantlets, mist should be used for 1-3 weeks while the plantlet grows new leaf tissue and some new roots. The temperature can be slowly adjusted in the greenhouse from the lab temperature to the normal cropping temperature for that particular plant. Perennial plants such as Echinacea, Heuchera, and Tiarella grow satisfactorily at cooler temperatures than annual plant species, and can be established from tc plantlets in 58-68 degrees F, while the annual plants are going to develop better from tc plantlets with a 68-78 degrees F environment.
Regardless, both groups will need to be acclimated to the target temperature range, and not merely dropped into that temperature. New leaves produced while being misted, will grow the proper cuticle for protection, and stomata will be operational. Root tissue grown ex-vitro will be functioning properly. Light should be reduced in intensity, especially from May to September, as the lab was probably growing the plantlets under fluorescent lamps.
The day length should be increased to 14-16 hours of light, during short days, and then can be reduced as the tc plantlets acclimate to the real world. This can be done with a number of different lamp types successfully. If a species (or cultivar) is triggered for growth under long days then that is what it should receive in the propagation period at the greenhouse. After the plant has finished in the mist system the day length can be reduced to the natural period. If you are doing this production in the late fall or winter, the perennial plants may go into dormancy with reduced day length.
Crop timing will vary by operation, and by environment, but the perennials started from tc will typically take 5-8 weeks to finish as plugs. Some species will take longer. Rapidly growing species could finish in less time with optimum light levels, and temperatures. An important consideration in success is the planting depth of the tc plantlet. It should be placed in the rooting medium at crown height, and plantlets that are too high will dry out while plantlets inserted too deeply into the medium will be likely to rot. On some species this is a very small difference between too low (or too high) and just right, perhaps as little as 1/8th inch. Plant propagators who do not pay strict attention to the height a microcutting is inserted in the medium can expect crop losses. Good attention should be paid to medium to plantlet contact.
A plantlet that is poorly seated in the medium can also dry out. Mist frequency and mist duration will vary by greenhouse environment, but the foliage should not get dry during the daylight period until the new leaves have been formed. The mist schedule should be set to re-mist the flats of plants just as the foliage begins to dry. A successful schedule on a sunny day in spring is 10 minutes between mistings and 20 seconds of mist on time, but this should be tested and altered to suit the growing environment in question. By early summer the interval between misting cycles may have dropped to 8 minutes, and on cloudy days will probably need to be extended to 15-20 minutes between misting cycles.
On a very rainy day frequency could be 20-40 minutes between cycles. Regardless of time of year and cloud cover the principal is the same, do not let the foliage get dry on the leaf surface. A leaf without cuticle is very easy to dry out (permanently). Growers new to tc finishing should expect to observe the crop frequently until they are more familiar with the crop needs and performance in their system. Once the microcuttings have grown some new tissue they can be weaned off the mist, but again this should be done gradually, not radically for successful acclimation.
Finished crop of tc plantlets Now ready for the "real world"
It is often required that plants need to be sized after the misting period has been concluded. Tissue culture producers generally do a good job in their part of propagation, but some variance in plantlet size is inevitable. While a propagator receiving the tc plantlets can size them before planting in flats, the operation is likely to need to do some sizing after the crop has completed the misting phase. By grouping the large plants, the medium, and the small in different flats there will be less total crop losses.
It is very inefficient to water plug by plug, so size them and water by flat. Once the misting phase is completed, you now have a successful stage IV plant, which means it is truly acclimated. Plugs finished from tc plantlets may require a week or two more growing time before shipment, so as to be well rooted into the plug tray, but they are basically ready for sale or potting up.