One of the best ways hobbyist greenhouse gardeners can keep their greenhouse continuously full of vibrant plants is to master the art of plant propagation. Plant propagation is the process of creating new plants from seed (sexual propagation) or clone/cutting/tissue culture (asexual propagation). Each method has its advantages and disadvantages but they are equivalent in the results they produce: more plants. The concept behind plant propagation is simple but the needs of a seedling or clone vary dramatically from that of a mature plant. Just as human babies require special attention, young plants are more sensitive and have different needs than their adult counterparts.
Although germination and rooting techniques vary depending on the specific plant species, there are some general guidelines that can be followed for propagating the majority of greenhouse ornamentals and vegetable plants.
A plant started from seed receives its genetics in a manner similar to the way humans receive genetics. Just as we are made up of a combination of our father and mother, a plant started from seed is made from the genetics of its father and mother. As with humans, some traits can skip generations and there is no guarantee that a plant started from seed will inherit specific traits from its mother or father. This is the main reason the majority of commercial plant propagation is done by clone or tissue culture. Starting from seed is unpredictable but it is also a chance to start a completely unique plant that will add more biodiversity to our wonderful planet.
Germination is the process in which the plant emerges from the seed to start its life. For most plants germination is best done in a very moist (high humidity) environment at a reasonably warm temperature, usually around 75-85 degrees F (the exception to this would be seeds that are generally planted in early spring when the ground temperature is much cooler). Propagation trays with humidity domes are great for starting seeds because they create a microclimate that is more easily controlled. Seeds can be placed directly into a moist medium in the tray and covered with the humidity dome. If the environment is not at least 75 degrees F consistently, it is advised to place a seedling heat mat under the tray to keep a constant temperature. Plants, in general, respond better to consistency and this is especially true with seeds or clones. Keep the top portion of the medium moist until all of the seeds have sprouted. Once the seedlings have broken the surface, lift the dome off periodically to bring in fresh air and also acclimate the seedlings to the lower humidity of the environment. Slowly increase the amount of time each day the dome is removed until it is removed entirely. Most varieties of plants can be acclimated in a matter of a few days. Follow the seed packet’s instructions for thinning, spacing, and transplanting.
Another popular germination method is the wet paper towel technique. Place your seeds in a damp paper towel and fold the paper towel over the seeds. Put the paper towel in a ziplock bag and place it on top of your refrigerator (toward the back; this keeps the seeds at a consistent temperature). Check daily by gently unfolding the paper towel to examine the seeds. Keep the paper towel moist; adding water if necessary. In a few days you should see the first root coming out of the seed (radicle root). Gently, using a tweezers if necessary, place the seed into the soil with the radicle root facing downward. Cover the seed and keep the top layer of soil moist until the plant breaks the soil’s surface. The paper towel technique is a fun way to teach children how plants start from seeds. This technique works best with larger seeds (melons, cucumbers, squash, corn, sunflower, etc.). Most smaller seeds, such as lettuce, are best planted directly into the soil.
Starting a plant from a donor or mother plant is a form of cloning or asexual propagation. Some plants naturally propagate themselves this way; a good example is the inch plant (wandering jew). Cloning has become the most popular method among commercial growers because the offspring are identical to the donor plant so the growers know exactly what they are going to get. It also allows the gardener to replicate plants with desirable qualities such as a certain fragrance, color, resistance to pathogens, or any other trait that could be deemed as beneficial.
There are two popular types of cloning techniques: cuttings and tissue culture. Hobbyist gardeners generally take cuttings to replicate their favorite plants because tissue culture is a more involved process that requires special equipment. African violets, pothos, gardenia, crepe myrtle, cyperus, geranium, wandering jew, coleus, impatiens, spider plants and hibiscus are all easily propagated by cutting. Many vegetables, including peppers, tomatoes, cucumber and squash, can also be propagated by cutting. Some vegetables, like cabbage, can be cloned via their root. Although there is a slight variance in cloning techniques (depending on plant species) the majority of the focus should be on environmental conditions as they are usually the determining factor in cloning success.
Like plants started from seed, plants propagated from cuttings respond best to consistent temperatures. Ideally the root zone (or potential root zone) is kept at a temperature of 75-85 degrees F. If the environment where the clones are kept has fluctuating temperatures it is best to utilize a heat mat or heat cables to rectify this problem and maintain consistent temperatures. For most species of plants, if temperatures are too low or continually fluctuate below the desired range, the plants enter what I refer to as a state of suspended animation. In other words, they remain green and healthy looking but fail to create roots or carry out any vital functions needed to stimulate new growth. Eventually these clones will die without ever creating a root.
In cases where humidity levels are high and temperatures are low a cutting or seedling will dampen off. Damping off is a horticultural disease caused by pathogenic microorganisms which destroy a seedling or cutting before it has a chance to grow. On the other end of the spectrum, clones that are kept in temperature ranges above the desired range will commonly wilt or rot and turn into mush long before they develop a new root zone. An inexpensive digital thermometer is a great way to monitor the temperature of the root zone. Make sure to monitor the temperature during the coldest and hottest points of the day to ensure you stay within the desired range even when the atmospheric conditions are the most volatile.
Humidity levels for cuttings should be kept high for at least the first few days. Although they are not yet established plants, cuttings still transpire moisture through their stomata and without the ability to absorb replacement moisture through a root zone a cutting can quickly wilt in lower humidity environments. Ideally, the humidity level for cuttings should be kept at 80-100% for the first few days. Humidity domes, plastic bags, or any way you can temporarily enclose the cuttings to retain a high humidity environment will suffice. I prefer the humidity domes sold at local gardening centers because they make it very easy to acclimate the plants to their future environment.
After you take your cuttings, lightly spray the inside of the humidity dome with water and place it on the tray. In the subsequent days remove the humidity dome for a few minutes each day to give the cuttings some fresh air; very similarly to the way you would acclimate seedlings. After four days, you can incrementally increase the amount of time you remove the dome each day until the cuttings are completely acclimated to their environment. Although some plant species are more finicky than others, generally speaking, the cuttings will have created their own roots in 7-14 days and at that time should be completely acclimated to the environment.
Seedlings and cuttings are more sensitive, not only to environmental conditions but also to pests and pathogenic microorganisms, than adult plants. Before planting seeds or taking cuttings completely sterilize all the equipment you plan on using. Diluted bleach or hydrogen peroxide are great ways to effectively sterilize your propagation equipment before each use. Make sure the medium you choose is sterile as well. Prepackaged propagation mediums offer the peace of mind of a sterilized medium and ensure the seedlings or cuttings aren’t being placed in a compromised medium.
There are many products on the market designed to aid the propagation process. Rooting hormones, seedling heat mats, aeroponic clone machines, humidity domes, sterile disposable scalpels and numerous prepackaged mediums are all products that can help achieve successful propagation. Just remember, there is no magic product that will automatically guarantee propagation success. These products teamed with sound environmental conditions are the keys to thriving seedlings and clones. Stable environmental conditions are the foundation for making any aspect of greenhouse gardening flourish and propagation is no different.
Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at Ehop@GardenAndGreenhouse.net.