Both plants and people can propagate through sexual reproduction, but obviously, this isn't true of asexual propagation: A severed human toe doesn't sprout a new person, nor does the person sprout a new toe!
Here we'll describe the most common types of asexual propagation methods used in the classroom setting: cuttings and division.
Taking a cutting involves removing a piece of a leaf, stem or root and placing it in a growing medium where it then develops the other parts that it left behind (i.e., a stem will then grow roots, a root will then grow a stem).
Rates of success with cuttings generally are lower than seed germination rates. For the best chance of success:
- Take cuttings with clean instruments
- Place them in moist, sterile, soilless potting mix
- Choose plants that root easily (see table below)
Cuttings of some plants root easily in vases of water, but others will rot before making roots if you place them directly in water. Pot those stem cuttings, as well as any root and leaf cuttings, in soilless potting mix. Listed in the table below are plants that grow well from cuttings and should provide you with a good success rate even in tough classroom conditions.
|Plant||Plant Part||Propagation Medium|
|Coleus||stem||water or soil|
|pothos ivy||stem||water or soil|
|geranium||stem||water or soil|
|African violet||leaf or stem,||soil|
|jade plant||stem or leaf||soil|
|English ivy||stem||water or soil|
|wandering Jew||stem||water or soil|
Have your rooting medium set up before taking cuttings. Use clean scissors and make sure that each cutting measures 4 to 6 inches long and has at least 4 leaves. Remove the bottom leaves from the cutting and immediately insert it in water or soil.
Caring for Cuttings
Cuttings need high humidity and warm temperatures to help them grow. Nursery professionals have mist beds that spray cuttings intermittently throughout the day to keep humidity high. You can create a similar effect by creating a tent with clear plastic wrap and then misting cuttings throughout the day with a spray bottle to keep the soil and the air around the cutting moist -- but not soaking wet. To make the tent, prop the plastic wrap off the surface of the planting mix and plant parts using popscicle sticks or other 'posts.' As soon as your plants establish a few roots (you can check by very gently tugging on the cutting to see if there is any resistance) you can remove moisture tents. You'll need to experiment to find the perfect balance for the humidity levels in your classroom.
Most of the plants listed in the table above root within a few weeks, but cuttings of some plants can take weeks or even months to develop the missing parts. Monitor plants regularly to check on progress. Are new leaves appearing? If the cutting is in water, can you see roots growing?
Further exploration: If your students have ever planted potato tubers, they've already practiced asexual plant propagation, since tubers -- underground energy storage units -- are a actually stems! Find out how one teacher used potato explorations to fire up inquiry with her class: It's All in the Eyes: Inquiry Up Close.
Some plants grow in a clumping habit, sending out roots and or stems (above ground or underground) that then produce new stems and leaves. As the term division implies, once the clumps exceed their original size, you can dig them up, divide the plant into smaller clumps of roots, stems and leaves, and replant the smaller divisions. Examples of plants that tend to clump include: lamb's ear, mondo grass, daylilies and chives.
Clumping plants are excellent for children's gardens because they need frequent division, giving more students the chance to have the planting experience. (Plus, you can sell divisions at plant sales to raise money to support the garden!)
Some plants grow new plants on aboveground stems. Although the new plants may or may not develop roots while on the parent plant, you can remove the new baby plants easily, plant them in soil, and new roots will appear quickly. Plants that commonly produce "babies" on above-ground stems include spider or airplane plant, strawberry geranium and mother-of- thousands.
Which Method to Use?
A plant's response to different types of asexual propagation varies. You can take a leaf of an African violet and plant it in soil to get a new African violet, but if you plant a leaf of a maple tree, you will end up with a shriveled up leaf. For asexual reproduction of a maple tree, you need to cut and plant a portion of stem that has 2 or 3 leaf buds on it.
So how can you find out which asexual propagation method to use with a certain plant? You can discover it through trial and error or search published resources for advice. An excellent source of information for common landscape plants is The Reference Manual of Woody Plant Propagation: From Seed to Tissue Culture: A Practical Working Guide to the Propagation of over 1100 Species by Michael Dirr and Charles Heuser. Additional resources are available via Internet searches.
There are many other forms of asexual propagation. Two that you can use to challenge older students are layering and tissue culture.
Propagation by layering involves coaxing a stem to grow roots while it is still attached to the mother plant. This happens naturally for some plants, such as black raspberries, when their stems touch the ground and produce new roots. Learn more about layering here.
Tissue culture involves growing an entire plant from just a few cells! The cells grow on nutrient medium until they have differentiated into leaf, stem, and root tissue and you can transplant them to potting soil. Tissue culture kits for school classrooms are available from many science educational supply catalogs.