Plant propagation is the creation of a plant from a plant‚ either from plant seeds or from plant parts (e․g․‚ a plant stem‚ root‚ or leaf)․ In Tennessee‚ a cutting or seed is only half of the equation․ If you know when to root the cuttings or germinate the seeds‚ and how to do it correctly‚ you can get dozens of new plants started by the time you finish with a good hydrangea plant or coneflower plant․
What follows will be a thorough discussion of the major propagation methods‚ when to use them‚ and what to expect trying them here in the mid-South․
Why Propagate Instead of Just Buying More Plants?
Cost is part of it․ But if your hydrangea has been in your yard for six years in Tennessee's clay soil and humid heat‚ a clone of that plant will do better than a plant grown from seed or purchased at a store․ Seed-grown plants from a store can't offer that guarantee․ Propagation can also enable you to fill gaps in a border without waiting for plants to come in at the nursery‚ or give you the chance to try out grafting‚ for instance to combine a hardy rootstock with a fruit variety that wouldn't otherwise survive our winters․
Vegetative Propagation: Cuttings‚ Grafting‚ and Layering
Vegetative propagation is an asexual reproduction method that produces a genetically identical clone of its parent from parts of the parent․
Stem cuttings are used for most shrubs and many perennial plants․ The fastest method is with softwood cuttings‚ taken from new growth in spring․ The cuttings should be kept moist until they root․ This can be done by placing them in a plastic bag․ In Tennessee‚ one can take cuttings in late May or June‚ when the new growth is firm but not fully woody․ Evergreens (boxwood or holly‚ for example) take semi-hardwood cuttings more successfully in late summer (August into September): cut a 4-6 inch (10-15 cm) shoot just below a leaf node‚ strip off the leaves on the bottom half‚ dip it in rooting hormone and plant it in a 50:50 perlite/peat mixture․ Softwood cuttings root in 3-6 weeks‚ semi-hardwood cuttings in 8-12 weeks․
Grafting‚ the joining of a scion‚ the top part selected for its potential fruit or flowers‚ with a rootstock selected for its hardiness or disease resistance‚ is the most common means of fruit tree propagation․ Most commercial apple and pear trees are grafted because seeds produce seedlings with undesirable fruit; whip and tongue grafting is the preferred method of laying down dormant scion wood in late winter‚ before buds are already open․
Difficult to root shrubs such as magnolias and rhododendrons can be propagated by layering․ Here‚ a low branch is wounded slightly on its underside and buried a few inches deep in the soil‚ and then pinned down to the underlaying soil․ It will root while still attached to the parent plant․ Depending on species and growing conditions‚ this takes anywhere from a few months to a full year․ You will know it is ready to be removed and transplanted when it resists a gentle tug․
Seed Propagation: Building Genetic Diversity
Seed propagation is the only form of propagation that offers genetic variability‚ an important quality to possess if you want to develop particular plant characteristics or improve the durability of your garden․
Most perennials and annuals simply require the same procedure: leave the seed heads on the plant until the flower head completely dries and turns brown (don't get impatient and pick before the seeds are completely ripe)․ After harvesting‚ make sure to store in paper envelopes (never plastic where moisture can result in molding) in a cool‚ dry place until ready for planting․
Germination conditions vary widely between species․ For example‚ many of the native wildflowers and perennials found in Tennessee‚ such as coneflower‚ milkweed‚ and black-eyed Susan‚ require cold stratification in order to germinate․ That means a cold-treated winter‚ either by sowing them in the ground outdoors in the fall‚ or a 4-8 week cold treatment in the refrigerator or icebox before starting indoors․ Warm-season vegetables and annuals thrive in soil temperatures between 70-75 °F (21-24 °C) for germination․ That usually means starting them indoors in March and planting them as transplants mid-to-late April when the frost risk is over (April 10-15 in Middle Tennessee‚ later to the east)․
Once seedlings develop the second true leaf pair and the root system has filled the starter container‚ harden them off in 7 to 10 days by incrementally increasing the amount of time they are outdoors before planting them․
Root Propagation and Division
However‚ some plants with fleshy‚ tuberous roots for storage of energy can be propagated by vegetative propagation rather than by cuttings from the stem or seed propagation․
Root cuttings can also be taken from perennial plants such as phlox‚ oriental poppies‚ and various ornamental grasses․ Cuttings should be taken when the plant is dormant‚ which would be late fall through the winter for most plants here․ To propagate‚ take a 2 to 3-inch piece of root‚ lay it flat on sandy soil‚ and store in a cool location (50 to 60 degrees Fahrenheit) until sprouts emerge (generally 4 to 8 weeks later)․
In propagation by tubers (e․g․ with dahlias or potatoes) the tuber is severed into pieces that contain at least one "eye"‚ or bud‚ from which the new plant will grow․ Dahlia tubers are usually divided in early spring‚ just before planting‚ because the cut ends often rot if stored at cool‚ damp temperatures for too long after being divided․
Division is often the most straightforward way of propagating many perennials and ornamental grasses‚ although it often merits only passing mention in propagation manuals․ It is simply a matter of removing the entire clump (an established hosta‚ daylily or ornamental grass‚ for example) and dividing the root ball into several sections․ In Tennessee‚ the best time to do this is fall and early spring because the plant will not be under heat stress in the summer as its roots reestablish․
A Few Practical Notes
Rooting hormone may improve rooting success rates for woody cuttings‚ but not for fast-rooting perennials․ Bottom heat (from a seedling heat mat) can be provided to speed up root formation for both cuttings and seeds: 70-75 degrees Fahrenheit․ This is particularly good advice if you're starting things indoors‚ given Tennessee's unpredictable spring weather․ It's no good to rush the transplanting; it will be much harder for a cutting or a seedling with a weak root system to be outdoors than one you gave an extra two weeks to․
And if you don't feel like taking a chance like that on a single variety‚ keep in mind that TN Nursery's established perennials‚ shrubs‚ and native plants often will be strong enough to divide or take cuttings from within a season or two of planting‚ so you're not choosing between buying and propagating so much as doing both․
