Gardener Within: Joe Lamp'l, a master gardener and author, shares how to to get plant cuttings to root using a "Forsythe pot."
Taking cuttings of your favorite plants while they’re still actively growing is a great and inexpensive way to perpetuate tender annuals, old friends and new acquisitions as insurance for next year, or simply to expand the garden or share your plants with others.
There are many methods to get plant cuttings to root, including suspending them in water. But one of my most successful ways has been the “Forsythe pot.” Cuttings started with this system develop strong, robust roots that are less likely to rot before they’re ready to transplant. They also take the move to potting soil better than “water” roots, which tend to form brittle clumps when removed from the water.
Start with a clean, 8-inch-diameter plastic pot. The nonporous plastic will help retain moisture. Sterilize with a solution of one part chlorine bleach to nine parts water, rinse and let dry. Cover the drainage holes with a piece of paper towel or large coffee filter and fill the pot to the rim with vermiculite, a fluffy, fast-draining but moisture-retaining mineral that lets new roots develop quickly while giving them lots of support.
Next, seal the drainage hole of a 4-inch terra-cotta clay pot to make a watertight reservoir. You can use a small cork or a bit of plastic from a milk carton or coffee-can lid covered with silicon tub caulking. Press and rotate the clay pot into the center of the vermiculite until its rim is level with the surface.
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Gently water the vermiculite until the water runs out of the drainage holes of the plastic pot. Once it’s saturated, this should be the only time you need to water the vermiculite directly.
Fill the terra-cotta pot reservoir with water, and keep the level topped off. The permeable terra-cotta clay will let water slowly seep into the vermiculite, keeping the cuttings moist without pouring water over them and possibly disturbing their growing roots.
Select the plants you want to propagate and take fairly good-sized cuttings from the ends of younger growth, if possible. Trim up to just below the point a leaf is attached to the stem. Many plants generate new roots from this node. Remove all but the leaves closest to the tip of the cutting. Too many leaves can dry the plant out before it has a chance to develop roots to take up replacement water.
Dip the end of the cutting, including the leaf node, in rooting hormone and gently tap off the excess powder. Then stick the cutting firmly into the moist vermiculite, covering the node deeply.
Place the Forsythe pot in a well-lit location out of direct sun. A north-facing window or under fluorescent lights is ideal. With no roots to take up water, the cuttings could dry out under bright, hot light. Some folks try to counter this by putting a large plastic bag over the pot to hold in moisture. I avoid this because it tends to promote rot. The slow trickle of water through the terra cotta lets the cuttings lose, and replace, all the water they need at their own pace. And it’s easy to see when the reservoir needs refilling.
In a week or so, start adding a quarter-strength solution of an all-purpose organic liquid fertilizer to the water when you top off the reservoir. A few weeks later, test root growth by gently tugging on the cuttings. They should feel firmly entrenched in the vermiculite. You can work the medium aside with a finger to see the healthy root growth.
At this point, you can remove the terra-cotta pot and slice the roots between the cuttings to transplant to a larger container filled with sterile potting medium. Water thoroughly with half-strength, balanced liquid fertilizer, one with the same percentage of all elements, such as a 5-5-5.
Now it’s just a matter of keeping the plants moist, well lit and warm. Using the Forsythe pot will give your cuttings the best possible start and provide all kinds of new plants for next year’s garden.
Joe Lamp’l, host of “Growing a Greener World” on PBS, is a master gardener and author. For more information visit www.joegardener.com.
Some plant may be considered noxious weeds in your area. If you’re not sure what plants may be invasive in your area, check with your local garden, horticultural center or noxious weed control board.