How to propagate spirea shrubs through cuttings
Spirea is an easy to grow shrub that can also be grown in containers to create a shady spot or privacy screen on your patio or deck. The shrub is also easy to propagate in June or July by cuttings, a clipped limb (cane) that is treated and planted in a unique soil mixture where it will root.
Prepare the Pot
Fill a pot (at least 6 inches tall) with coarse sand, or a mixture of half sand and half peat, or a mixture of half peat and half perlite. You may have some success using sterile potting soil instead. Moisture the planting mix until water starts to trickle out the bottom of the pot.
The cutting should be made in early morning when the limb is full of moisture. Choose a limb that is new growth at least 6 inches long and on the upper portion of the bush. Rooting results are better with a lateral limb, which is a limb that grows off a terminal (or main) limb. Make the cut using sterilized garden clippers or a knife. Garden tools can be sterilized by dipping them into a 1:9 mix of bleach to water (roughly 2 tablespoon bleach in 1 cup water). Remove the leaves from the bottom 1/3 of the cutting.
Dip the bottom 1/3 of the cutting into rooting hormone. Rooting hormone, a chemical compound that promotes root growth, is available at lawn and garden centers for less than $10. An alternative root hormone that has been shown to be somewhat successful is honey. To avoid the potential of contaminating the container of rooting hormone, transfer some of the compound to another container, like a sandwich bag, and dip the bottom 1/3 of the cutting into the bag. If the powder does not cling to the cutting, dip the cutting in water and then re-insert it into the bag. Shake off the excess compound. Multiple cuttings from the same healthy shrub can be dipped into the same bag. Discard the bag with any left-over rooting hormone.
Use your finger or a pencil to create a hole in the planting mix to a depth equal to 1/3 of the cutting. Insert the cutting and water lightly to compress the planting mix around the cutting to hold it upright. Cover the cutting and container with clear plastic. A clear, plastic food storage bag can be used; synch the bag tight around the container. Place the container where it will get non-direct sunlight. Direct sunlight will make the mini-green house too hot.
Check the planting mix once or twice a week to see if more water is needed to keep it moist. There should always be moisture on the plastic. The cutting may lose leaves. If the cutting starts sprouting new leaves, then roots have started. Even if no new leaves appear, as long as some leaves from the initial cutting are still clinging, the cutting may be rooting.
Allow the cutting to continue to grow in the pot until the next late spring (remains in the pot roughly 10 months), at which time the cutting can be planted outdoors. During that growing stage, if the cutting starts pushing against the plastic bag, the bag can be removed.