If you've ever put an apple in a paper bag to help ripen other fruits, you've played around with plant hormones. As they ripen apples emit ethylene gas, which is a hormone that accelerates ripening of fruit.
Ernesto Sandoval, director of the Botanical Conservatory at UC Davis, talked about plant hormones and their practical applications at this year's San Francisco Flower and Garden Show.
Sandoval focused on two main plant hormones that are produced in the growing tips, or meristems, of plants. Auxins, produced in shoot tips, stimulate root growth. Cytokinins, produced in root tips, stimulate shoot growth.
So if you dig up a plant from the ground, you inevitably prune the root tips. A healthy plant will need to grow new roots, and to do this it needs auxins it gets from the shoot tips.
But wait, what if you've pruned the top growth at the same time as you dug up the plant? Sandoval said that if you get rid of both actively growing parts of the plant – the shoot tips and the root tips – at the same time, you've removed both sources of guidance. The plant doesn't know what to do and won't grow.
So what do you do if you want to move a nondormant shrub or tree and it's too big? The solution is to plan ahead. Prune it in advance, then wait for new growth to appear. Cutting top growth stimulates cytokinins, which travel upward from the roots and cause dormant buds to grow new shoots. Once new shoots have appeared, you can dig it up. The new shoots will have a good supply of auxins, which will promote root growth.
The exception is roses. You can do whatever you want and they'll still grow, Sandoval said.
The most surprising thing I learned was that auxins are sensitive to gravity. They travel from the growing tip to the roots. So if you take cuttings, you will get better root growth if you keep the cuttings upright, rather than laying them horizontally as you prepare the planting media or carry them from one place to another.
Sandoval almost never uses purchased rooting hormones for propagating any more. I choose good material at the right time, and hold it vertically so hormones build up at the bottom, he said. An actively growing part with young leaves ensures a good supply of auxins. Research shows that cuttings need a continuous source of auxins, so he retains the leaves and the plant tip, rather than cutting them back to reduce evaporative loss.
A dormant plant or wimpy branches won't root well, he said. He makes a straight cut just below a node, which gives more even rooting than a slanted cut. The exception is woodier stems, which he cuts at an angle to get more surface area. He gets better rooting with softer tissue, which has more auxins, but uses woodier tissue if a particular plant has soft tissue that rots before it can produce new roots.
Some plants can be propagated from leaves, which contain meristematic tissue that's not very organized. Lachenalia leaves, for instance, can be cut in strips and yield clusters of bulbs at the base of every cutting. I've seen some kale plants propagated from single leaves, and now I know why it's possible.
On a multibranched shrub such as lavender, it's easy to identify the actively growing branch tips that will be rich in auxins. On this variety, they have soft green leaves, which will turn more silvery as the season progresses.
When you take cuttings, place them vertically so that the auxins, which stimulate root growth, will accumulate in the bottom of the cutting. Don't lay the cuttings on your work surface, like the example in the foreground. Instead, place them in a container where they can stand upright, like these sage cuttings.
When you prune a plant, choose the most vigorous cuttings to propagate. The first cutting on the left is barely alive. The second and third don't look like they're actively growing. The last two are healthy, with lots of new leaves, so they should be able to provide enough auxins to grow good roots, but the one on the right is the best choice because it is less woody.
© 2014 Tanya Kucak