Sciency Answers: Rooting cuttings

Today's question is from Loree, of Danger Garden fame:

I have a couple of Echium that I've grown extremely attached to. I know there is no chance they are going to winter over here and I don't have any more space to dig them up (they've gotten huge!) and pot them with the hope of their surviving. I'm wondering if you can talk me through the process of trying to propagate from a cutting. I have both soft wood and hardwood on my Echium fastuosum, “Pride of Madeira" (at least I think I do!?) what is the best to take a cutting from? And then what do I do. Hope you can help,

Indeed I can! Here is my Sciency Answer:
Rooting cuttings is a pretty good trick. You take a piece of a plant lacking a very important organ, roots, and convince it to grow new ones. This is possible because plant cells are 'totipotent' meaning one type of plant cell can change into a different type of plant cell. Our cells can't do that -- our skin cells can only produce more skin cells, etc, and when you cut off one of our legs, we can't grow a new one. The only human cells which are totipotent are those embryonic stem cells you keep hearing about. But all plant cells are essentially embryonic stem cells. When you take a cutting, the plant goes, "Whoa! No roots!" and starts trying to make some. At first, all the cells are busy being specialize to form bark or stem or buds. But some of those cells de-differentiate into generic cells called callus, and then those callus cells change into roots. This process takes time, of course, and during that time the plant has no roots, so is very vulnerable to drying out and dying.
To deal with that reality, all the procedures for rooting cuttings are designed to do two things: Speed up root formation, and keep the plant alive in the mean time. As a general rule, younger, softer, tender growth is quicker to make roots, but also quicker to wilt and die, while older stems may take longer to root, but tend to hold up better during the wait. When you see instructions for taking cuttings that recommend soft wood or hard wood cuttings, you are just seeing what, in other people's experience, is the stage of growth which best balances, for that particular plant, rooting quickly and not dying. For some easy-to-root plants, this is merely a suggestion. For others, one particular stage of growth may be the only time you have a reasonable chance of success. To move from the general to Loree's specific question, it appears that for echium, the best stage is softwood, meaning new, tender, freshly sprouted growth.
Once you've taken your cutting, you need to provide a special environment to help it survive. The first requirement is usually extremely high humidity to keep the plants from drying out before the roots get going, unless the plant in question is a succulent or a desert plant able to cope with very low water. Commercially, this is achieved on benches in a greenhouse which are automatically sprayed down with a fine mist every couple minutes. This is excellent, but not practical for most of us. At home, my default method is a zip-lock bag. I put some moist (not wet) potting soil in the bag, shove my cuttings in, and seal the bag. I should add here that though some cuttings will root if placed in pure water, I would never recommend it. It looks cool, but everything that roots in water will root more happily in good potting soil, and roots formed in water are physically different (because of a lack of oxygen) than roots formed in soil, so transplanting from water to soil can be a shock for the young cutting. Moist vermiculite or water-holding florist foam (eg, Oasis) are both also good alternatives to standard potting media. I always use potting media because that is generally what I have around, but you may want to experiment.

(I had fully intended to have a cute little photo of happy cuttings in a zip-lock bag at this point in the post. But I didn't get around to it. So you can use your imagination instead.)

With your high humidity provided to ensure your cuttings don't wilt and die, you now need to encourage the things to grow roots. There are a couple things you can do to help this process.

Light: Light is essential. Growing roots requires energy, so the cuttings need to photosynthesize as much as possible. This can be a balancing act -- too much sun can dry out the cuttings, or overheat ones in a sealed bag. I tend to aim for light, dappled shade under a tree in the summer. Florescent lights also work great because they produce a fair amount of light without a lot of heat.

Bottom heat: If possible, keep the bottom of the cuttings warmer than the tops by placing them on something warm, like a seedling heat mat. Warmth speeds up cell division at the bottom of the cutting, shortening time to rooting. Keeping the tops of the cuttings cooler keeps them growing slower and using less water while the roots are developing.

Rooting hormones: The plant hormone auxin promotes new root formation. You can buy auxin preparations specifically for rootings cuttings either as a powder or liquid you dip the end of the cutting in before sticking it in the media. Many cuttings will root on their own, but hormone treatment will usually speed up the rooting process and increase your percent success. In many species, the exact ideal concentration of hormone to apply for maximal rooting has been identified. I doubt anyone has done that study for echium, so you might want to try treating at different rates and see what works best for you.

Wounding: This may seem counter-intuitive, but for many species, cutting or even crushing the end of the cutting promotes rooting. In essence, this exposes more cells to the hormone treatment, and gives more cells a chance to divide and switch into roots. Depending on the species, sometimes this helps, sometimes not, as wounding can also promote rotting. Again, you might want to play with it and see what works best for you.

Then you just wait for roots to form. Telling if there are roots or not can be difficult because, obviously, the roots are going to be out of sight (I guess this would be the only benefit to rooting in water). The oft repeated advice is to tug gently on the cutting and see if it resists or not. The problem with this advice, at least for me, is that it only works for plants I don't care about. When I'm rooting a few odd coleus, I generally forget about them for a few weeks, then give them a solid tug which is solidly resisted by their roots and I know they are ready. For precious plants I end up fussing with them practically day, generally tugging too gently to be able to tell if they've rooted or not, sometimes tugging to hard at just the wrong time and ripping off just nearly developed roots, which is fairly devastating. So I use other clues. A good one is to look for new growth. Until roots form, the cutting will probably not really grow, so new leaves usually (not always) means new roots. If you are rooting in a zip-lock bag, you can also watch for roots to show against the clear plastic. If in doubt (and I frequently am), gently lift the cutting with some soil, and gently wash or shake the soil away. If the end of the cutting looks just like when you stuck it in, you've got a while to wait. If you see lumpy whitish or brownish bumps on the bottom, you are in luck, that is callus, the first step to roots. If you see roots, of course, you are home free.

Once your cutting has rooted, you need to slowly acclimate it to the harsh real world. Commercially, this is done on a bench next to the mist benches where humidity is still high, but the cuttings aren't getting directly sprayed. If you've rooted in a plastic bag, first open the bag, then gradually, over a course of days, roll the top down until the cutting is out in the real world. With cuttings I care about, I do as I've just written. With things I don't care so much about, I just open the bag one day, nothing gradual about it. Generally things take this just fine, provided they've had time to develop a good root system. Then you can pull the rooted cuttings out, stick them in pots, and smile at them with smug satisfaction.

If, by the way, you are interested in learning more about the art and science of plant propagation, Ken Druse's Making More Plants is excellent. Very thorough, very readable, and, like all of Ken's books, very beautifully photographed. For an even more in depth but much drier presentation, the standard textbook on the subject is Hartmann and Kester's Plant Propagation.

Have a question? Get a sciency answer! Just e-mail me: engeizuki at gmail dot com