I get a lot of questions about a lot of different products and practices. New topics send me to the scientific data bases and that’s where I went for today’s posting. One of my garden writing colleagues asked me about colored glass rooters – glass containers in different colors that can be filled with water and a plant cutting. The conventional internet wisdom, according to my colleague, is that green and blue glass rooters are the best.
The first mention I could find of such a practice is from an 1801 publication called The Cottage Gardener. In it, we’re informed that for rooting cuttings “such coloured glass is useless; it has no influence over the production of roots.” Nevertheless, 200 years later web postings like “I have found that cuttings placed in colored blue or green glass root faster than clear glass” are taken as solid evidence that blue or green glass containers are best for rooting cuttings.
There is science behind different colors of light and rooting, but it’s a little more complicated. Chlorophyll absorbs red and blue light best, so plants whose leaves are exposed to red and blue light grow well and tend to produce a healthy flush of roots. On the other hand, plants whose roots are exposed to blue light have decreased root growth compared to those under white light conditions. In this case, the photoreceptor called cryptochrome might be responsible for inhibition, as it is a blue light absorber. Similarly, plant roots exposed to green light do not grow as well as those exposed to white light.
In my opinion, this is another example of aesthetics trumping science. Of course colored glass rooters are more attractive that plain old glass jars. And that’s a perfectly valid reason to use them as part of one’s home decor. But it’s not science, nor is it necessarily the best way to encourage rooting.
What seems to be most important in rooting cuttings in water is to use indirect lighting (north-facing windows in the northern hemisphere, for example) so that the water doesn’t get too hot. And keep in mind that not all species root well from cuttings.
Shouldn’t cuttings be rooted in a medium other than water?
It would seem to me that the rooting of cuttings in water is a useful educational enterprise and sometimes serves ones curiosity – rooting in colored water might be interesting. But, if one wants to produce a plant from a cutting then, after one knows what one is doing, one should root the cutting in some solid medium
Plant propagation can be a vexing activity, because species are so variable in their abilities to be rooted. It also depends on their life stage – some root better from woody tissue (i.e. stems that are over a year old) than soft tissue, for example. Some species are just generally difficult and require dipping in rooting hormone and placement in a mist chamber. Others are ridiculously easy and can be rooted by simply putting cuttings in plain old water. I’ve found one of the easiest ways to root cuttings is by placing them in a ziplock bag (to maintain high humidity) with a damp paper towel. In any case, you can experiment
with different propagation methods. (Kenny, the Sustainable Landscapes book you bought has an entire chapter on propagation.)
My grandmother swore by rooting cuttings in brown glass (she used empty yeast jars) and I have found her to be right. My sister sent me 3 dozen cutting of hoyas several weeks ago. I put the majority of them in brown jars of water. I didn’t have enough so I put some in clear glass and put them all in the same south-facing window so they get exactly the same amount of light. Within 2 weeks I had measurable roots on the brown glass cuttings and none on the clear glass ones. After 6 weeks I’ve got cuttings from the brown glass that are ready to plant, the ones from clear glass still only have minimal roots. I’m moving them to the brown glass when I get all those potted up.
You’ll note that I cautioned against south-facing windows because of the heat and intense light. It’s more than likely that you’re seeing the negative effects of this environment – not anything in particular about brown glass. But if you are stuck with a south-facing exposure, then clear glass is not a good choice. You’d have just as good of luck in a ceramic (opaque) jar as a brown glass one.
The color we see is the color of light that is reflected. Therefore blue or green colored glass would be reflecting green and blue light wavelengths away from the inside of the vessel where your developing roots are. This makes sense why they might root better in these colored glass vessels if blue and green light hinder root development.
You’re partially right.
Wavelengths of light are absorbed or reflected/transmitted. The light you see is what’s left after other wavelengths have been absorbed. If you have an opaque container (i.e., no light passes through the glass), then inside the container it’s black. If you have a translucent or transparent container, then the inside of the container is the same color as it appears on the outside. So a blue glass container is blue on the outside and the inside.