Is there any way to tell if a cutting has put down roots? I've had cuttings with healthy-looking green leaves that just never create roots (despite using root powder). I'd like to discard cuttings that don't root, but I also don't want to break off young roots by tugging on the stems. Is there any trick for this that won't injure the plant?
I assume you're talking about rooting plants in soil.
I'll describe the system I like to use, and tell you how I generally decide when they have roots. I imagine the way to tell may differ slightly depending on your rooting system, which is why I'm explaining mine.
- I pot the cutting in fresh seed-starting mix. I add a granulated 7-7-7 fertilizer and rockdust to the soil before potting, since seed-starting mix has pretty much no nutritional value. Whatever you do, don't add a fertilizer with higher nitrogen than potassium and phosphorus: this will not help the cutting root. I normally use a large cutting (4-16" long) as opposed to a tiny one (about 2"), although this should work for tiny ones, too. I usually put 3-4 inches of stem beneath the soil, but less has worked, too.
- I water with water that has about a teaspoon of potassium sulfate per gallon of water. I actually water liberally, although that might sound counterintuitive for a cutting, and you probably don't need to water it so much. I believe the potassium sulfate makes it safer to use more water.
- Keep the cutting away from bright light for two days.
- Put the cutting in bright light after two days, and keep it there.
- When the plant starts growing very noticeably, it probably has roots. (Cuttings usually won't grow much without roots; correct me if I'm wrong, but that has always seemed to be the case in my experience.) Given the bright light, this should not take a really long time (at least for the kinds of plants I've been propagating: tomatoes, peppers, watermelon, tomatillos, cucumbers).
If you don't use potassium sulfate you can certainly expect the cutting to wilt after two or three days. However, it should perk up again within a few more days, and by then it likely has roots. However, if you expose the cutting to bright light before two days have passed, it will likely wither fast and not recover, or else be stunted for a while.
I like to use cuttings from indoor plants to avoid outdoor pathogens and insects (like aphids).
The main obstacle I've found in rooting cuttings is damping off disease. This is why I use fresh seed-starting mix (to avoid the pathogens). I've heard sprinkling cinnamon on top of the soil helps, as an alternative to seed-starting mix, but I haven't tried this yet. I do know, however, that 2700k CFL bulbs help to ward off damping off, as does increased sunlight. So if you make sure some of the light you use on the third day is 2700k (not all of the light, though), it will probably increase your survival rates.
Removing lots of leaves from your cutting prior to potting is not necessary if you keep it out of bright light for two days. Otherwise, it's pretty much essential, so as to simulate a less bright environment.
So, with my method, you just tell if it has roots by whether it's growing or not. Also, the lack of damping off pathogens will greatly influence the odds of inevitable roots. Using my method above, I have never had damping off with my cuttings, and I have had a 100% success rate, so far, which I understand is remarkable. I don't guarantee you'll have a 100% success rate with this method. Without this method I have had maybe one out of twelve cuttings survive (including cuttings rooted in water, which also succumbed to damping off after potting, notwithstanding their elaborate roots), and then the only ones that survived had bright light on the soil to prevent damping off. Bright light on the soil may actually be all you need, but I would need to experiment with infected soil more to be sure, and it's perhaps a task to get bright light on the soil without getting it on the leaves the first two days. Maybe cinnamon would help there instead.
I would further note that potassium sulfate will likely greatly increase your success rates, and rooting speed, too. I recommend it if you're into cuttings. I imagine it speeds root growth. It certainly makes plants stronger.
You do not need hormone rooting powder with this method (at least for tomatoes, peppers, tomatillos, cucumbers and watermelons). You may or may not need it for other stuff.
As an alternative to this method, if you're taking cuttings of houseplants, like spider plants, golden pothos, aloe vera, ivy, creeping Charlie, wandering Jew, Christmas cactus, Easter cactus, and such, in my experience, rooting them in water and potting directly into potting soil afterward works reasonably well (at least it did when I was a teenager when I tried it last). When rooting in water you can see the roots to know they have roots, of course. Potting soil doesn't work so well with vegetables by comparison, probably due to the higher nitrogen content. I don't know if the potting soil I used as a teenager was pre-fertilized.
It should be noted that seed-starting mix and potting soil are not the same thing, even though they look pretty much the same.
Excellent reply and thank you. All I have to add is that when those babies are coming along happily into another seasonal expected change, such as mid autumn or winter, I let them be in their original container and pot up when the weather is forecast as to be more consistently warm.
I'm in a temperate Australian climate and here, now, we are in a warm autumn and headed for a warm winter. I know that in the next weeks the day time temps will still be hot (29-32 deg. C) but night time will be considerably cooler or cooling. Yes, sure I can put my little ones (a month old) in to the ground but night temps may cause issues as may any unusual here, frost. Watch the long term meteorological forecasts by government agencies, not just the news!
Best way to tell if it has put down roots is to note whether it has died. Sound obvious but sometimes a green cutting can last for a long time without roots, but eventually it will dry out and turn brown. If you see any new buds or growth, that's an indication that something is going on down below. I root in clear plastic so I can see root development. If I see a white root, I know I'm good. Another way to tell is to gently tug at it after a month or so. If it slides right out, you are out of luck. If it resists, you're good.
I usually get roots after six weeks when rooting woody plants like trees and roses, but never 100 percent, so root more than you need.
But to increase your odds - if you're rooting woody plants you don't need any fertilizer or sulfates or anything similar. And you want to remove most of the leaves on the cuttings. And you don't want the cuttings in direct sunlight.
Because in the first place you're trying to get the plant to grow roots. Fertilizer will not help that process. Fertilizer is only useful once you have an established plant with roots and leaves.
In the second place, roots take up moisture and nutrients. Without roots, there is no water uptake. Leaves evaporate water. if you have lots of leaves evaporating water and no incoming water through the roots, your cutting will die.
Finally, direct sun will only help once you have new growth. Prior to that, the leaves you've left will be fine with indirect sunlight.
You will know the plant has rooted if after a month or two it is still alive, the leaves haven't fallen off, and new growth is apparent. After a few weeks, you can gently tug at the cutting and if it sticks, you're OK. But I think it's better to just be patient. Then, when you re-pot, don't plant too deeply in the new pot. Keep the roots near the surface so they don't suffocate.
And there's some research suggesting that putting your cutting into a larger pot helps the roots "sense" how much room they have and you get larger plants by putting the small, newly rooted cuttings into much bigger pots than you were planning to. In other words, a 16 oz pot vs a gallon pot.