Propagating Trees and Shrubs in the Winter Post-Webinar Q&A

In a recent webinar from UNH Extension and UMaine Cooperative Extension, Dr. Bryan Peterson and Kate Garland answered questions about about propagating trees and shrubs in the winter months.

We didn’t get to all of the questions viewers had for us, so we are sharing relevant questions and answers here as a written Q&A. Enjoy!

What about cuttings of broadleaf evergreens (ceanothus, manzanita, laurels, for instance)?

Recommendations from Hartmann and Kester's book Plant Propagation (7th edition) are as follows: Ceanothus softwood tip cuttings or semi-hardwood cuttings work well when dipped in 1,000ppm IBA; Manzanita (aka Arctostaphylos) dormant cuttings should be taken in March from short side branches from the middle of the plant rather than long end runners and dipped in 8,000ppm IBA; Kalmia appears to be much easier to propagate from seed than cuttings (cold stratification for 8 weeks is recommended).

Is it true that willow cuttings in the water will help other cuttings root?

The literature on willow extracts is murky, but willow extracts (or sticking willow branches in the rooting water) definitely promotes rooting of other cuttings in the water. The exact reason for this is unclear, but it may relate to the presence of salicylic acid, which can improve the quality of roots on cuttings. Some studies show salicylic acid promotes rooting, while others show it doesn't--it probably depends on what plants are being tested, since different plants respond differently.  Overall, the research is pretty limited on exactly what compounds in willow encourage rooting; it might be as simple as a high rooting hormone content in willow, or some other compounds that aren't well identified. The biology of root formation from cuttings is extremely complex, and not that well understood on a chemical level. To my knowledge, nobody has ever exhaustively characterized the chemistry of willow extracts and their effects on rooting.  

I used a rooting gel this year, and it worked well. When do I move plants out of gel and into soil?

I have never used rooting gel, but I've seen it for sale as an alternative to talc formulations of IBA rooting hormone. If this is the correct product, cuttings are just dipped into the gel before sticking into a rooting medium. Generally, we like to move cuttings out of the propagation environment into a "normal" growing medium or soil as soon as they have enough roots to survive the transplanting.  For most cuttings, it is when roots are well formed and at least a half inch long or so. I can better answer this question with more detail; feel free to email me at bryan.j.peterson@maine.edu. 

How can you tell when the cutting rooted successfully?

Top growth can be a sign of some root development, but isn't always a reliable indicator. Some propagators gently tug on cuttings to see if there seems to be some establishment, but it's best to simply be patient and wait several weeks before giving that a shot. If you've been waiting a long while (say 10-12 weeks) and there's no sign of growth, gently tip the pot and take a look at the base of the cutting. Sometimes I'm confident that cuttings are rooted based on some resistance when I tug on them, but then I lift them out of the rooting medium (perlite, peat, etc.) and discover no roots at all! If that happens, just put the cutting back into the medium and give it more time.  Sometimes, cuttings will root really robustly, and sometimes only a few wimpy roots. If your cuttings produce only wimpy roots no matter how long you give them, then transplant what you have to a good growing medium and start fertilizing in hopes of promoting more root growth. But some cuttings are just kind of duds and start to decline and die before they get enough roots to thrive!

Isn't Pussy willow one species that has a male and female shrub?

Yes, it's a dioeceous plant meaning a plant will only have either male or female flowers. Both male and female plants will produce the fuzzy buds we all enjoy in the spring, but the buds on males tend to be a bit more showy.

Any special treatment for Aronias (arbutifolia and melanocarpa)?

Softwood cuttings in late June dipped in 4,000ppm IBA. Keep misted, consider cutting top leaves in half to reduce surface area and use bag to maintain humidity.  I've rooted softwood cuttings of Aronia very nicely using 3,000 ppm IBA, although I suspect you can probably get away with the standard 1,000-ppm powder (0.1% IBA) that you can get at garden centers and big box stores. I always collect these cuttings before they stop their growth for the season; I'm not sure if they would root as well if you wait any longer.

I’m trying to propagate climbing hydrangea, cut recently, very budded cuttings, any help?

I don't have firsthand experience propagating climbing hydrangea from dormant cuttings, but I do know it roots readily from softwood cuttings taken in June. I expect it would also root quite easily from dormant cuttings, so I'd suggest being patient and welcome you to let us know how it works out for you.

How can stakes in late winter (cut now) be stored until planting in spring (for live stakes)- directly from cutting to planting the stakes? Do they dry out? Or better to wait longer before cutting to be as close to planting as possible, like late April?

It's best to stick your live stake material when you take the cuttings, but they can be stored in a bucket of water or wrapped in wet burlap to keep moist until planting. With that in mind, it's best to aim to take your cuttings when the ground has begun to thaw, but before bud break. You could collect now as long as you store them cold and moist; any warming temps would cause bud break. By far the best choice is to collect them and stick them into the ground on the same day.  Once buds break in the spring, I think your chance of success is reduced quite a bit (but maybe still worth a try!) 

I have read somewhere you can use cinnamon as a rooting hormone. Does it work too?

I've also come across this claim. The evidence on cinnamon is mixed, and pretty weak overall. My impression is that it doesn't make a notable difference for most plants, but some studies have suggested an effect on rooting for some plants. Some people have speculated that antimicrobial properties of the cinnamon improve success by preventing cuttings from decaying, but I don't think I've found any studies that confirmed this. It may just be the case that plants that root well with cinnamon treatment would have rooted just as well without any dipping treatment at all. The same is true for honey, which I also see often as an alternative to rooting hormone. In any case, results will be more reliable and consistent with a commercially available rooting powder, since it is known to have the most reliably beneficial active ingredient - the rooting hormone IBA.

How should I take cuttings for winterberry?

Recommendations in the literature suggest using cuttings with 4-5 nodes taken in early July dipped in 5,000ppm IBA. Keep misted and out of direct sunlight until it begins to push new top growth. Wounding the base of the cutting by simply stripping off the lower leaves and scratching with your fingernail before dipping in hormone can be helpful.  I've found this to be a good approach for our native Ilex mucronata (mountain holly or catberry), and I think it would be very reliable for the other native winterberries.  One caveat - you want to find nice vigorous shoots; in years where I tried using short shoots, rooting was very poor.  In years where I was able to find some plants with great new growth (6"+), rooting was much better!  I want to stress that rooting hormone concentrations (1,000 or 5,000 or 8,000) are just recommendations based on what others have tried and found to work.  It's often OK if you use a hormone concentration that is lower or higher than the recommended amount.

Is there no need to mist or bag deciduous cuttings that are not leafed out?

There's no need to mist or bag stuck cuttings that are not leafed out yet.

Any tips for propagating boxwood?

Softwood cuttings in June dipped in 2,500-5,000ppm IBA is recommended.

What about propagating Speckled Alder?

I think softwood cuttings should work well as long as they are not allowed to dry out. I'm not sure about hardwood cuttings! Simple layering could work pretty well too, I think. To do this, bend a branch of the shrub down to the ground or into a pot and bury a few inches of it in the soil, leaving the last 4-5" of the stem exposed. Maybe set a rock or brick over the buried portion to stop if from springing back up. Check for roots in the buried part of the stem every few weeks. Once the stem is rooted, you can cut it off the parent plant, dig it up, and transplant it. I suspect mound layering would be another option, but it is a much more involved technique! North Carolina State Extension has a nice introduction to layering if you're interested, keeping in mind that the two approaches that I think are most likely to succeed are simple layering and mound layering.  https://content.ces.ncsu.edu/plant-propagation-by-layering-instructions…

I tried to root some bearberry and read somewhere that it should be done with a heel cutting to the bottom. Is there any general rule about the type of bottom cuts to cuttings?

Heel cuttings, where you include a small section of older wood attached to the new growth you're using for the cutting, can be beneficial in some species. However, straight cuttings work well in most situations. For bearberry, a dormant straight cutting (taken right now) will work very well. I've never propagated bearberry, so I have no recommendations from personal experience with the plant. I wouldn't be surprised if a straight cutting or a heel cutting worked about equally well, or if a heel cutting worked better as it does for some evergreens. However, heel cuttings may be worth a try if you don't mind the extra bit of damage it causes on the stems of the parent plant. Heel cuttings are easy to collect: simply identify the branch you want to use as a cutting, grab it firmly at the base, and jerk the stem downward so that it breaks off of the parent plant; some of the wood of the older stem that your cutting was attached to comes along for the ride (that's the heel). I think a higher hormone concentration might be the most important factor for success with bearberry, and heel vs. straight cuttings maybe not as crucial.  

Thank you, everyone, for the great questions!