I've had several questions about success rates, or lack thereof, using this method in hot weather. I'm also finding high heat not only disadvantageous to wrapping itself, but also in succeeding in bringing them from the callus stage to a rooted plant.
I'm doing my best not to use extra electricity this summer running the air conditioning when regulating windows and fans are keeping it livable. The extra 50% increase in power costs to artificially cool the place really makes a difference! The extra power also offends my sustainability senses. Whatever the source of the power being used to create the electricity, I'm not sure I want to use it, saving it for when I can't get by OK without it. So far, so good.
All this is shared to explain this...even the lovely, dark, cool work room isn't able to remain in the temperature range I've found most beneficial for callusing without using the a/c to keep it there. My experiments with refrigerating cuttings haven't been any better than the reports I've received. I'm not sure if the average fridge keeps them too cold for callus to form, but that is seeming the case. I don't have any kind of root cellar nor anywhere to create a temporary one which might provide the appropriate temperatures required, so I'm waiting until cooler weather periods to wrap any more material.
Even if I could successfully callus cuttings now, the heat we're experiencing is not conducive to being able to carry them further. When day temps remained around eighty degrees F or lower, it wasn't an issue. They formed their roots and began growing acceptably. Those patterns are gone for a while. Today, it's supposed to be 104 F here, with up to 107 by the end of the week, and they're threatening to add humidity to the mix, something we traditionally don't have to worry much about.
These hotter temperatures have sucked the life out of many of the cuttings which had successfully callused not that long ago. There are several potential causes I can think of.
It appears more dormant to semi dormant material calluses more successfully than softer, more actively growing ones do. Perhaps it's due to their containing higher levels of nutrients stored in the wood which would otherwise have been used to break dormancy when the weather triggered it? It could be the softer material doesn't have these stored resources and are more susceptible to using up what they do have just to remain alive and aren't able to form the necessary roots before they collapse. Almost like trying to force a bare root into growth too quickly, stimulating it to use up its stored resources to leaf out, form flowers and collapse. I've had many soft wood cuttings during the hotter summer months using other methods leaf out, set blooms and die, so the pattern seems to hold.
Perhaps the softer, more actively growing material has too high levels of auxins and hormones which stimulate the faster growth at the expense of rooting? My experiences using mist propagation showed the actively growing cuttings rooted in as little as seven to ten days from stems which had just dropped their flowers this time of the year. Harder wood cuttings often wouldn't root under mist in weather like this, but would root in winter at much slower rates, sometimes all winter. Those cuttings were under a regular mist of fog which prevented them from drying out and scorching and were in full sun so all green parts of the cutting were providing photosynthesis to maintain it while it formed roots and began supporting itself. Wrapping the way I have been doesn't provide the mist or very high humidity, and I have found planting the cuttings deeply in the cups or pots prevented them from drying out before the roots formed. This would reduce their ability to provide themselves food via photosynthesis while they root.
It could be there are pathogens at work we don't see as much when temperatures are lower. Botrytis and Downey Mildew can both significantly reduce the success rate of propagation no matter what method you use. Some of the results have almost looked like Fire Blight on some of the cuttings. The weather has been suitable for that issue, cool and damp, hotter and damp, repeat. The evergreen Pears are definitely showing those effects in the neighborhood.
I don't spray anything. Not a value judgement if you do as I know there are many environments you can't grow roses in without chemical assistance. Spraying just doesn't fit in with my climate, wind, allergies and sensitivities, nor budget. If it won't grow here without chemical intervention, it isn't going to hang around long. I won't spend the money for them and I'm sure not going to spend the money on doctors visits resulting from the reactions I have to chemicals with increasing regularity.
It might be possible to prevent whatever potential pathogen might be at work on the cuttings by spraying with a fungicide, or not. I won't know as I don't own any fungicides. I can't spray any with these temperatures and wind and I'm not willing to experience any potential effects I might (probably will) have from exposure to them. If you are exploring this method in a hotter area and having the same disappointing low to zero success rate and you spray, please consider including the cuttings in your regimen and report back what the results were. It might make important contributions to the knowledge base about it.
I think I'm also finding specific varietal differences to wrapping in hot weather as I know to be the case in cooler weather. The last batch of unnamed China cuttings I callused and planted have had significantly different results from each other. One rooted and began developing into viable plants, at a much lower percentage of success than earlier in cooler, damper weather, but they have seemed willing to perform. The other, handled and treated exactly the same as the first and done at the same time, limped along, complaining the entire time and all but one cutting has turned black and failed. The jury is still out on the lone survivor. Perhaps the second variety just isn't as successful at creating its own roots and possibly would be better budded than own root? Perhaps it isn't as forgiving about being pushed to root in hot weather as the first? Maybe it had a higher level of pathogen exposure then the other? I honestly don't know. This is definitely going to take a lot more exploration, experimentation and observation.
I will be very interested hearing others' experiences from other climates and from those who spray and will include the cuttings in their spray program. None of us can provide all the potential variations in climate, culture or potential fungi and bacteria. Pooling the results and observations should help to lead to the discoveries of what may work in these hotter times. At least, I hope so and I'll definitely enjoy hearing others' experiences, theories and results. Stay tuned!