Hardening off

[ruskinPalms]

Is hardening off for real? Or is it just a rationalization for vastly decreased winter time growth that occurs in places with prolonged periods of cold? Is it really beneficial for palms and other plants to 'go dormant'? Or is it better for them to genuinely continue to grow through the winter months even though they are likely to see brief true freezing weather? Has there ever been any real research on the subject?

I don't know the answers to my questions.

Somtimes I curse my V. arecina for putting out lush new growth during a warm spell immediately followed by a night of freezing weather in winter. Yet, I praise them for putting out lush new growth immediately following a night of freezing weather in winter during the inevitable ensuing warmup. It is a roller coaster down here...

Any thoughts?

[Turtile]

Hardening off helps palms survive winter cold.  New growth can be damaged quickly by a quick swing in temperature.  A hardened off tree can be up to 10-20F more resistant to cold.

Hardening off resists:

Drought

Winter cold

Winter burn

[PalmGuyWC]

Bill,

I asked the same question recently and don't think I got an answer. In N. Calif. where I live, the temps. drop gradually in the fall, each night getting just a little bit cooler, until finally it's cold and we get our first frost, usually around the middle of Nov. I can almost always expect frequent frosts in the winter, but most of my palms are cold hardy and can take it.

The way it was explained to me is.......as growth slows down due to shorter days and chilly nights, the cells in the growing part of the palms are more compact with thicker cell walls, and presumeably tougher, and more able to withstand colder temps.

Another phenominon I've observed is when temps. fall below freezing and there might be frost on the fronds, dark splotches may show up on the fronds and it looks pretty scary. I have observed this on many different cold  hardy species including Rhapis. The dark splotches are usually angular rather than being round. I was told this was caused by the cells releasing moisture through the cell walls which would concentrate the salts within the cells and lower the freezing temps which would keep the cell walls from ruptureing. The dark spots are causd by moisture that is outside the cell walls and usually runs paralell to the veins in the frond, the reason they have angular straight sides.

Presumeably, with less fluid inside the cells, there would be less pressure inside the cells and rupturing of the cell walls cause by the expansion of freezing.

I think it only stands to reason that rapidly growing cells with thinner walls would be more vulnerable to damage from freezing. I doubt that tropical palms would have this defense mechinism, and maybe that's the reason some palms are cold hardy and others aren't.

I've noticed a few hours after the temps. rise above freezing, the dark splotches go away as the cells reabsorb the moisture they released.

I've never seen a scientific paper on the subject concerning palms, but maybe most palm scientiest don't live where it gets so cold.

There are a lot of palm growers who believe adding a low nitrogen fertilizer about a month to six weeks before the first anticipated cold weather will increase the salts inside the cells and lower their freezing temps. Makes sense to me, but I always forget to do it.

Dick

[ruskinPalms]

Thanks Dick! Your explanation makes a lot of sense. It might be a nice area of research for some of you botany students out there seeking your PhD.

[jonnymac]

I have heard that gradual fall cooling can increase the hardiness of some broadleaf evergreens like citrus and Eucalyptus, but my understanding is that palms don't respond to cold harding like this.  A palm tree's metabolism never really stops, but it can come to a very slow crawl.

Now hardiness can be gained by certain palms that have strategies that help with drought tolerance.  Some palms can respond to drought conditions because of modified stomates, thickened cell wall and thicker wax layering.  All strategies that help the palm reduce transpiration rates, but also with the added bonus of giving increased cold tollerence.  

This is the reason mature P. dactylifera, P. canairensis and W. robusta are growing in Z8 places like El Paso, Las Cruses and St George.

Here is a science paper on the subject.

Link

[2cold]

I live in the North, and "make" the weather for my plants in

my sun room. I notice that the plants are very quick to

"detect" longer days, even when the temps in the sun room

are unchanged, and still cool. The night temp is set for 45f, this time of year, which causes the palms

to go dormant. Most of the plants just seen to slow down and rest. We cut back on the water, and just keep an eye on them.

But right around this time of the year, we see changes, but our temps outside,

(and inside the sun room) are still very chilly. My Washy is starting to push a spear up, very slowly, but there is some

growth. Same with the tachy. The citrus are forming buds, etc.

So the day length is really a key to plant growth. Once the plants "detect" the longer days, spring starts in my little

Northern Jungle. Even though it is still 20f outside, and very cool inside! It takes a bit of will power to hold off increasing the temperature for another month!!

[PalmGuyWC]

Michael,

I had to laugh at the remark Dr. Larry Noblick made at the end of his discussion about cold hardy palms, that some just have the "right stuff." It just shows that some (but few) scientiest have a sense of humor. I've had the pleasure to have known many scientiest in the field of botany, and many of them take themselves to seriously, but I've found, offer them a little good wine (or a martinni) and they loosen up like us ordinary mortals.

It's interesting that Dr. Noblick mentions that drought tollerent palms also seem to be the most cold hardy. I did a quick mental check of my cold hardy palms, and sure enough, most have a thick textured frond with a glossy wax finish to withstand drought or cold. I also noticed that some are glaucious underneeth the fronds, and I wonder if this has anything to do with "cold hardiness?"

One palm I can grow and thrives in my hot/cold climate is Chamaedorea microspadix which is very cold tollerant. It doens't seem to fit the mold as it has a thin textured frond and is not glossy, but has a matt finish. But...even it has a wax on the fronds.  I've found when it's extreamy hot here (over 100F) that the emerging sphers will sometime not want to open as the wax on the fronds tends to melt in extream heat and the blades stick together. Even Rhapis do this sometimes.

I guess it's no accident that palms that will take drought and heat will also take the cold.

Dick

[ruskinPalms]

Another cold hardy palm that does not seem to fit the description to me is A. cunninghamiana. They don't seem to have a very robust leaflet, no wax. Hyophorbe do fit the description and should be tougher but are not. They do have a thick, waxy leaflet. W. birfucata recently performed well during a high 20s freeze. It has a tough, somewhat waxy leaflet. How is D. decipiens?

[PalmGuyWC]

Bill,

I only have two Madagascar palms that might grow in my harsh climate and they are all in pots now. I have two 5 gal Dypsis decipiens. They are extreamly slow but they have continued to push out new spears this very cold winter. I also have two R. xerophilas, another very slow growing palm which was cold tested last winter down to 25F with lathe protection.

Both of these palms have tough fronds and are waxey. R. xerophila fronds have the texture of plastic and are very stiff. I suspect R. xerophila might be the most cold hardy of the Raveneas, but it needs excellent drainage and full sun.

Dick