I have been staring achingly at the temps this AM which just cant seem to get above 30 F. There doesn't seem to be much frost on the trees though. The dew point is listed at 23 F. Does this mean that frost won't appear until temps approach 23? How is the dew point calculated? Does JM damage occur from the frost or from the plant cells freezing, or both? (I am hoping that someone says that the lack of visible frost is a good sign!) Any input from budding metereologists welcome!
I don't know much about it, but I do know orange growers in Florida will spray their trees with water when a freeze comes. This puts a coating of ice on the plants and keeps frost off. Evidently the plants can take 32 degrees, but frost must do something extra to kill plant tissue. Waiting on the experts....
Definition of DEW POINT, from Wikipedia: The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point. When the dew point temperature falls below freezing it is often called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition. The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates that the dew point is equal to the current temperature (and the air is maximally saturated with water). When the dew point stays constant and temperature increases, relative humidity will decrease. At a given barometric pressure, independent of temperature, the dew point indicates the mole fraction of water vapor in the air, and therefore determines the specific humidity of the air. TIPS on Frost Protection: When the inevitable occurs and a frost is predicted, there are several things which you can do to protect your plants. Water the garden thoroughly before nightfall. The soil will release moisture into the air around your plants during the night, keep the air somewhat warmer. Even a slight breeze will prevent cold air from settling near the ground during the night. You can help keep frost from forming by providing this breeze artificially with an electric fan. Be sure to protect the fan and all electrical connections from water and the elements. Cover up before dusk! By the time it gets dark much of the stored heat in the garden has already been lost. If you have time, build a simple frame around the plant, or row of plants. (Even a single stake can be used in many cases.) Then drape a cover of newspaper, cardboard, plastic tarps, bed sheeting or any other lightweight material over the frame to create a tent. If you don't have time to create a frame, lay the protective cover directly onto the plant. This will help to slow the loss of heat rising from the foliage and the ground. Remove the covers in the morning, once the frost has thawed, to let the light and fresh air back in, and to prevent overheating by the sun. For smaller individual plants you can use glass jars, milk jugs with the bottom removed, paper cups upside down flower pots as heat traps. Don't forget to remove these covers in the morning. You can collect heat during the day by painting plastic milk jugs black and filling them with water. Place them around your plants where they will collect heat during the day. Water loses heat more slowly than either soil or air. This collected heat will radiate out throughout the night. Potted plants are particularly susceptible to frosts because the roots are also unprotected. If you are unable to move your container plants indoors or under cover remember to also wrap the pot in burlap or bubble wrap, or simply bury the pot in soil in addition to protecting the foliage.
I'm no meteorological expert either, but I do know that it is the sharp ice crystals in the frost that cause the damage to the tender shoots, not the temperature itself. (unless it were to fall very low) If there is little or no frost due to the temperature being above the dew point then it sounds like your luck is in. Time will tell. I don't see how watering your garden before a frost would help in this situation. It would just make the air more humid and the formation of frost more likely, because the increased humidity would raise the dew point. Edit: Also early morning sun while the frost is still on the plant tends to make the damage much worse. I have heard this process compared to freeze-drying, not the healthiest process for living tissue. We suffer relatively little frost damage on our Acers, but when we do it is invariably on ones exposed to early morning sun.
I have no leafing out yet, but have been repotting here in Zone 5B. I have not put all of them back in the ground. At what point do I need to worry about root damage in pots? Out of the back of my mind I thought about 25 degrees F. was the worrying point. Mike
Mike, After repotting I think you're safe down to ~<20F because the roots are more insulated and since it's Spring the length of time when the temps dip low are limited. I have potted trees somewhat exposed through the winter down to <5F. Gil
Some friends bought an established Japanese maple in a multiple gallon container on sale from a local outlet that may have left it out in the air through this last somewhat nippy winter. The sales yard is located on a flat site with no air drainage and native stands of paper birch - a cold climate tree not seen everywhere in this region. I took the plastic tub off and found most of the outer roots to have a discolored and inactive appearance, tufts of new white roots visible only at several points making it appear most of the outer roots had frozen and died. The tenderness of the younger, outer roots of a containerized plant make it so that the whole plant should be considered about 20 degrees F less hardy than the same specimen would be in the ground.
Ron based on your friend's tree you're theorizing that we should shift our zones up 2 levels (20 degrees) when considering their hardiness in our area?
Plants in pots are more tender because the roots are not in the ground. The hardiest part of a woody plant is the top. When plants are brought out of the ground the whole plant becomes more vulnerable to cold because the roots are less hardy; if the roots die the whole plant is lost.
The dew point is the temperature at which the air becomes saturated with water, so water condenses out of the air as dew or ice crystals if cooling continues. Frost damage to plants isn't related; the damage is caused by water within the plant tissues freezing, not by condensing water from outside the plant. So if the dew point is e.g. -5C, and the temperature falls to -4, you won't get any frost crystals condensing out of the air, but you are still liable to get freeze damage to any tender new growth on the plants.
This is a subject that comes up year after year in all gardening forums, see for example http://forums.gardenweb.com/forums/load/maple/msg0112460517647.html?27 for a recent discussion. People should use the search function to reach the information. Gomero
Point well taken. But there is also a social element to these forums -- I was pleased, for example, to see the post from Mike, a fellow Mainer.
From Wikipedia: If spring shoots on my maples reached -4C I would much prefer them not to have frost on.
Gomero, No offense, but your making the assumption that I didn't search this forum and others. I have read that specific thread previously, and do not find it to be conclusive. I rather trust this forum and it's members with better experiencial knowledge of the plant. I'd rather anger a few people and be safe with my collection of sticks. I'd also like to know at which point I should worry. I had some dieback this year on plenty of exposed and sheltered plants, so I am being cautious about our New England weather affecting this years potential growth. I beleive last year I had a lot of late secondary growth, didn't harden off enough before it got cold in my area. Mike
Mike, Sorry, I did not mean to offense anybody. You can also read, if you have not done it already, this UBC post where you can retrieve information on the role of Pseudomonas as nucleation agents for ice formation. http://www.botanicalgarden.ubc.ca/forums/showthread.php?t=37156&highlight=gomero Gomero
