Great question...Lou, hope you don't mine...but while you're asking, what about watering the inground trees before a freeze(tropical fruit trees). I've heard two different opinions and always wondered which is the "right" way...Thanks!

Trees that are somewhat drought-stressed are more cold tolerant. The reason is that the concentration of solutes in the cells of trees that are not fully turgid is greater than the concentration in cells of turgid trees. Since it's the solutes in bound water (water in cells) that act as nature's anti-freeze for your trees, they are better left unwatered before a freeze. The exception would be if you somehow had knowledge the freeze would be so protracted (measured in weeks or months) that water would be locked in frozen soil for an extended period and unavailable for uptake.

Al

Thanks Al :o)

Thanks Al.

It's supposed to get to 28*F tonight and 25*F tomorrow night and stay at freezing temperature for 2-8 hours. It did rain yesterday along with a bit of snow. People says to water deeply in the ground if it hadn't rained for quite a while but I wasn't sure about above ground. Now that you mentioned about trees being drought-stressed being more cold tolerant. Interesting... I had 14 and 12 inches of rain in September and October. Not so much in November but still very moist in the ground.

I don't let my trees dry out completely during the winter, but I do keep them pretty dry. Of course, my winter is prolly a little different than yours. ;o)

Al

lou_midlothian_tx, concerning inground plants there is a case for watering before a freeze if there is warm sunny weather preceding the event. The reasoning is that water has the highest specific heat which means that it takes a long time to heat up and and a long time to cool down; therefore the soil moisture might retain enough calories at root level to tide the plant over the freeze. It is credible in light of the fact that the difference between life and death for a plant is a critical temperature over a critical period.
The watering must be done long enough in advance to absorb the calories from the sun.

What would we realize if we assume a soil that was saturated in the AM, then assume a day-long sun load, and further assume we are dealing with trees planted in bare soil (if it's not bare soil, the vegetation would shade the soil and a relatively insignificant amount of passive heat would be generated at the soil surface)? That's three assumptions we need to make before we start, so we're starting to look like the model set-ups that "proved" global warming. ;o) We would then also have to assume that there actually is a way to practically apply the idea that there is a critical temperature that kills the plant. I'll explain why that is technically true, put practically speaking - false.

Even if we were dealing with trees in a bare soil, how much water remains in the soil by days end if we water it in the AM so it has a full day to 'absorb' heat? Only a minute amount in micro-pores and a microscopically thin layer on soil particulates - so not much of an addition to the soil as a heat sink. We also need to take into account that we KNOW the cooling effect of evaporation is greater than the sunlight that strikes water's surface. Case: The sun that passes through a water droplet on a leaf is capable of generating only .2 calories per minute, while a drop of water absorbs 3.75 calories as it evaporates; so the net effect of a a sun load on a wet surface is to cool the surface considerably as long as it's wet. When the surface dries and starts to warm, it increases the evaporation rate and heat absorption at increasingly deeper soil levels. Soil is a fairly poor conductor, and becomes an increasingly poor conductor as the volume of air in it increases. So, as what water remains in the soil evaporates, it reduces the effects of passive solar gain by absorbing a good portion of that heat.

As the sun load decreases (dusk) the soil loses heat fairly rapidly, primarily through conduction and radiation, but also aided to some degree by convection. I'm unable to find where watering before a freeze would make a significant difference in soil temperatures over a cold night.

We also need to examine the idea that "... the difference between life and death for a plant is a critical temperature over a critical period."

I'll say briefly that the 'critical period' part is far less important than the 'critical temperature'. When bound (intra-cellular) water freezes, cells die as ice crystals destroy the cells. This usually happens in any plants planted out in the hopes they will survive the areas low temperatures (Lou's) at some temperature several degrees below freezing.

While we can allow there is only 1 degree of dead in any organism, it's not practical to assume that the organism simply collapses at a given temperature. Neither roots nor shoots die in concert, they die incrementally. The softer/newer/more herbaceous tissues are the first to succumb to chill injury, with the injury progressing through more lignified and resistant tissues as the temperature falls. This fact effectively eliminates the thought that we can practically apply the idea that any finite temperature = the end of the organism. The best we can practically claim is that IF watering in-ground trees has any value at all, it would be limited to reducing the extent of chill injury. If we fall back and try to reinforce the idea that death is death, we need to look at the moment immediately before viability ends. At that point, what we are left with is not a tree we are likely to want.

Al

Here is some additional information about watering before a freeze...a Florida freeze that is. We do things different here.

Here is a link that might be useful: According to the University of Florida

Al,

I'm unable to find where watering before a freeze would make a significant difference in soil temperatures over a cold night.

While I cannot argue that watering before a freeze contributes an overall net positive heat differential (or is even "good" for in-ground or container trees), I can attest to what I see and feel when I use the slow-water method under a tree on a night where temps are below 30. The immediate heat is evident to both senses. If I recall correctly, my lay description is because water from the hose is clearly above 32 degrees by some margin, and when it hits colder air/ground the affect of cooling that margin results in heat "loss" into the air above (which then rises to the leaves). I understand the slow-water release method during a freeze night is not the same method as heavy soaking before a freeze. But are these concepts the same, just one is immediate and the other is slower going? (heat must be "released" when cooling a wet warmer ground).

While your explanation of evaporation may be correct (it sounds good to me ;-) I cannot help thinking while reading it "exactly how much water is going to evaporate on a cold humid night?". Whatever the amount, the effects would have to be greater than the heat "released" when cooling.

It seems the more I read about pre-freeze soaking (not in forums, mind you) the less it seems to be favored as a primary protective method -- even when it ignores the other impacts heavy watering has on a tree during winter.

I perused the article Laura linked to. Since I wasn't sure if she supplied it as a counterpoint to what I said, I didn't bother replying. There really isn't anything in it significantly different than what I said further upthread.

This from the UF link: "A well watered soil will absorb more solar radiation than dry soil and will reradiate heat during the night. This practice elevated minimum night temperatures in the canopy of citrus trees by as much as 2F (1°C). However, prolonged saturated soil conditions damage the root systems of most plants."

I would only comment that this will only be effective if the soil is bare and even then only offers a 2* range of protection.

As you read further, you'll find: "Ornamental plants can be protected during a freeze by sprinkling the plants with water."

This means you would need to continually coat the leaves with water during periods of freezing weather, and I don't think this is what the OP was asking about. He simply wanted to know if watering his "... trees in the container ..." would benefit from a good watering before the freeze. The answer is no. They would likely be less resistant to freezing when they are fully turgid.

Let me clarify the fact that when I referred to evaporation, I was replying to Ron and referring to the fact that while under a sun load, the soil (that was holding the water) would also be experiencing a certain degree of evaporation, which would tend to at least partially inhibit a rise in soil temps from passive solar gain. I wasn't referring to any cooling effects of night time evaporation.

Wetting the soil before a freeze can provide up to 2 degrees of protection. How it works: Water contains heat that is released, and a wet soil allows geothermal heat at lower soil depths to be tapped. The water that fills the soil pore spaces helps conduct this subsurface heat upward. California regularly uses a flood or furrow system to condition soils in fruit plantings by wetting them before freezes. However, this requires relatively flat ground and a large volume of water. As noted above, this is only marginally effective over a very narrow critical range of temperatures - only a couple of degrees.

Keep in mind too, that your perception of an increase in temperature after watering the soil may be partially deceiving because of the increase in humidity. Most literature cites the same "up to 2* rise in temps" as the link UF Link.

Al

Al: Never mind... I think I understand where you're coming from now. There is what you are watering (soil vs. foliage) and what you intend to protect (roots vs. limbs/foliage vs. fruit). My climate never experiences "deep" freezes; the most common event is radiant frost and most are concerned about protecting citrus fruit. My thoughts rarely gear toward ground temps but more to ambient air temps (for young trees) and protecting mature tree fruit.

That UFL article linked above barely mentions heavy watering in-ground over extended periods in winter is NOT good. Wouldn't even a handful of "heavy waterings" before freezes for ground trees during winter be considered a relatively extended period, except in the most arid of locations?

I know watering has almost no impact on ambient heat temps unless the area is covered from ground up, and even when covered has much less impact on standard form trees than other lower-to-ground foliage. Makes you really consider whether it's worth deep watering.

Anyway sorry this is off topic since the OP was specifically asking about containers for his specific trees.

Al, I'm in the same zone you're in, so I keep my potted trees, perennials, and shrubs fairly dry over winter, too. Like you, I throw a little snow over the pots when everything gets really dry, and I'm amazed that the containers don't dry out sooner than they do!

As I mentioned in another thread, I group everything in plastic baby pools, and keep them inside the unheated garage all winter.

So far, so good!

lou_midlothian_tx, heat is transferred from a body of more heat to one of less heat. Earth, soil, potting mix etc. will reach a higher temperature than the moisture the pore spaces over the same time on a sunny day. The heat absorbed by the solid fraction is transferred to the moisture fraction by conduction and convection. This will continue until everything reaches the same temperature. As the sun sets, the heat source loses intensity and heat begins to move in the opposite direction; from the moisture in the growing mix to the solid particles to the surrounding air and container if the plant is in one. One can argue the insignificance of this process in the greater scheme of things but it is one of a very few tools to protect plants from "killing" temperatures. It is based upon empirical science but it can be made to appear nebulous when framed by different parameters. "Water boils at 212 degrees F" is accurate only when the atmospheric pressure is 14lb./sq.in.
At the end of the exercise, I trust that you are a bit closer to an answer to the question you posed.

As noted, if you are in that critical range where raising the temperature only a couple of degrees, or keeping the temperature from falling only a couple of degrees is the difference between viable tissue and necrosis, it can be effective. It is generally used as a desperate measure to save an in-ground crop when it's been planted in a zone-pushing location or during freak freezes, and not a very practical tool for the hobby gardener's bag of tricks. My reply was also based on empirical science, and the only reason it was framed as nebulous is because it IS nebulous in all but a set of very specific circumstances and parameters.

As noted, when considered as a possible preventive of cold injury in a container culture scenario, the increased threat of cold injury due to increased turgidity outweighs the extremely minute (immeasurable, by conventional methods) increase in temperature of an extremely small heat sink (soil in the container), which would make the practice counter-productive.

Al

tapla, two things strike me about this exchange.

1. You do not appear to have read my posts as carefully as I have read yours; as indeed I read all posts.
2. You are trying to make a point which eludes me. So I have decided to address this one specifically to you. It is not my customary style and therefore slightly painful.
   Are you saying that you are right? You are!
   Are you saying that I am wrong? I am not!
   Are you saying that my first response to the OP was not applicable or relevant? I do not know! I am beginning to think that it was not. But of this, be sure. Had I known the answer beforehand the post would have been phrased very differently.
   You see, I hesitate to make assertive, let alone dogmatic statements. This lack of confidence is a result of my training and experience. I sometimes sum it up in the statement from an esteemed mentor, "Mother Nature bats last".
   No retort/rebuttal is necessary.