Water or not before hard frost?

steve22802(7a VA)February 27, 2009

Regarding watering in cold weather, what's the best approach to take this time of year when you have seedlings coming up or that have at least germinated or overwintered crops (spinach and bunching onions) when a hard frost (20 degree overnight low) is predicated. Is it better to let the soil dry out a bit for a couple days ahead or is it better to water everything thoroughly just before the night of the low temps?

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Well, you'll probably get several different opinions, some supporting each possibility. Personally, I've always felt and heard, at least anecdotally, that plants are better able to withstand cold if they are somewhat on the dry side. The theory, I guess, is that a somewhat waterstressed plant has less total moisture in its tissues, and that the moisture that is there is dispersed more in the inter-cellular spaces, where freezing does less damage.

This makes sense to me because cold damages cells by freezing the moisture in them, and having the ice crystals rupture the cell (think of a can of pop freezing, bulging, and rupturing in a cold car or in your freezer).

Another thing which may help is to fertilize a day or two ahead with fertilizer that is high in a certain type of urea (although I can't remember what it is, low _______ urea) and high in potassium. This makes sense to me too, because the higher the concentration of salts in a solution, the lower the freezing point.

However, while these theories are all good and fine, I don't think there is all that much practical effect to them. After all, when massive freezes threaten Florida citrus or similar crops, you don't hear of the growers going out there and fertilizing or drying the plants out. If these approaches had a major practical effect, I would think someone would be doing it on a big scale.

The good news is that 20 degrees shouldn't hurt either spinach seedlings or onions significantly. My onion plants came last year from Texas in very early April, were planted, and then went through a night in the low teens and a day of snow with no ill effects at all. And, spinach can be successfully overwintered here in the north with moderate mulching from a fall sowing, to grow again in the spring, and believe me, it gets MUCH colder than 20 degrees here in Michigan. I have baby romaine lettuce plants out in my garden right now that have been in the cold all winter, and they look just fine.

    Bookmark   February 27, 2009 at 6:43PM
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dave_f1 SC, USDA Zone 8a(7b)

Steve, it's definitely a better idea to keep the plants well-hydrated in anticipation of very cold weather. Moist soil will act as an insulator, absorbing heat slowly and releasing it slowly. Watering during a sunny day will be most helpful. By the way, plants that freeze slowly and thaw slowly will be damaged the least. Anyway, wetting the soil will help, but covering tender stuff is best. Spinach and onions don;t fall in the category...they're very tough. I subject my spinach and onions to temps in the teens all winter.

    Bookmark   February 27, 2009 at 7:51PM
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steve22802(7a VA)

I'm not worried about the spinach and bunching onions surviving since I planted them last fall and they have survived all winter already. It's more a matter of trying to understand how to keep them in best condition and help them get growing again as the weather fluctuates wildly here in the spring. It was around 60 degrees both yesterday and today but the upcoming lows for Sunday, Monday, and Tuesday nights are all around 20.

I have new crops of turnips, beets, peas, carrots and more bunching onions and spinach that have all germinated but haven't done much growing yet and they are the ones I'm really trying to help along thru the cold. I actually presprouted most of these seeds to try and get them going a little sooner but now I'm worried that they will have a hard time during the cold snap. I was hoping to have finished building some mini tunnels by now to give a bit of protection but I haven't gotten that project finished yet. Maybe tomorrow I can give it a push...

    Bookmark   February 27, 2009 at 8:24PM
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Water before a long-lasting freeze (days).

When the soil is frozen the plants can't take up water easily...when it's that cold the plant's metabolism also slows down so it's a double-whammy as far the plant putting water back into itself.

If it's a short freeze, don't worry about it unless they need watering anyway right before the freeze.

    Bookmark   February 27, 2009 at 9:01PM
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A moist soil will retain more heat than a dry one as water is a tremendous heat sink. However, if your soil is 60 deg the evening before the frost and you put alot of 45 deg water on it you are sort of defeating yourself. The warm soil works best if the wind is very calm. Capturing some of that soil radiated heat around the plants with sheets, plastic, row cover or something else helps.

    Bookmark   February 27, 2009 at 10:30PM
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I agree with watering before a big freeze or frost if the soil is on the dry side, but I would not fertilize as it may stimulate new growth during a warm spell and new growth is tender and tender growth is easy to damage

    Bookmark   February 28, 2009 at 12:30AM
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Yet another vote for "water before a freeze". I work at a nursery and we always make sure the plants are watered and covered before a freeze, even the hardies. It is true that moist soil holds more heat than dry soil, and moist soil also helps keep the roots from freezing. As you probably know, a plant with a frozen top can rebound a lot better than one with a frozen top AND damaged roots.

    Bookmark   February 28, 2009 at 9:10AM
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I'm with knittlin - the reason for watering before a freeze is the heat-energy that's in the water. This applies to mild zones where plants are active and the freeze will be light and the groundwater is relatively warm which well-protects the roots.

Not 7a, IOW. I'm amazed that you have seedlings coming up outside. I'm 7a and we are six weeks from that point at least. The ground is cold and wet, watering would be as pointless as the ole coals to newcastle...

    Bookmark   February 28, 2009 at 9:43AM
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dave_f1 SC, USDA Zone 8a(7b)

Steve, again I wouldn't worry much about those seedlings. Even new seedlings of cold-hardy crops like carrots, spinach, turnips, peas are very tough. Low 20's aren;t a problem. I would just cover them with something light, if anything. I have carrot, pea, and spinach seedlings growing in Jan here, and it gets into the teens and low 20's quite a bit.

    Bookmark   February 28, 2009 at 10:45AM
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steve22802(7a VA)

Pnbrown, the reason I have seedlings up so early is that I presprouted the seeds indoors first and then planted them out. This is the first time I've tried doing this to get an extra early start so I'm not sure how effective it will be and of course now I'm worrying about whether or not they will survive this coming cold snap.

Dave_f1, thanks for the input, that's exactly what I was hoping to hear! ;) I'm now going to stop babying these seedlings so much.

    Bookmark   February 28, 2009 at 2:28PM
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Dave is encouraging, but don't forget he's speaking from a much milder area. I'm in florida at the moment working in my southerly gardens - it's ever so easy to forget how it is in different climes.

If your 7a va is anything like my 7a maritime mass (and based on what I see driving through this time of year it's very like it)then unfortunately your transplants won't make it. Maybe if you are south of richmond or right in the tidewater. It's kinda fascinating how the war was fought between richmond and washington, and every year driving back that's where the weather changes from blissful southern spring to dreary wet cold late winter. Usually I have to stop along the maryland turnpike and change into warm clothes...

But I applaud your hopeful enthusiasm! How about a quick poly cloche over those puppies?

    Bookmark   February 28, 2009 at 2:42PM
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dave_f1 SC, USDA Zone 8a(7b)

Well no, my area isn;t much milder at all. Very NW corner of SC in the foothills. Anyway, I did mention that I had these seedlings in the ground in January. It's almost March now. We have teens and low 20's frequently in Jan and Feb, when I sow the seed for these cold-tolerant veggies. Steve, it may be a good idea to invest in some sort of lightweight rowcover if you'll be planting early in the season. And have a bunch to cover. They last at least 2 years for me. Some of my carrots and spinach seedlings are covered, some aren;t, and they all look ok. Covering them up for the really cold nights will give you some comfort. There are no guarantees. I do keep the rowcover on my lettuces all the time so I can keep them actively growing all winter. Uncovered, they survive but don;t grow very fast. Dave

    Bookmark   February 28, 2009 at 3:01PM
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