Strawberry Plasticulture - Freeze Advisory (2/25/02)
Vol. 3 No. 11 - Barclay Poling, Editor & Ext. Small Fruit Specialist, NC State

In this issue:
-1. Severe freeze expected on Thursday and into Friday (NC)
-2. Temperature minimums have been lowered; ENC needs to pay attention
-3. Across the Southeast - Big Worries
-4. Further comments on our Clayton strategy

-1. Severe freeze expected on Thursday and will continue into Friday
We will have the two coldest mornings of the year this Thursday and Friday (2/28-2/29) across North Carolina (Figs. 1 and 2).

-2. Temperature minimums are lower!
In checking some minimums for across the state, I was surprised to see how the Monday forecast has dropped temperatures by at least 3 F from what was reported here last Friday (Feb 22 - black; Feb 25 - blue):

North Carolina Minimums on 2/28 (Thu):
Western NC, Foothills, Western Piedmont:
Asheville - 17 (12)
Hendersonville - 17 (13)
Lincolnton - 22 (17)
Salisbury - 21 (18)
Statesville - 17 (14)
Waynesville - 15 (11)
Winston-Salem - 19 (16)
Yadkinville - 17 (14)
Central Piedmont
Burlington - 20 (17)
Henderson - 17 (14)
Louisburg - 15 (12)
Pittsboro - 17 (14)

Low temperatures are now a problem for Eastern NC as well on 2/28
Burgaw - 22
Elizabeth City - 25
Jacksonville - 21
Kinston - 21
Rocky Mount - 22
Shallotte - 22
Tarboro - 20
Williamston - 23
Wilmington - 24

As a point of interest, a number of growers have been finding that their temperature minimums are often running several degrees below various forecast products. As a rule, it is probably a good idea to look at the temperature minimums listed here and take another 2-3 degrees off, and then you may be closer to what is actually going to be the case at the ground level on your farm.

Changing forecast for Clayton: It was forecasted last Friday that we would have a minimum of 24 F (for 2/28), and then today it was dropped to 22 F. More surprising was another forecast product that is indicating 15-18 F on both Thursday and Friday mornings! This is a pretty big difference! I will plan on going with the 15-18 F, and hope we only get 22 F!

Implications of mid-teens at Clayton:
Today, we made a decision to apply row covers to our entire research acre at Clayton. We are not planning to do any irrigation on top of our covers at Clayton, as our goal is to mainly protect the flower buds inside the crowns and hopefully a number of the emerged flower buds, but not all. You can see that for Camarosa (Fig. 3), there are several newly emerged flower buds just above my index finger - the smallest of these could survive temperatures down to 18-19 F, but no colder. The larger ones will be killed in the low 20s.

We are actually quite fortunate that Camarosa is not any further along than it is! Even if we lose these emerged buds, there are still lots more to come (especially this year). What we don't want is a temperature below 15 F - then, you start losing flower buds inside the crown. At 13-15 F you start getting injury to the crown as well. At 10 F and lower, you get severe crown injury and very heavy crop losses.

Fig. 3. These flower buds just above my index finger have the potential to make some "decent berries" and should be protected with row covers - if available! All of these should still be alive - even after recent cold nights. These will be killed if temperatures drop into the very low 20s and upper teens. In the mid-teens I am concerned about injury to the flower buds that have not emerged, and in low teens down to 10 F you can get crown injury! Unfortunately, there are areas in TN which are heading for such lows (next figures).

-3. Across the Southeast - Big Worries!

Fig. 4. High pressure system with dangerously low temperatures moving into TN (Wed)

Fig. 4. Minimum temperatures expected Thursday morning (2/28). Note the pink section is indicating temperatures in the single digits! Western NC will see temperatures close to 10F.

Fig. 5. Minimum temperatures expected Friday morning (3/1).

Further comments on our Clayton strategy…
We anticipate having the row covers down by noon (Wed). It will likely be too windy tomorrow (Tue) and we are hopefully going to get a nice lull in winds Wed morning. The other reason that we decided not to apply covers on Tue is because of relatively warm temperatures expected here during the day (76 F). However, we may still have difficult winds to deal with Wed morning, and extra personnel will then be needed to get the covers down. The reasons we are not making plans to do any watering on top of the covers are: 1) early crop stage - we just aren't that far along, and 2) with row covers we should be able to keep temperatures above 22-23 F, even if it gets to 19 F outside (weather shelter was at 4 ft). Without row covers, we could see temperatures at the ground level that are well below the air temp at 4 ft (note the 15.3 F at 7am). At that point in the night (3am), the row cover was providing 7.6 F protection over the non-covered crowns. That makes the investment in row covers for this kind of freeze VERY WORTHWHILE. We could go colder than 19 F on 2/28 and 3/1. But, as long as we keep our temperatures beneath the covers above 20 F, I am not going to worry about it.

Consider this data set collected on Jan 1, 2002 in Chesterfield, VA:
(Jan. 1, 2002)
time temp-shelter (4ft) crown w/cover exposed crown

3am 26 F 25.5 F 23.7 F
4am 24 24.6 22.9
5am 23 23.7 22.0
6am 21 22.9 17.3
7am 19 22.9 15.3
8am 19 24.6 21.0
9am 23 31.4 27.2
10am 27 43.7 32.2

For growers in areas with temperatures going into the low teens and single digits, it is much harder to say how cold it will get under the cover without some additional data. I did manage to get this information together for this advisory that relates to Jan., 2, 2002 at the same location as above:

I have not had any nights colder than 14 F to monitor in 2002 - may get this opportunity on Thursday! However, I will close this advisory by mentioning the fact that on Jan 4th during the "big snow" (it started snowing at 5 pm, Jan 3)), we had an outside temperature of 14 F at 7am, but the temperature of the snow covered crowns stayed in the low 30s the whole night and morning (33 F with row cover; 31.4 F without row cover at 7am). This is very interesting data as it suggests the potential benefits that could result from an ice blanket that acts like a snow cover. We can create an ice blanket with overhead irrigation on top of covers. The problem is that we don't have enough experience with this technology to understand the "downside" - what happens if you lose your irrigation system in the middle of the night, and the winds are very high (as being predicted). How many hours of protection will an ice blanket provide without irrigation on top? Could the ice blanket make matters worse because of EVAPOROTATIVE COOLING EFFECTS - if you go enough hours without irrigation on top? Is there any benefit to running some water during the day? More on the problems and risks associated with irrigation on top of covers in Tuesday's advisory.

Have a good evening!
Barclay Poling

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