"In trying to decide whether now is a good time to pull covers, or not, I always begin by thinking about the "crop stage" and how hardy the plants may be in mid-December.
Unlike last year, we have not had as much good hardening weather, and so I believe the idea of delaying the cover application has been justified. Right now, I think uncovered Chandler plants could handle cold temperatures in the upper teens, but mid-teens and lower would make me nervous.
In looking ahead into next week, I see that areas of the Piedmont like Gibsonville, NC (near Burlington) could experience a minimum of 18 F on Wed morning (12/23), whereas at Clayton (southeast of Raleigh) we will reach a minimum of 20 F. Leading up to Wednesday we will have a series of what I consider ideal hardening off temperatures (40's in day, 20's in night). It is best to allow the plants to be fully exposed to these kinds of outdoor air temperatures before pulling the winter blanket. In the milder Clayton area, we will not pull covers at all this winter unless there is a severe freeze threat, and a forecast in the low teens or single digits will tell us to go ahead and cover. We will then leave the covers in place through January, provided that we do not encounter a week of abnormally mild temperatures. The row covers can really elevate temperatures during the day, and if the forecast is for a week with daytime highs in the 60s we will pull the covers to the side.
In colder areas, like Gibsonville, NC and through the Piedmont, foothills and mountains, it is typical for strawberry plasticulture growers to pull covers in mid-to-late December. Pulling this late will help harden plants off for winter - all those days in late Nov and early Dec with day temperatures in the 40s, and nights in 20s are perfect for hardening off.
Other important factors in deciding "when" to actually pull the cover (if at all), depends on the variety and how far along the crop is right now. Chandler is a very cold hardy variety by comparison to Camarosa, and it it can grow "too well" under row covers if you are not careful. If you have large Chandler plugs that are nearly touching in the row the last thing you want to do is stimulate more plant development with row covers. Row covers work against you they cause the plant to produce too many crowns and ultimately flowers. As a rule, if I see a plant with 5-6 inch diameter on the plastic at Thanksgiving, I am quite satisfied -- this is what I refer to as a nicely balanced plant. If I see plants that are 8-9 inches in diameter, then I know to hit the brakes! Row covers will force even more plant development and I have seen situations where row covers can cause so much branch crown development that the flower numbers per plant are truly excessive. When bloom counts on Chandler get above 80 in the spring, you're in trouble. Ideally we would control plant development well enough so that you would not get in excess of 60 berries per plant and many growers would much prefer a plant with around 45-50 fruits (better berry size). At Thanksgiving time it has been my experience that the 6 inch diameter plant on the plastic is just about right.
So, to summarize this discussion on when to cover up:
1. Look at plant development in late Nov (Thanksgiving) and determine if your plant size is small (3-4 inch dia), medium (5-7 inch), or larger (8 inches and higher)
2. If the plant size is small, then you would want to apply rows covers earlier than later. If the plants are in the right range of around 6 inches, then the main idea is to simply offer "cold protection" as needed. If the plants are relatively hardy in mid- December, then you are will want to protect the crop from temperatures in the mid-teens and lower as they may not be hardy below this level. But, if the temperatures are not threatening, then there is no hurry to cover. In this advisory I have indicated that with temperatures in Gibsonville heading into upper teens in the middle of the week next week, I would probably then plan to cover by next Tuesday (12/22). At Clayton, we are not going to worry about low 20s. When covers are applied in late December in Piedmont, Foothills and Mountains, it is customary to leave the covers on for about the next 5-7 weeks (unless we get a Jan. thaw of some kind). I do not like to leave covers on too late into the winter. Ideally, you would get them off again in late winter to keep the plants in more of dormant condition (usually 2nd week in Feb in Raleigh area). I generally don't recommend that covers be left on through early March (when you begin to see new blossoms in this area). With larger plants (e.g. 8 inches and higher), you need to be extremely careful with Chandler - it can become a "beast" by next spring and your pickers are not going to like picking all that small fruit. Thus, with larger plants at this time of year you need to use the covers strictly for protection against extremes in temperatures and then try not to leave the covers on for any extended period.
3. Row cover weight - we still stick to 1.5 oz covers for the Mountains and 1 to 1.2 oz for the piedmont and sandhills. In eastern NC we do not use row covers for extended periods - just for protection against serious freezes. By the way we have seen some excellent yield benefits associated with pulling covers in mid-December at the Upper Mountain Research Station in Laurel Springs. We seem to get the best overall yields and fruit size with 1.5 oz covers pulled in mid-Dec vs. mid-Nov, or mid-Jan., in this colder location (3,000 ft elevation). It is interesting that last year we did a Camarosa trial in Salisbury, NC with 1 oz covers pulled in mid-Nov, mid-Dec and mid-Jan, and there was no difference! We are repeating that trial again at Salisbury in 2009-2010.
4. Chandler can grow too aggressively in winter under covers, and so if you already have large plants now, watch out! With the more cold sensitive Camarosa, the practice by growers in the Piedmont has been to pull covers even in early November. We have a lot more to learn about with managing Camarosa with row covers in colder regions, and I look forward to the data set that will be coming out of the Salisbury (Piedmont) research station next spring to give us some better guidance on row cover timing and duration with this variety (we also have different weight covers in this study).
I'll be back with some actual photos from Clayton in an advisory this Sunday. Please email your questions. For the next few days I will be traveling (again) and the cell number is best.
Have a good weekend!