Satsuma plum in bloom. Photo: T. Baugher
According to Penn State's Weather World, which goes back to 1897, this has occurred only a handful of times. The most recent year this happened was in 1984. Other years included 1960 and 1932.
Reported Dates of Green Tip for the Cornell MaluSim Carbohydrate Model
Nearly all the orchards with NEWA weather instruments have reported reaching green tip (GT), most within the last week. The lone hold-outs are Lackawanna and up in Erie county. Table 1 shows the dates the growers recorded green tip. I have also listed the green tip date in 2016. A positive number indicates GT this year was later than last year, while a negative number indicates an earlier date than the previous year. We have one orchard in Butler county that reports a significantly earlier GT than last year; however, most show similar dates as last year.
The importance of GT is that it starts the Cornell MaluSim Carbohydrate model. You can also start following the model by going to Cornell's Network for Environment and Weather Applications (NEWA) and choosing a weather station nearest you and entering your green tip date. Once you have entered your GT date the model will utilize that date and keep track of the carbohydrate balance. The next biofix for the model will be full bloom. Note that if the difference between green tip and bloom is less than 21 days the results of the model may not be reliable.
Table 1. 2017 Reported dates of green tip on apples at different locations and comparison to 2016.
|Location||County||Green Tip||2016 GT||2016 FB||+/- days vs 2016 GT*||Reported Cultivar|
|Piney Mtn.||Adams||27-Mar||23-Mar||20-Apr||4||Gala etc.|
Winter Damage to Flower Buds?
Some growers have mentioned that, in looking at their flower buds, some do not seem to appear normal. We did have quite a rollercoaster ride in temperatures from January to the middle of March. Figure 1 is a graph of the daily maximum and minimum temperature from our NEWA weather station at Rock Springs. We have annotated a few specific days when exceptionally high temperatures occurred followed by very low temperatures. The old rule of thumb was if the drop in temperature between a high was 40 degrees or more there is the potential for flower damage to occur. In Table 2 we show some of those instances. In some cases there may have been a long enough cooling down period. Normally we like to see a 7 to 10 day period after an unusually high temperature and a severe drop in temperature. As you can see there were several periods that may have caused some flower damage at Rock Springs.
However, we must also emphasize that we do not need a large percentage of viable flowers to set more than a full crop on all species of tree fruit. The best course of action now is to watch and wait until the flowers develop more fully.
Table 2. Selected maximum and minimum temperatures at Rock Springs in January through March.
|Date||Max. Temp.||Date||Min. Temp.||Days||Change|
Figure 1. Maximum and minimum temperatures at Rock Springs.
Critical Temperatures for Various Tree Fruit Crops
The temperature at which fruit buds are injured depends on their stage of development, and as fruit buds emerge, growers will be concerned about critical temperatures that may kill 10 percent and 90 percent of buds if they are exposed for 30 minutes.As we learned in 2016, the critical temperature tables are guidelines only, and consideration should also be given to weather conditions preceding cold nights.
Assessing Fruit Bud Survival and Crop Potential
Mike Basedow, Edwin Winzeler, and Tara Baugher assessed bud survival on shoots collected from the Penn State Fruit Research and Extension Center and nearby orchards on March 22nd and 23rd to obtain baseline data for the new season. As expected, apricot and plum varieties approaching full bloom had significant damage (Table 3). Cherry bud damage ranged from 2% to 25%, and the number of viable buds per 6 inches of shoot length ranged from 14 to 26. Buckeye Gala at green tip had no bud damage. When assessing crop potential, it is important to focus on bud survival by using a technique adapted from strategies to adjust crop load at thinning time (Schupp, 2016).
Table 3. Fruit bud viability in Biglerville, PA following temperature fluctuations in February and March, 2017.
For stone fruit, shoot length was measured for assessments of number of viable flower buds per 6 inches, and for apples, shoot diameter was measured for assessments of percent of a full crop load (using an "Equilifruit" fruit thinning gauge).
|Cultivar/fruit||Percent alive||Viable buds per 6 inches|
|Santa Rosa plum||16%||1|
|Black Amber plum||0%||0|
|Black Gold cherry||93%||17|
|Cultivar/fruit||Percent alive||% Full crop|
|Buckeye Gala apple||100%||1068|