Category Archives: Flower buds

Freeze Injury Updates (Winchester-March 29): Sweet cherry blooms lost to freezing temperatures; apples and peaches made it through.

Temperature profile of orchard blocks in the AHS Jr. AREC’s research farm (Winchester, VA) on March 28-29, 2022.

Our digital temperature data loggers recorded temperatures as low as 17 oF that continued for > 2h this morning (Tuesday, March 29). Some of our apple cultivars, including Gala and Pink Lady, were at the tight cluster and first pink; whereas, others like Fuji and Honeycrisp were between half-inch green and tight cluster. Temperatures below 21F were enough to wipe out the flowers of these cultivars completely, but we were pleasantly surprised to find out that all apple cultivars we assessed this afternoon survived this frost event with minimal damage. We recorded between 15 – 35% bud mortality in the king blooms and less than 15% in the side-blooms of Honeycrisp, Gala, and Pink Lady, with the latest showing the highest percentage of damage. Similarly, the four peach cultivars (Rich May, Sunhigh, Red Haven, and John Boy) we evaluated showed less than 15% flower mortality.

I generally believe that 10-30% flower mortality after frost is not a bad thing and could even be considered desirable as a natural thinning process and to ensure a good return bloom next year. It is generally evident that thinning during bloom has a very positive impact on return bloom; and whether this thinning is done chemically (e.g. liquid lime sulfur), mechanically (e.g. hand thinning), or by mother nature, the impact on return bloom will probably be the same.

On the other hand, our sweet cherry cultivars failed to sustain these low temperatures, and despite being still in the tight-cluster stage, they were severely damaged, with an average bud mortality rate of around 80% in Ebony Pearl and Selah. Regina, a relatively late-blooming cultivar, was still at the swollen bud stage, showing 0% damage.

It is also worth noting that flower development can vary among cultivars, species, and locations in the state. Also, temperatures recorded this morning in Winchester were seriously low, but they might not be the lowest in other areas. So, you can depend on this report only if your trees are in the same developmental stage and your temperature didn’t go below 17 oF this morning. If otherwise, I suggest that you evaluate the damage in your block as per the method and images I shared in previous posts.

Flower buds were collected from three apple cultivars (Pink Lady, Gala and Honeycrisp) and assessed for damage after the frost event that occurred on the morning of March 29, 2022 in Winchester/Frederick county area.
Flowers were collected from four peach cultivars (Sun High, Rich May, John Boy and Red Haven) and assessed for damage after the frost event that occurred on the morning of March 29, 2022 in Winchester/Frederick county area.
Flowers were collected from three sweet cherry cultivars (Ebony Pearl, Regina and Selah) and assessed for damage after the frost event that occurred on the morning of March 29, 2022 in Winchester/Frederick county area.

MaluSim Model for Central Virginia, 27 April 2015

Today, I ran the first Central Virginia MaluSim model for 2015. I will start the Winchester models later this week.

For Central Virginia, I use data from a weather station set up at Silver Creek Orchards and managed by my colleague, Dr. Mizuho Nita. A big thank you to Dr. Nita for allowing me to install the necessary instrumentation and for maintaining the station and associated software.

This year, I will be using forecast data from the National Weather Service. In past years, I used data because they provided 10-day forecasts with cloud cover predictions. Intellicast no longer provides the cloud cover data in an easy to use format, so I am switching to the National Weather Service’s forecasts. Note that these forecasts only project five days into the future.

We will discuss the details of the model at Tuesday’s meeting at Saunders Brother. Peck Central VA MaluSim 4_27_15

However, this first model run suggests that the relatively cool weather, and mostly sunny weather that has occurred since bud break has resulted in a moderate carbohydrate deficit in apple trees. Growers can expect average results from chemical thinners applied the last couple-few days through to the weekend. In blocks where significant thinning is needed, growers should apply carbaryl at petal fall and then look to the warmer weather that is predicted for the weekend and early next week for their 10 mm application.

Central Virginia weather data, MaluSim data, and an interpretation chart are in the pdf linked to below:

Peck Central VA MaluSim 4_27_15

MaluSim Carbohydrate Model for April 30, 2014

Growers in the Central Virginia growing region are reporting that their bloom is stretched out quite a bit with some side bloom still in flower and some king bloom fruit approaching 5 to 6 mm in fruitlet size. In general, most chemical thinners are more active on the smaller, slower growing fruit, so this spread may prove to be advantageous.

Peck Central VA MaluSim 4_30_14

For those Central Virginia growers who are starting to map out their thinning schedule, I’ve started running the MaluSim carbohydrate model. I will discuss the details of how the model works, as well as my research results on validating the model at the meeting on Thursday, May 1.

The MaluSim model is most effective at predicting thinning efficacy at the 10 mm timing, and has a much lower predictive value for flower, petal fall, or 20-25 mm thinning applications.

As of today, the long-term forecast shows a long string of days with mild temperatures and lots of sun. From these forecasted data, the MaluSim model <<pdf>> is showing no extremes in carbohydrate supply, meaning that chemical thinners will have a “average” response. For a detailed look at the model, as well as interpretation table, download the above linked pdf.

For the Winchester area, I will assess fruit development on Friday to decide when the first model run is warranted.

A big thank you to John Saunders for helping with the MaluSim modeling work this year. Last year, Dr. Mizuho Nita (grape plant pathologist at our AREC) installed weather stations at Silver Creek and set up a network relay system that allows me to download meteorological data from Silver Creek right to my computer. If you work with Mizuho on grape disease related issues, please thank him as well!

Risk of Frosts this Week

The weather forecasts for the coming week show three days with potential frosts. For Wednesday morning, the National Weather Service is forecasting a low of 29F in Winchester and a low of 30F in Tyro. I’m sure other locations in VA are also at risk of frost.

In the Winchester area, apples are 1/4″ green tip with some cultivars showing pink. Peaches are at first pick to some early blooming cultivars having a few open flowers. The phenology in Central VA is a bit more advanced, with peaches having been in bloom for nearly two weeks. According to the critical temperature charts, we can expect a small amount of damage from these temperatures but not a complete crop failure.  So, fingers crossed that it doesn’t get any colder than predicted. However, even if flowers are not killed by the frosts, there is also the risk of russets and frost rings. Temperatures are forecasted to warm up towards the end of the week.

Below are  links for the Washington State University fact sheets (as pdfs) on the critical temperatures for flower buds:

Michigan State University has adapted the WSU critical temperature fact sheets into this easy-to-read fact sheet.

MSU has also put together an excellent set of resources about freezes and frosts in fruit crops.

Additionally, work done by Dr. Steve McArtney (NCSU) and others has shown that applications of Promalin (gibberellin 4+7 plus the cytokinin 6-BA) at 25 ppm immediately following a frost event may help increase fruit set and cropping. In essence, the Promalin sends signals to the developing fruit that the seeds are still intact and growing, even though the frost has killed the embryo. The result is a crop of parthenocarpic (seedless) fruit. This “rescue treatment” is not a full proof way of setting a crop, and it is still unclear under what environmental conditions and for which cultivars it works best. If the temperatures drop much below 28F, each grower will have to make a decision as to whether or not it is worth trying to rescue the crop with Promalin.

You can read more about the “rescue treatment” with Promalin in this Good Fruit Grower article.

Fruit Bud Damage in the Winchester Area

The winter of 2013-14 will surely go down as one of the colder, snowier winters in recent years. Many growers have been asking about potential flower bud damage from the cold temperatures that we’ve had. As a general rule-of-thumb, peach and sweet cherry buds are hardy to about -10F, and near complete bud loss and perhaps some branch damage will happen when temperatures are below -20F. As I mentioned in a previous post, plants are not affected by wind chills, so we are talking about absolute cold temperatures, which will be warmer than the wind chill temperatures that are reported in weather reports. Apples and pears are generally hardy down to -25F. However, hardiness is much less when: there is a warm spell preceding the cold temperatures, when trees have already been pruned, when the cold temperatures persist for an extended period of time, or when trees were under water or nutrient stress in the previous season. Under these circumstances, peach and sweet cherry buds might be damaged at 0F or even warmer. During this past winter, peach buds may have been killed during multiple events.

Looking at the absolute cold temperatures from August 1 to present, we see that many of the fruit growing regions in the mid-west and Northeast likely have moderate to significant damage to their peach and sweet cherry crops.


Zooming into the Mid-Atlantic, we see that there were some areas along the western edge of Virginia that sustained temperatures below -10F this winter. Many other areas of Virginia have had temperatures in the minus single digits.


To get an idea of how much peach bud damage is in the Winchester area, last week my lab  collected 10 peach branches from each of seventeen different blocks of peaches. Many of the blocks were located at the AREC, but we also collected branches from some nearby farms. The branches were kept in a vase at room temperature for 24 hours, and then every bud on every branch was dissected from the tip to the base to look for damage. Healthy buds are green and look moist, while damaged bud are brown and translucent.

Table 1. Peach bud damage from the winter of 2013-14. Branches were collected on March 4 and analyzed on March 5.


Percent of Flower Buds Alive on Shoot

Total Number of Buds Dissected


83.0 317


30.8 289

Loring 1

0.5 195

Loring 2

0.0 180

Loring 3

91.0 144

Red Haven 1

94.1 187

Red Haven 2

92.5 253

Red Haven 3

83.2 232

Red Haven 4

96.0 173

Red Haven 5

97.8 90

Red Haven 6

86.6 313


0.0 197


65.0 163

Sugar Giant

79.0 372

Sweet Breeze

41.7 103


45.9 204


50.0 142

From these data it is clear that there are both variety and site differences in bud damage. A few blocks are likely to have no or little fruit this year, but most blocks have the potential to set a full crop. Remember that it only takes 15-40% of the buds to make a full crop. The bottom line is that before you prune, it would be a good idea to assess how much damage you have in your trees. Only in situations where severe bud damage occurred should growers leave extra wood when pruning.

Penn State did a similar analyses of peach buds, and found that only 22-32 percent of the peach buds were dead at the Biglerville Fruit Lab.

Fruit Bud Damage from Cold Temperatures

Cold temperatures in the winter, such as we’re experiencing over the next couple-few days with a polar vortex can cause damage to plants, people, and livestock. Tmin forecast 1_6_14_midatlanticAt this time of year, fruit buds on our main tree-fruit crops (apple, peach, cherry, and pear) are still in their winter dormant state (endodormancy). During endodormancy, buds have a very low water content and tend to be more cold tolerant. However, absolute lows are not the only factor to consider. If warm temperatures precede a cold spell, then tree buds tend to be less cold tolerant and are more likely to be damaged. There is also considerable variation amongst species and cultivars. For this reason, critical temperature thresholds, like those developed for spring frost damage to flower buds, are not well defined. However, from an ongoing discussion among pomologists in the Eastern part of North America, the consensus seems to be that peach flower buds start to be damaged at -10F and complete crop failure and/or tree loss occurs at -20F. Cherry and plum flower buds are slightly more cold hardy than peach buds and damage will likely occur at -20F. Apple flower buds can withstand temperatures down to -25F. As of now, the forecast for most of Virginia does not show temperatures dropping below 0F. Hopefully, this means that fruit buds in Virginia will not be damaged by the polar vortex.

Plants are not affected by wind chills (plants do not lose heat to the wind). But people and animals will be impacted by the heat loss from the high winds that are accompanying this storm. I’ve seen several wind chill forecasts for Tuesday in the -10 to -20F range. If your pruning crews are outside, make sure they are dressed appropriately for the cold. Also, make sure that your farm animals at least have shelter from the wind. For more information on protecting farm animals and equipment from the cold, read Cory Childs’ (VCE-Warren Co.) blog post from earlier today.



MaluSim Carbohydrate Models for May 13: Winchester and Central Virginia

Slide3Slide3The MaluSim carbohydrate model was run on May 13 for both Winchester and Central Virginia.

In Central Virginia, many later blooming cultivars are around 10 mm fruitlet size and can still be chemically thinned. From applications made over the past weekend, growers can expect average responses to chemical thinners. As we move through the week, greater and greater carbohydrate deficits are predicted, which will increase the potency of chemical thinners.

For Winchester, the model is predicting mild carbohydrate deficits early in the week, and much more severe deficits towards the end of the week and over the weekend. Applications made this past weekend through the middle of the week will likely have an average to slightly aggressive response. However, applications make towards the end of the week may result in very aggressive thinning activity. If the weather forecasts hold up, growers should consider reducing rates for applications made towards the end of the week.

I will run the model again on Thursday, May 16.

Fruitlet sizes from the Winchester AREC:

  • Honeycrisp: 8.8 mm
  • York: 8.0 mm
  • Fuji: 8.6 mm
  • Suncrisp: 8.4 mm
  • Pink Lady: 9.2 mm
  • Empire: 11.8 mm

One more note, there are widespread frost/freeze predictions for most of Virginia for Tuesday morning. If a significant frost/freeze event occurs, growers should be cautious with their thinning applications until they can assess if damage has occurred. However, the forecast for Winchester predicts the temperature will only go to 32F, which will not be cold enough to cause significant damage. The forecast for fruit growing regions to the south of Winchester appears to be in the mid-30s to low 40s.

To see more information, click on the linked pdf files below:

Peck Winchester VA MaluSim 5_13_13

Peck Central VA MaluSim 5_13_13

MaluSim Carbohydrate Models for May 6: Winchester and Central Virginia



MaluSim models were run on Monday May 6 for Winchester and Central Virginia (Piney River). The outputs from both locations were fairly similar, with carbohydrate deficits predicted over the next six or so days. This is mostly due to the cloud cover from a series of storms that will pass through the region. With sunnier conditions and warmer temperatures, the model predicts a carbohydrate surplus by Saturday.

Apple trees are still at late bloom to 4 mm fruitlet size in the Winchester area, and growers will probably not need to start thinning until later in the week or early next week.

In Central Virginia, fruitlets are ranging from 8 to 15 mm. Growers should consider reducing rates over the next few days as the model is predicting an aggressive response to standard rates of chemical thinners. However, as we move towards the end of the week, growers should use full rates to ensure an effective response.

It is unclear how much “natural” thinning will occur from this prolonged stretch of cloudy weather. However, work by Dr. Byers suggests that 3 days of cloudy weather with temperatures at 70F (this was a controlled study in growth chambers) can cause 50% of the fruitlets to abscise. With the prolonged stretch of cloudy weather that we are experiencing, growers should consider being less aggressive with rates until the end of the week.

Peck Winchester VA MaluSim 5_6_13

Peck Central VA MaluSim 5_6_13

MaluSim Carbohydrate Models for May 2: Winchester and Central Virginia

MaluSim Central VA 5_2_13


MaluSim Wincheter 5_2_2013



Today, I ran the first MaluSim Carbohydrate models for 2013. This includes a simulation for Winchester, and a simulation for Central Virginia (Piney River, VA). Thanks to Bennett Saunders for sending us the data to use for Central Virginia.

The Malusim model was developed by Drs. Alan Lakso and Terence Robinson at Cornell University to estimate the carbohydrate balance in an ‘Empire’ apple tree based upon many years of detailed research and information found in the scientific literature. In recent years, the model has been used by researchers and extension specialists in New York as a tool to help assess the application timing for applying thinning materials at the 10 mm fruitlet size. Carbohydrate status, and therefore the MaluSim model, is not as important for other times when chemical thinners might be applied (i.e., bloom, petal fall, 20 mm), so it is not as clear how well the model does to predict the response of chemical thinners used at timings other than 10 mm.

For the past several years, I have been running trials at the Winchester AREC to see how well the MaluSim model performs under conditions found in Virginia. Many researchers, including myself, are still working towards understanding the best way to use the model. From what we have found, when weather forecasts are used to predict the future carbohydrate status of an apple tree, the MaluSim model can provide a warning as to when there might be a chance for severe under- or over-thinning. This is particularly true when there are three- or four-day trends with large carbohydrate surpluses or deficits predicted. For this reason, in addition to the data points for each day, I show a four-day running average (the black line on the charts). To be consistent with other researchers and extension specialists, the four-day running average is calculated from the date the data is show to three days into the future.

Please keep these caveats in mind if you are using the MaluSim model to guide your thinning application timing:

  • We’re still learning how to best use the model. At this point, it is just one more tool in the toolbox. There is no tool that is better than your own experience with chemical thinning on your own farm.
  • Future predictions of carbohydrate status are based upon weather forecasts from for Winchester and Piney River. Temperature forecasts are probably pretty good for 3-5 days, but cloud cover (which is how I calculate the future solar radiation) is much less reliable. Although I show predictions out 10 days, my confidence that they are correct decreases significantly after 5 days. Nonetheless, it is useful information–just remember to keep looking for updated models so that you have the most up-to-date information. I will do my best to post a model for Central Virginia and Winchester twice a week.
  • Look at the day that you want to spray, and the 2-3 days after that. If there are periods of severe surplus or deficit, consider adjusting the rates or timing of your thinning applications.
  • The model is based upon the phenology of ‘Empire’. This is because the researchers in NY had a large body of data on Empire to use when they created the model. Of particular note, you should be aware that the model “clock” starts at silver tip for Empire. Cultivars with much earlier or later bud break may not correlate as well with the model values.
  • The models that I am running use data from the Winchester AREC, and Saunders Brothers Orchards in Piney River, VA. You need to consider whether or not those locations represent the weather in your orchard.
  • The model only accounts for temperatures and sunlight. Other weather events, such as rainfall or frosts may cause other impacts on the efficacy of your thinning materials that are not going to be represented in the model.

In the below pdf files, I provide a chart of the weather data that are used to run each model (daily maximum temperature, daily minimum temperature, and daily solar radiation), a chart of the MaluSim model with day-by-day data and a four-day running average, and a chart with the potential response a chemical thinner might have for different levels of carbohydrate balance in the tree.

So, what do today’s models show? Some very advanced blocks of early blooming apple cultivars in Central Virginia are getting close to the 10 mm thinning stage. From applications that occur today and tomorrow, you should expect “normal” to “slightly aggressive” response from your standard thinning rates, but nothing that suggests sever overthinning. However, there is a period of unsettled weather predicted for Mon-Wed of next week that could cause the carbohydrate levels to drop in the trees. It all depends on the amount of cloud cover we get over those days. Temperatures are going to remain moderate over that period, so even with the potential for some cloud cover, it doesn’t look like a severe carbohydrate deficit. Cloud cover is the hardest thing to predict, and predictions are only good for 3 days at best.

In Winchester, we are still at late bloom to petal fall, so there’s probably another week before the carbohydrate model data needs careful interpretation.

I will talk more about the model at tonight’s In-depth fruit school in Winchester.

Peck Central VA MaluSim 5_2_13

Peck Winchester VA MaluSim 5_2_13

Potential for Cold Temperature Damage in Northern VA

The National Weather Service is predicting temperatures to drop to 22F by the morning of April 4 in the Winchester area. For the bud phenology that I’ve seen in the area, we will probably be spared from having very much damage.

Apples are still dormant with a few early blooming cultivars at silver or green tip. Even at green tip, there should only be 10% damage when temperatures drop to 18F. Peaches with a little pink showing may be susceptible to a low level ~10% of damage. Cherries are still in stage 1 or 2, and should be able to withstand temperatures down to 17F with minimal damage. So, fingers crossed that it doesn’t get any colder than predicted. Temperatures are forecasted to be more “spring-like” next week!

Here are pdf links for the Washington State University fact sheets on the critical temperatures for flower buds:

Additionally, Michigan State University has adapted the WSU critical temperature fact sheets into this easy-to-read fact sheet.

MSU has also put together an excellent set of resources about freezes and frosts in fruit crops.