Madeline Wimmer- UMN Fruit Production Extension Educator
Image: Golden Russet apple variety showing flowers in full bloom at Hoch Orchard, located in La Crescent, MN (Houston County; Zone 5a; 05/06/25).
After the growth stage pink and the unfolding of the king flower have occurred, other blossoms on the apple cluster begin to unfold until the tree is in full bloom. Bloom is a critical period for apple production. Because apples are insect pollinated and cannot self-pollinate, success for pollination depends on weather conditions, the availability and activity of pollinators, and having a good pollen source to cross-pollinate a particular variety.
An apple tree that is used as a pollen source is referred to as a pollinizer; however, most orchards usually grow multiple apple varieties already. As long as the variety bloom times overlap, pollination can generally work out, and purchasing designated trees to be pollinizers is usually not necessary.
Most apple trees complete their bloom and pollination window within 10 days of the first bloom date, though this period may be shorter with higher temperatures. Orchards in southern regions that were in bloom this past week experienced temperatures that were slightly warmer and drier than average* (see newsletter end notes), which can lead to a shorter bloom window.
When possible, it's recommended to avoid insecticides and miticides at bloom time to maintain good pollinator health among populations that are present. Some growers choose to extend their window of caution through careful choice of pesticide type, application timing etc. during petal fall by choosing pesticides with minimal risk to bees. Recall that broad spectrum insecticides, like carbaryl and pyrethroids are higher risk products. Refer to tools like UC IPM’s Bee Precaution Pesticide Ratings system to receive guidance on risks of particular pesticide products.
Additionally, some recent studies have shown harm to pollinator health by non-insecticide pesticide applications. Research looking at these topics is ongoing, and impacts to pollinators by products likely varies based on products, their formulations, how they are used, and the rate and amount of time pollinators are exposed. It’s generally best to apply pesticides during low pollinator times (i.e., late evening, or before sunrise) and when the risk of drift is low unless directed otherwise by label instructions.
Tools and resources for further learning:
Pesticide Applicator Best Management Practices (BMPs; Pesticide Environmental Stewardship)
Image: Varieties like Honeycrisp and Haralson that have biennial bearing tendencies benefit from early-season fruit thinning done during bloom or petal fall. The above photo shows two rows of apple trees, both in bloom, with low return bloom on the right side.
Early-season fruit thinning is especially helpful for apple varieties like Honeycrisp, McIntosh, and Haralson, which are known for biennial bearing with low return bloom occurring every other year. Last week, an article was posted on bloom thinning apples and understanding the pollen tube growth model for growers who are interested in learning more about that method. Petal fall is the next opportunity to fruit thin.
The products available to use for petal fall thinning include hormonal (i.e., plant growth regulator) products like NAA (e.g., Fruitone N, PoMaxa, or Refine), or 6BA (e.g., Maxcel), which interrupt natural development processes causing more developing fruitlets to drop. Carbaryl (e.g., Sevin and Sevin XLR+) is a different product available for use, which acts as a mild thinner based on a small amount of phototoxicity induced. It’s important to recognize that carbaryl is a broad spectrum insecticide that can harm pollinators and should be used with that consideration in mind for application timing. It’s possible for growers to combine multiple thinning products, or add adjuvants when tank mixing, to increase product efficacy. Thinning will also be more effective earlier when products are applied before apples reach approximately 12 mm in size.
Resource: For more information on fruit thinning at petal fall, refer to the Midwest Fruit Pest Management Guide, starting on page 44.
If it’s your first time thinning fruit, it might be relieving to hear that many growers think that they have overthinned initially. Recall that only one fruit is needed per fruiting spur, or tip, and a healthy distance between fruits—from one spur to the next—is about 4-6 inches. Thus, thinning may appear dramatic when first practiced until later in the season when the fruit load is easier to see due to an increase in fruit size.
Images: Two different grape varieties located at the same site showing two different growth stages: budburst with leaf tips visible (left), and 2-3 leaves separated with growth between 2-4 inches. Growth stages referenced from the modified E-L Growth Stage Guide. Photos taken at Little Hill Berry Farm located in Rice County (Zone 5a, 05/12).
Images: High bush blueberries are in bloom at Little Hill Berry Farm located in Rice County (Zone 5a, 05/12/25).
Blueberries present some challenges for production across much of Minnesota because of their requirement for a lower soil pH, between 4.5–5.2, and usually require pre-planting soil amendments with sulfur or peat to lower soil pH in most Minnesota soils.
There are multiple types of blueberries: high bush, half-high/hybrid, and low bush, or wild blueberries. Of these three types, half-high (hybrid) blueberries are more suitable to Minnesota, but some farms still successfully grow high bush varieties like Patriot and Bluegold, although winter dieback can occur, even in Zone 5a regions.
The UMN Fruit Breeding Program has developed a number of blueberry varieties that are more cold hardy, including Superior, St. Cloud, Pink Popcorn®, Chippewa, and Northcountry. These varieties grow as high as four feet and can spread to a little over three feet wide. They still require acidic soil, but are hardy to Zone 3.
Image: A photo of the UMN hybrid blueberry varieties Pink Popcorn (left), and St. Cloud (right).
The above information captures the average temperatures and precipitation throughout Minnesota and compares it to previous years. Key: 1) cd: Climate Division, regions within a state used for summarizing weather and climate data; 2) temp and prcp: this growing season’s average temperature (°F; temp) and precipitation (in; prcp) from the dates provides (May 7th through May 12th, 2025); 3) norm: the average temperature (°F) or precipitation amount (in) based on previous years data from weather tracking; 4) dev: the deviation of the tracked temperature and precipitation to the average (how far from normal is it?). This information can be found through cli-MATE, under Climate Division Data: Estimated Between Two Dates.
**
Maps showing accumulated precipitation in inches for the state of Minnesota (left) and the departure from past years precipitation accumulation (right), showing that Minnesota has received less rainfall this year from May 1st to May 13th, than prior years, on average.
Thank you to our farm and ag professional partners for contributions to the UMN Fruit Update series. Non-credited photos in this article were either taken by Madeline Wimmer or within the UMN Extension system.
- Apples
- Growth stage update: Bloom
- About apple insect pollinators
- Apple fruit thinning, continued (post-bloom)
- Grapes:
- Growth stage update: Budburst/bud break through early leaf separation
- Early-season shoot thinning
- Strawberries:
- Article: Day-neutral strawberry planting: Key points
- Blueberries:
- Growth stage: Bloom
- End notes
Apples
Growth stage update: Full bloom
Image: Golden Russet apple variety showing flowers in full bloom at Hoch Orchard, located in La Crescent, MN (Houston County; Zone 5a; 05/06/25).
After the growth stage pink and the unfolding of the king flower have occurred, other blossoms on the apple cluster begin to unfold until the tree is in full bloom. Bloom is a critical period for apple production. Because apples are insect pollinated and cannot self-pollinate, success for pollination depends on weather conditions, the availability and activity of pollinators, and having a good pollen source to cross-pollinate a particular variety.
An apple tree that is used as a pollen source is referred to as a pollinizer; however, most orchards usually grow multiple apple varieties already. As long as the variety bloom times overlap, pollination can generally work out, and purchasing designated trees to be pollinizers is usually not necessary.
Most apple trees complete their bloom and pollination window within 10 days of the first bloom date, though this period may be shorter with higher temperatures. Orchards in southern regions that were in bloom this past week experienced temperatures that were slightly warmer and drier than average* (see newsletter end notes), which can lead to a shorter bloom window.
About apple insect pollinators:
Bees are common insect pollinators of apples. This includes honey bees, bumble bees, and mason bees. Whether you are managing hives on-site, or encouraging pollinator activity through other ways, like building up pollinator habitat, knowing best practices for protecting pollinators is important for orchard management.When possible, it's recommended to avoid insecticides and miticides at bloom time to maintain good pollinator health among populations that are present. Some growers choose to extend their window of caution through careful choice of pesticide type, application timing etc. during petal fall by choosing pesticides with minimal risk to bees. Recall that broad spectrum insecticides, like carbaryl and pyrethroids are higher risk products. Refer to tools like UC IPM’s Bee Precaution Pesticide Ratings system to receive guidance on risks of particular pesticide products.
Additionally, some recent studies have shown harm to pollinator health by non-insecticide pesticide applications. Research looking at these topics is ongoing, and impacts to pollinators by products likely varies based on products, their formulations, how they are used, and the rate and amount of time pollinators are exposed. It’s generally best to apply pesticides during low pollinator times (i.e., late evening, or before sunrise) and when the risk of drift is low unless directed otherwise by label instructions.
Tools and resources for further learning:
Pesticide Applicator Best Management Practices (BMPs; Pesticide Environmental Stewardship)
Apple fruit thinning, continued (post-bloom):
Early-season fruit thinning is especially helpful for apple varieties like Honeycrisp, McIntosh, and Haralson, which are known for biennial bearing with low return bloom occurring every other year. Last week, an article was posted on bloom thinning apples and understanding the pollen tube growth model for growers who are interested in learning more about that method. Petal fall is the next opportunity to fruit thin.
The products available to use for petal fall thinning include hormonal (i.e., plant growth regulator) products like NAA (e.g., Fruitone N, PoMaxa, or Refine), or 6BA (e.g., Maxcel), which interrupt natural development processes causing more developing fruitlets to drop. Carbaryl (e.g., Sevin and Sevin XLR+) is a different product available for use, which acts as a mild thinner based on a small amount of phototoxicity induced. It’s important to recognize that carbaryl is a broad spectrum insecticide that can harm pollinators and should be used with that consideration in mind for application timing. It’s possible for growers to combine multiple thinning products, or add adjuvants when tank mixing, to increase product efficacy. Thinning will also be more effective earlier when products are applied before apples reach approximately 12 mm in size.
Resource: For more information on fruit thinning at petal fall, refer to the Midwest Fruit Pest Management Guide, starting on page 44.
If it’s your first time thinning fruit, it might be relieving to hear that many growers think that they have overthinned initially. Recall that only one fruit is needed per fruiting spur, or tip, and a healthy distance between fruits—from one spur to the next—is about 4-6 inches. Thus, thinning may appear dramatic when first practiced until later in the season when the fruit load is easier to see due to an increase in fruit size.
Grapes
Growth stage update: Budburst/bud break through early leaf separation
Budburst marks the beginning of shoot growth for cold climate grapes. While the time from sap running to bud break can feel like a long time period, growth can happen quickly at this point, and it’s important to make plans for early-season management tasks like shoot thinning and tucking shoots into catch wires, which can easily be missed.
We’re entering the time in which management for early-season grape diseases will become critical. The past week has been drier than average** (see newsletter end notes), which contrasts the frequent rain events in early parts of the 2024 growing season. Having a drier period with lower relative humidity puts less pressure on spore release and infections for many pathogen types, which may give way to the need for fewer overall fungicide applications for the season. Keep in mind that most fungicides are proactive in their management; the first application should generally be made when shoots are between 1-3 inches for fungicides listed in the Midwest Fruit Pest Management Guide (beginning on page 157).
Article: To learn more about early-season grapevine diseases, check out the UMN Fruit Update from April 30th, 2025.
We’re entering the time in which management for early-season grape diseases will become critical. The past week has been drier than average** (see newsletter end notes), which contrasts the frequent rain events in early parts of the 2024 growing season. Having a drier period with lower relative humidity puts less pressure on spore release and infections for many pathogen types, which may give way to the need for fewer overall fungicide applications for the season. Keep in mind that most fungicides are proactive in their management; the first application should generally be made when shoots are between 1-3 inches for fungicides listed in the Midwest Fruit Pest Management Guide (beginning on page 157).
Article: To learn more about early-season grapevine diseases, check out the UMN Fruit Update from April 30th, 2025.
Early-season shoot thinning
Images: Photos showing before and after shoot thinning of a spur-pruned, high cordon grapevine. One method of thinning is to reduce the number of shoots to around 6-8 shoots per linear foot of cordon.
Early-season shoot growth can happen quickly during this time, especially with warmer weather in the forecast. When they first emerge, grape shoots are tender, and become stiffer as they grow to around one-foot in length. Before then, they can easily be removed by hand, which makes shoot thinning faster and easier.
Most cold climate grapes do well when they are thinned to 6-8 shoots per linear foot of canopy. While pruning is the first step to limiting the number of shoots per linear foot of canopy, there are a number of reasons to come back for early shoot thinning. Having a balanced number of shoots in a given amount of canopy helps bring vines into balance and prevent overcropping. It additionally allows for good airflow and sunlight throughout the canopy.
When grapevines have been slightly under pruned, there tend to be a number of options to select from while thinning off unwanted shoots. Secondary shoots that simultaneously emerge with primary shoots for some grape varieties should also be removed in most scenarios. Be mindful of any buds that are growing from the cordon that could make a good future replacement for an old spur, for example. Keep these, of which many will not fruit, with the option to remove fruit from any that do this growing season.
The process of thinning out shoots should be precise enough to do a good job, but does not require a lot of precise measurements or deep thinking. Make a plan, know roughly what one foot looks like, start slow, and speed up as you get into a pattern with your canopy shoot density and layout preferences.
Videos:
Article: Day-neutral strawberry planting: Key Points
Early-season shoot growth can happen quickly during this time, especially with warmer weather in the forecast. When they first emerge, grape shoots are tender, and become stiffer as they grow to around one-foot in length. Before then, they can easily be removed by hand, which makes shoot thinning faster and easier.
Most cold climate grapes do well when they are thinned to 6-8 shoots per linear foot of canopy. While pruning is the first step to limiting the number of shoots per linear foot of canopy, there are a number of reasons to come back for early shoot thinning. Having a balanced number of shoots in a given amount of canopy helps bring vines into balance and prevent overcropping. It additionally allows for good airflow and sunlight throughout the canopy.
When grapevines have been slightly under pruned, there tend to be a number of options to select from while thinning off unwanted shoots. Secondary shoots that simultaneously emerge with primary shoots for some grape varieties should also be removed in most scenarios. Be mindful of any buds that are growing from the cordon that could make a good future replacement for an old spur, for example. Keep these, of which many will not fruit, with the option to remove fruit from any that do this growing season.
The process of thinning out shoots should be precise enough to do a good job, but does not require a lot of precise measurements or deep thinking. Make a plan, know roughly what one foot looks like, start slow, and speed up as you get into a pattern with your canopy shoot density and layout preferences.
Videos:
- Shoot thinning VSP vines (John Thull, UMN Vineyard Manager and Researcher & Minnesota Wine and Grape Alliance)
- Shoot thinning high cordon (John Thull, UMN Vineyard Manager and Researcher & Minnesota Wine and Grape Alliance)
- Shoot thinning high cordon Petite Pearl grapevines (YouTube; UW Fruit Program)
Strawberries
Day-neutral strawberry planting: Key points
Are you planning to plant bare-root day-neutral strawberries this year? Learn about a technique helpful for growers planting into mounds with plastic mulch that includes a short demonstration video.Article: Day-neutral strawberry planting: Key Points
Blueberries
Growth stage: Bloom
Images: High bush blueberries are in bloom at Little Hill Berry Farm located in Rice County (Zone 5a, 05/12/25).
Blueberries present some challenges for production across much of Minnesota because of their requirement for a lower soil pH, between 4.5–5.2, and usually require pre-planting soil amendments with sulfur or peat to lower soil pH in most Minnesota soils.
There are multiple types of blueberries: high bush, half-high/hybrid, and low bush, or wild blueberries. Of these three types, half-high (hybrid) blueberries are more suitable to Minnesota, but some farms still successfully grow high bush varieties like Patriot and Bluegold, although winter dieback can occur, even in Zone 5a regions.
The UMN Fruit Breeding Program has developed a number of blueberry varieties that are more cold hardy, including Superior, St. Cloud, Pink Popcorn®, Chippewa, and Northcountry. These varieties grow as high as four feet and can spread to a little over three feet wide. They still require acidic soil, but are hardy to Zone 3.
End notes
*The above information captures the average temperatures and precipitation throughout Minnesota and compares it to previous years. Key: 1) cd: Climate Division, regions within a state used for summarizing weather and climate data; 2) temp and prcp: this growing season’s average temperature (°F; temp) and precipitation (in; prcp) from the dates provides (May 7th through May 12th, 2025); 3) norm: the average temperature (°F) or precipitation amount (in) based on previous years data from weather tracking; 4) dev: the deviation of the tracked temperature and precipitation to the average (how far from normal is it?). This information can be found through cli-MATE, under Climate Division Data: Estimated Between Two Dates.
**
Maps showing accumulated precipitation in inches for the state of Minnesota (left) and the departure from past years precipitation accumulation (right), showing that Minnesota has received less rainfall this year from May 1st to May 13th, than prior years, on average.
Thank you to our farm and ag professional partners for contributions to the UMN Fruit Update series. Non-credited photos in this article were either taken by Madeline Wimmer or within the UMN Extension system.
Comments
Post a Comment