Spring Frosts and Fruit Tree Buds

 

By Amy Lentz, Boulder County CSU Extension

One can grow several different types of fruit trees in Colorado with some performing better than others depending on where and at what elevation you live. Along the Front Range, apples and cherries used to be staple industries prior to the 1950’s. A few rough weather years along with changing market forces moved the industries to other states, but these types of fruit trees along with pears and plums can be quite fruitful (pardon the pun) along the Front Range. Other types of fruit trees will struggle in this region to produce a crop, whereas if you are on the Western Slope, several different types of fruit trees will give you a dependable fruit crop due to the benefit of warmer winds…perfect for growing those coveted peaches.

A lot of the potential to get a decent harvest from a fruit tree in Colorado boils down to phenology. What’s that, you ask? Phenology is a branch of science dealing with the relationship between climate and periodic biological phenomena (definition from Merriam-Webster) such as bird migrations and, in this case, flowering patterns of plants. In more simple terms, this is often referred to as bloom stage or growth stage. Fruit trees will go through various bloom stages as they wake up each spring. For example, apple trees have nine stages of blooms from dormant to fruit set as seen in this pictorial chart from Cornell University:

Apple Phenology Chart - Cornell University

Back to why it’s not easy to have a productive fruit tree in Colorado…springtime frosts! It’s safe to say that you never know when that last spring frost will come in Colorado. And when it does, how low will it go? This is the key to whether or not you will get a decent fruit crop from year to year – it’s the combination of the low temperatures experienced in the spring and the stage of bloom at that time. For example, if your leaves and/or flowers are just starting to emerge (at the ‘green tip’ stage of development), your tree’s flower buds can take some pretty low temperatures and can easily survive a light frost. However, if you tree is in full bloom, you will lose many of those flowers if the temperatures dip a just a few degrees below freezing. The following chart shows the various bloom stages for tart and sweet cherry and peaches:

% Bloom Loss vs. Temperature vs. Bloom Stage - Utah State University

Notice for tart cherries that even at temperatures down to 15° F, only 10% of the blooms will be damaged or killed if they are just swollen and have not broken through yet. Conversely, with that same tree in full bloom, you will see around 90% of the blooms (and future fruit) damaged or killed if the temperature gets down to just 25° F. The relationship is similar for peaches and other fruit trees, as well. Simply put, the further along in bloom, the more susceptible the buds are to being killed by a spring frost.

Undamaged peach flower bud compared to damaged bud after exposure to very low temperatures (-15°F). Photos: Amy Lentz

Throw in crazy Colorado weather and it’s a fool’s bet! The timing of these stages will vary from year to year, making it hard to predict when your tree will be in full bloom. The following photos were taken in the Fort Collins area in 2017 for an apple tree. The dates of each stage of flower are marked above each picture taken of the same branch over time. Just one year later in 2018, the same tree was more than two weeks later in its timing for each bloom stage.

Pink Stage occurred on April 12, 2017. In 2018, the same stage occurred about 16 days later, showing up on April 28th. Photos: Amy Lentz

There are not many options to outsmart Mother Nature and prevent your fruit tree from receiving some damage during a spring frost and some trees will fare better than others. Apricots are usually the first to bloom, followed by peaches, cherries, plums, pears and apples will usually be last. Here is one tip if you haven’t yet planted your tree…place it on the north or northeast side of a structure or house; close enough that it’s shaded in the winter and far enough that it’s in full sun later in the summer when the sun is higher in the sky. This can provide more consistent winter temperatures, hopefully keeping the tree dormant for just a bit longer and delaying flower development. Even a week of two can make all the difference!

Weather can be our Friend or Foe.

 

Color to come, hopefully.

Last October, Colorado saw some crazy weather with cold weather come in after really warm weather.  Several days in early October were in the 80’s.  First there was a big wind event.  Then temperatures hitting below normal around Oct 13^th, hitting just above freezing, followed by temperatures as cold as 11 degrees F and ended toward the end of the month with a big snow storm.  Of course the moisture was needed and appreciated, but rain would have been better.  Yearly average temperatures in mid to late October in the Grand Junction area are 67 / 41 Degrees F.

Plum Tree, pre-pruning, L. Masters

Plants, like people, do not like quick change.  Many perennial plants like trees had not had the chance to appropriately harden off, or acclimate, so they were ready for winter.  There are still trees that still have their leaves and not the normal ones like Oak trees.  This is because the trees did not have time to form the hormone that creates the abscission layer, causing leaves to drop in fall.  I predict we will see samples of tree damage from the cold for months to come.

Our fruit tree growers and the CSU Western CO Research Center in Orchard Mesa was tracking how the fruit trees were acclimating, or in this cause the lack of acclimating.  Here is what David Sterle reported in early December from the Western CO Research Center: “We are seeing between 10 and 40% dead (peach) buds after the freeze depending on variety. The bigger worry is probably the shoot tissue, and an important thing about the shoots is that from the middle of the shoot to the base there is a lot less damage. The damage in the tip 1/3 of the shoot is about 3 times as common as on the base. So from that point on I will only be looking at buds from the basal half of the shoot, where there are more live buds. We are recommending spraying Captan / Topsin / lime sulfur if possible to prevent cytospora from getting into the damage shoot tissue.”  The good news is this varied by location and peach variety and some trees did go on to acclimate as winter went on.  And of course last year we saw a late frost that impacted the number of peaches that were available.  So I predict peaches will be at a premium again this year.

Checking for life, WCRC

Many of the growers have seen twig death.  No studies have been done on the combination of the cold combined with the drought, but certainly the drought has not been helpful.  Many of the peach growers worked to prune out the dead.  Typically over 50 % of the tree is pruned to stimulate new growth for next year, so it just took some modifying to do more tip pruning to remove the obvious dead.  Pending not having any damaging spring frosts, we should still have a peach crop.  But the wine growers have not fared as well.  Many wine growers have moved to grafted Vitis vinifera varieties of grapes, native to the Mediterranean area, that are hardy against phylloxera, which is basically a root eating aphid that slowly kills the vines.  However, these grafted grape varieties are not as hard.  It is expected up to 100% of these winery hybrid grapes are gone.  And sweet cherries are a very similar story.  Several of the local sweet cherry growers had just recovered from drought damage on their fruit trees.  Drought stress can affect trees for several years.  Areas in Delta County have irrigation that is very dependent on natural precipitation levels.  Recent drought years they have seen no irrigation in late summer or fall.

drought and insect damage, S. Carter

In doing multiple orchard visits, many of the orchards have decided to wait on pruning the grapes and cherries to see if they sprout.  CSU Research specialist Horst Caspari did a session for grape growers on how to prune and what to look for the cold damage.  Many will be depending on the vines sprouting from the lower portion of the vine, or may be replanting and starting over.  Last summer the grape growers in the area had to deal with possible smoke taint from the local wildfires.  Combine that with a decrease in visits to tasting rooms, and it has been a very bad wine year.

So, appreciate your local fruit and wine producers as it has been a tough year for them beyond what the rest of us have experienced.  Colorado climate can be a challenge to grow plants in, but when we are successful nothing tastes better.

By Susan Carter, Extension Horticulture and Natural Resource Agent, Tri River Area

The Basics of Fruit Tree Pruning

By Amy Lentz – Weld County CSU Extension

The season for pruning fruit trees has been in full swing lately. My colleagues and I have been busy teaching classes, talking with the public and helping growers “get their fruit tree pruning on”. It can be an intimidating task for some - "What if I prune out too much?" or "What types of cuts should I make?" With a little extra patience, basic research and confidence, you can do it...I promise.

Weld CMG's helping teach residents of a community.

Larimer CMG's learning to prune.

Here are some questions and answers that might help you get started (or finished) with your pruning needs.


1. Are fruit trees different than shade trees?

Yes! Fruit trees are pruned differently than shade trees. With fruit trees, you are growing a crop, maybe even for monetary gain. Therefore, you will be manipulating the tree’s growth more than you would when pruning shade trees. The three D’s (disease, damage and dead wood) still apply, but you will also be making cuts for other reasons, such as to increase fruit production or to fill an empty space with more branches. When trying to decide what to prune and what to not prune, look for strong branches with wide branch angles, which are more capable of holding a heavy load. You want to keep those! It's better to remove branches that have a very narrow, or weak, branch angle. Also, you would not be pruning your shade trees as often or as heavily as you would fruit tree! 

2. How should I prune a young tree vs. an older tree?

Young  fruit trees are much more forgiving than older fruit trees, so go lightly if you are pruning an older tree that needs a good deal of thinning. Spread out your pruning cuts to open up the canopy by pruning it lightly over several years. You can prune your young trees (say up to 5 years old or so) a little harder - and they are easier to train when they are young. So start with young trees if possible. Here is an example of how to train a tree over 5 growing seasons from a one year old whip (spreaders were added after the 4th spring):

3.  How do I know where the fruit will be?

Most fruit trees often bloom on old wood. Apples and pears, for example, grow on spurs which can begin developing after 2 years and last a long time, up to 20 years. Plums and cherries also have fruiting buds that occur on old wood, at least 2 years old, either on spurs or along the stems. Peaches, on the other hand, are developing fruit buds on last years growth, so annual pruning to encourage new growth is important for peaches. It's important to know the difference between fruiting buds and vegetative buds. You will be removing some fruiting wood during the regular pruning process, but you don’t want to inadvertently prune off all those little spurs, which will lead to fruit during the upcoming season.

Photos from University of Maine Cooperative Extension: https://extension.umaine.edu/fruit/growing-fruit-trees-in-maine/pruning/

4.. When is the best time to prune my fruit trees?

With the warmth of spring happening more often lately, it’s time to get outside and finish up those pruning jobs if you haven’t already. The best time to prune is in late winter or early spring when the trees are still dormant (which is hard to judge in Colorado!). Either way, you want to stop pruning when the buds first begin to swell and break. It’s best to prune older trees first to make sure they have enough time to heal before spring and leave the younger trees until later in the pruning season. 

Another reason we prune when the trees are dormant is to reduce the spread of diseases like fire blight, that affects those fruit trees in the Rose Family, such as apples and pears. This particular disease can easily be identified and pruned out in the winter. 

For more information on Fireblight, see the following fact sheet from CSU: https://extension.colostate.edu/topic-areas/yard-garden/fire-blight-2-907/
Another good source of information on fireblight, more for the commercial orchard, can be found here from Washington State University: http://treefruit.wsu.edu/crop-protection/disease-management/fire-blight/

5. What are my goals and objectives when pruning fruit trees? 

Approx. 5 year old cherry trees after pruning to thin the canopy.

Your main goal when pruning fruit trees is to open up the canopy of the tree to allow increased light penetration and increased air movement in the center of the canopy, where fruit spurs are located. 

You want to encourage outward growth on the tree to get branches to grow into open space. This is done by pruning to an outward facing branch or bud. Along with the three D’s (diseased, dead or damaged), look for crossing branches, branches that are shading or crowding one another and branches pointing back toward the tree. Don’t remove more than 25% a single pruning season, less for older trees so you don’t stress the tree too much. This will help with fruit development, as well as, decreasing the prevalence of insects and disease. 

To do this, remove crowded branches by making reduction or thinning cuts back to branches that are facing outward, away from the trunk. Reserve heading cuts (where you cut back to a small bud) for only those branches where you want to encourage new growth, such as lower branches or young branches. As you remove some of the twigs and branches to allow for more space in the tree, you will also, in effect, be reducing the number of fruit buds on the tree as you remove wood, so you will also be increasing the energy allotted to the fruit buds that remain – creating larger fruits that have ample sun and lots of space around them. This is the goal of growing fruit trees!

Finally...How can I build my confidence to prune my fruit tree?

Pruning fruit trees takes time, patience and a little critical thinking. A hands-on approach helps a great deal to make these concepts sink in. Definitely look beyond this blog because there is a lot more information than what I could provide here. When doing your research, be sure to look toward education- or research-based information and watch videos to help you make sound decisions when pruning. You can always remove a branch…but you can’t put it back on! Take your time, think about the branches you want to remove and what you want to keep. You can do it!

For more information on pruning fruit trees, check out the following sources from Colorado State and other universities:

Cornell Cooperative Extension - Pruning Apple Trees
http://chemung.cce.cornell.edu/resources/pruning-apple-trees

CSU Fact Sheet - Training and Pruning Fruit Trees 

https://extension.colostate.edu/topic-areas/yard-garden/training-and-pruning-fruit-trees-7-003/

Utah State - Pruning Fruit Trees: Clip with Confidence!

https://extension.usu.edu/yardandgarden/ou-files/PruningHandout-OY-Mar2017.

Educational Video Series from Montana State 

https://mtmastergardener.org/news/19079/educational-video-series-on-growing-small-fruit-and-tree-fruit-in-montana

Dormant Oil and Pruning Fruit Trees

This is a tough year.  With dry, warm weather some of our trees are confused.  Many of our fruit trees never really hardened off as reported by our local research station in Grand Junction.  And now many of the buds are swelling on trees in the Grand Valley.   I always tell homeowners to wait to prune their fruit trees later than the commercial orchards so they are closer to spring for the trees to heal.  Growers have to start early due to the sheer volume of pruning they need to do.  And I prefer to spray dormant oil after pruning but this may be one year I decide to do the opposite on my own trees.

Colostate photo-Our goal-Fruit!

I think everyone that growers fruit trees should spray with dormant oil.  This product used to be made from petroleum products but is now usually made from cotton meal seed so it can even be used on organic trees.  Yes, organic trees are sprayed, but that is a whole other article.  This oil is very thick and is only used in winter versus horticulture or summer oils that can be used during the season.  The reason we spray this is to suffocate any insect eggs or overwintering insect adults on the trees to limit pest pressure on the fruit tree in the coming season.  It is an especially good control of aphids.  Make sure the weather is warm enough to spray and that your buds have not broken (not showing any color).  The oil can burn the contents of the bud if open.  Always read the label, it’s the law.  And be sure to cover as much as the tree as possible.  Here is our fact sheet on dormant oil.  It can also be a good control for some insects on ornamental trees as well.  http://extension.colostate.edu/topic-areas/insects/insect-control-horticultural-oils-5-569/

So why do we even prune fruit trees?  Well of course to get better fruit.  Pruning a fruit tree is vastly different than pruning an ornamental or shade tree.  Many of those rules go out the window because with fruit trees you are actually trying to stress them to induce fruiting.  So our goal is to capture enough sun in the fruit tree canopy without sun burning the fruit.  The best shape to do that would be an upside down umbrella.  Now if you do not want to have a tree that looks like an upside down umbrella in your landscape, you can prune to have a central leader with spaced side limbs.  This is an older method of pruning apple trees and cherries also do well with this system of pruning.

Before pruning different types of fruit trees it is important to understand what year wood gives you the fruit.  With peaches the fruit occurs on one year old wood, the new growth from last year.  On apples, it is typically 2^nd to 4^th year wood but some depends on variety.

With pruning these trees we want to remove 50-60% of the tree if it is young and used to being pruned that way.  Don’t take an old tree and do this, it will stress too much.  Talk to your local agent if you have an older tree.  Here is a link: http://planttalk.colostate.edu/topics/fruits/1210-pruning-mature-fruit-trees/

Open Center Peach Pruning- Photo by Susan Carter

So young trees have their central leader removed when they have established root systems.  Here is a a diagram from UMN. 

Now back to in their prime trees; when pruning you want to think of the side shoots of the limb together looking like a feather.  So there are very few branches going up or down.  Branches underneath tend not to get enough light and too many branches going in or up can shade other branches and leaves.  But you want a few interior and upright branches to produce future growth and to provide some shade for your trunk and main branches or they may develop sunscald.  http://planttalk.colostate.edu/topics/weeds-cultural-problems/2111-sunscald-trees/

There are different types of cuts used to prune fruit trees.  My favorite to help keep the limbs going out instead of up is called a bench cut.  This cut removes the tip of the upward growing branch and brings is back to an outward facing side branch.  This side branch will then become your growing tip and will help keep the height of the tree down and to create that upside down umbrella shape.  Dutch  cuts are vertical cuts that are made on side limbs of mainly apples to induce dormant bud break below.  Something you would never do on an ornamental tree. 

Apple tree with dutch cut- photo by Susan Carter

So to summarize, start pruning and training your fruit trees when young;  Spray with dormant oil prior to bud break;  Don’t  be afraid to remove half the tree yearly;  Know the age of the wood that produces fruit so you don’t remove too much of it. 

Lastly, look for a local training on fruit trees thru Extension.  The Tri River Area offers a fruit tree class as part of their Master Gardener training.  I promise a future article on picking which fruit tree is right for you or should you support the orchards and let them do the labor.  By Susan L. Carter, Horticulture Agent, Tri River Area

Cherries, Apricots and Peaches, Oh My!

Fruits of the Grand Valley are abundant this year.  A mild spring with good rain actually gave us cherries and apricots this year.  Depending on who you talk to and where they are located, people will tell you that we only get a cherry crop every 4-10 years due to late frosts.  So cherish those sweet cherries, hopefully we will have lots of them next year, but only Mother Nature knows.  Cherries in our area do require control of the Western Cherry Fruit Fly.  If not controlled, you get little worms in your cherries, not very appetizing.  Here is how to control them.  https://extension.usu.edu/files/publications/factsheet/western-cherry-fruit-flies06.pdf.

The apricots due get Coryneum blight, also called shot hole disease because spots form on the leaves and then drop out.  The fruit gets red spots mainly on the upper surface where the rain sits.  Unfortunately, the best time to control this disease is in the fall.  Removal of infected twigs and branches helps too.  Here is our information on this fungal disease.  In the home garden, you can cut out these bad spots in the fruit, but it certainly does not make the fruit saleable.

http://extension.colostate.edu/topic-areas/agriculture/coryneum-blight-2-914/

I did one site visit very early on to a peach orchard that had very early signs of coryneum blight and the grower was able to do some spraying to control it.  Luckily it was only in one low lying corner of the field.  Where plants grow really does make a difference.  A few very early peaches are coming to market now but we will see the bulk of early yummy Palisade peaches hit the market in about two weeks.  Bob Hammon, our Entomologist tells me we will also see Olathe Sweet, Sweet Corn about July 11^th.  Can I say again, YUM.  Nothing better than fresh produce.

Back to the peaches.  I am on a Working Group to help find disease controls for Cytosphora which is a fungal disease that has no cure.  We believe many stressors’ contribute to this disease starting with proper planting techniques, insect control, care and maintenance and cleanliness.  A new Plant Pathologist, Jane Stewart, on campus is studying the effects of different fungicides as a control.  Samples are being sent to her from our area.  But we still need to find out the why.  My big point to you, the reader, is that fruit trees are very hard to grow, take a lot of time and responsibility to grow and not spread any pests.  In fact, I would say fruit trees are the highest maintenance plant one can grow. 

Yes, I do have some fruit trees on my property, but luckily my husband also has a horticulture degree so he does most of the insect control, we split the pruning duties, both pick and guess who does most of the canning?  Thou he has gotten into drying.  He dried cherries last weekend.  So in the end, appreciate all the hard work that went into the fresh fruit, support your local growers and a good thank you would go a long way too.  Enjoy.

Susan Carter
CSUE Tri River Area Horticulture Agent