Critical Temperatures for Frost Damage on Fruit Trees

Each spring I post the spring critical temperatures chart from Utah State to help you determine at what stage your fruit trees may be at as spring progresses.  With the prediction of temperatures in the mid 30’s for tonight in northern Illinois, I thought I would repost the “Critical Temperatures For Frost Damage on Fruit Trees” now for those who grow their own fruit at home and for commercial growers.  Our trees here at Royal Oak Farm Orchard should be far enough along that the low 30’s should not have much effect on the current clusters and blooms.  But here is an explanation of how the fruitlets, clusters or blooms can be effected if the temperatures get low enough.

As the trees develop in the spring and buds start to swell, they lose the ability to withstand the cold winter temperatures that they could withstand in dormancy during the cold winter months. The young, actively growing tissue can then be damaged or even killed. Swollen fruit buds can better withstand temperatures in the teens without any damage. As the buds open, temperatures in the low 20s can cause harm, but sometimes leave other buds undamaged.  As growth moves from green tip to 1/4” green to 1/2” green to tight cluster to pink in apple trees, temperatures in the upper 20s can cause considerable harm to an early blooming tree. Near bloom, the range between slight and severe damage can be very small. So the stage of bud development determines how susceptible any given fruit crop is when freezes occur.  For more information on what those critical temperatures are that can cause freeze damage to trees during development, I have uploaded two charts from Utah State University that you can download in PDF format by clicking on either chart above.

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Spraying Guide and Spray Timing

It's time to actively examine a spray protocol  for your fruit trees! The question everyone needs to ask before spraying is “Do I want to spray or not?”   Well, unfortunately, in northern Illinois we have four main pests that we will almost always have to spray for.  The decision to spray or not depends on how much fruit loss you are willing to take. That is your threshold.  If you can accept some fruit loss, then the need to spray diminishes greatly.  But if you only have a few trees and some fruit damage may mean losing half your fruit, then spraying becomes more important.  Let’s meet our top four pests in northern Illinois and southern Wisconsin.

The four main pests that we face here in our climate zone are apple scab, plum curculio, codling moth and apple maggot, in that order. But how do we know when to spray for them if it is a last resort to protecting our fruit?  

All tree fruit have several distinct growth stages as the fruit matures.  Knowing and identifying those growth stages is  very important for the home grower because recommendations and spray timing for spray applications are linked to these specific growth stages.  The chart below shows the common growth stages for apple trees.  

Since the average home grower does not have access to their own weather stations or degree day calculators, the fruit tree development stages play an ever greater  role in pest management for the average grower.  Most spray schedules (protocol) follow the tree development stages to aid in the timing of sprays so they are most effective.  It is important to note that many diseases and some insects can only be controlled by spraying before they can be seen like apple scab.  Spraying less frequently or at the wrong time will typically result in poor results.  And, spraying more frequently will not necessarily give greater control. 

The tree developmental stages or tree phenology gives us a guide as to when to spray, but what do we spray if we have to spray?  If we consult some of the various spray guides available to the home grower, we will find that most of the spray guides provide us with the tree’s development stages (phenology) and the insects or diseases that frequently  occur during each of theses stages.  So the tree phenology serves as timing guide when the application of a particular spray is recommended in order to control specific insects or diseases at the right time.   The following spray guide for apple trees will give us the time to spray based on tree phenology, the pest to spray for and the product recommendation for that pest(s).

This particular spray guide is included in “Managing Pests in Home Fruit Plantings” from the Purdue University Extensions Publication web site as a free download.  It goes into detail as to the various products available for spraying that include both conventional and organic alternatives.  The publication includes apples,  pears, peach, cherry, grape, strawberry and raspberry guides as well as the phenology charts for each fruit type.

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How to Do a Scratch Test on Trees & Plants

by Stark Bro's

Here is a great article from Stark Brothers Nursery on testing your fruit trees or vines to see if they made it through the winter. 

When spring finally arrives, plants and trees start to wake up, and the gardening world gets exciting! It is also the time of year to evaluate what survived the winter. One common misconception this time of year is that all trees and plants wake at the same time. When we compare dormant plants and trees to things that are already waking in our gardens, it’s like comparing apples and oranges.

Today, we’re going to equip you with one of the most handy tools that you can use to determine what’s still living: The Scratch Test.

1. Scratch Test: Fruit Trees

The most telltale way to determine if your young dormant trees are still alive is by checking the cambium layer under the bark. The task is simple!

What you need:

• Your thumbnail or a smooth knife

How to do a scratch test:

1. With your thumbnail or knife, lightly scratch a small spot into the bark of the tree’s trunk (in a location about halfway up the tree).
2. Look for wet tissue beneath the bark layer that is scratched back. It should have a greenish hue — this is living tissue.

If you find that the cambium layer beneath the bark has become dry, brittle, and brown, then it indicates that the tree has failed to live.

Top: living tree; Bottom: dead tree [click images to view larger]:

Sometimes, after performing a scratch test, you may discover the tree’s trunk shows no signs of life even though new growth still sprouts from the roots. This happens in grafted trees if the top-portion (the grafted variety) dies while the rootstock goes on living. If this happens, then it is best to replace the tree. Letting just the rootstock grow will result in a tree that lacks the qualities of the grafted variety you originally chose to plant. Rootstocks are used for characteristics like dwarfing and hardiness, and are often not ideal candidates for fruit-bearing trees.

Things to avoid when performing a scratch test:

• Do not cut a large wound into the tree to determine whether or not it is living. A small spot will suffice.
  • Large wounds cut into your tree will require more effort to heal over.
• Do not perform a scratch test on a branch/limb of the tree.
  • Testing the trunk is necessary. Limbs can break/die without determining the status of the rest of the tree.

2. Scratch Test: Berry Plants & Vines

For berry plants and vines, you can still attempt a scratch test to identify the living tissue. Simply follow the steps above for a scratch test on trees, and adjust to accommodate the size of the berry plants’ canes or vines. Pick a spot on the young cane/vine that is a few inches above the soil level to scratch.

 

Many berry plants send new growth up from their roots, so a dead cane may not determine a dead plant; however, a living cane will determine a living plant. With that in mind and, since some plants may feature thorns, you may prefer pruning in order to look for living tissue in your berry plants and fruiting vines.

3. Prune Back: Berry Plants & Vines

What you need:

• A pair of sharp and clean pruning shears

How to prune to check for living tissue:

1. With your pruners, cut from the tips of the canes, working your way back toward the ground.
2. Cut the canes back little by little and check for signs of life after each cut.
3. Stop cutting back once you reach green, living tissue.

You are still looking for greenish, wet, living wood, but it may be necessary to cut dry dead tips back to find it. Cutting this dead wood away also helps the plants to sustain healthy growth during the growing season.

If your berry plant exhibits no living tissue in the canes above the ground, and you are also not seeing any new growth sprout from the root system well into the growing season, then your berry plant is likely no longer living. Replacing the berry plant is recommended in this case. It would also be a good time to assess water drainage, soil nutrients, and soil pH in that location if the cause of death is unknown.

Now that you know how to check to see if your plants and trees are living, it’s also important to know that, if they are still dormant, they simply need more time to break dormancy — especially if this is their first growing season in your yard! Each year, the seasons and the weather are slightly different, but living plants and trees will always wake up when they’re ready.

 

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