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There is a common problem with fruit trees in temperate climates where the trees flower before the last frost. Generally speaking, if the tree has flowered, and the temperature drops below freezing, the flowers die and do not produce fruit.

This is certainly not a new problem. It's my understanding that the problem affects almost the entire fruit farming industry in temperate climates.

I've heard that fruit farmers have large-scale commercial techniques that they use to mitigate the issue, such as:

  • Covering
  • Fog or smoke
  • Wind Machines
  • Sprinkling
  • Heating

While these methods might be practical for commercial farmers, not all of them are practical for small-scale urban farmers like me.


One technique that us gardeners often wonder about is:

Instead of actively protecting the open flowers, would it be possible to trick fruit trees into not flowering as early? In other words, keep the ground frozen longer than it would be naturally, to delay flowering (I'm making an assumption that trees don't flower when the ground is frozen).

While such a technique would likely be impractical for large-scale farmers, it might be resonably practical for small-scale gardeners.

Note: I imagine that there are more factors at play than just ground temperature. For example, the amount of sun, angle of the sun, and the temperature of the air might also be factors when it comes to the flowering timing.

I suspect that prolonged frozen ground conditions could significantly delay the tree from flowering (maybe by a couple of weeks) which would make a huge difference when it comes to avoiding late frosts from killing flowers.

Can I prolong frozen ground conditions to trick fruit trees into flowering later?


Idea:

An idea would be to insulate the ground around the tree with snow. The theory being that the insulation would prevent heat from the sun from warming up the ground where the roots are.

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However, an obvious problem with snow is that it melts, and not always when you want it to. From past experience, I'm guessing that this pile of snow will mostly melt before it is helpful to me (before late spring).

Edit:

I'm dealing specifically with plants in the prunus genus:

Examples:

  • Prunus tomentosa
  • Prunus besseyi x prunus salicina hybrids
  • Prunus avium x prunus salicina hybrids
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    $\begingroup$ Research “ice house” as many houses had a stock of ice collected in winter to keep things cool before refrigerators were invented. $\endgroup$
    – Solar Mike
    Commented Dec 30, 2017 at 16:51
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    $\begingroup$ You should also consider keeping the sunlight off... $\endgroup$
    – Solar Mike
    Commented Dec 30, 2017 at 16:55
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    $\begingroup$ You might want to check with fruit tree experts regarding whether putting a lot of snow for prolonged periods against a tree trunk will adversely affect the tree. A large cold mass against the bark for a long time might affect how water & nutrients from the roots travels up the trunk, after all, the critical part of the trunk is just below the bark - ring bark a tree & it dies. $\endgroup$
    – Fred
    Commented Dec 31, 2017 at 1:48
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    $\begingroup$ One crazy idea, dig a circular ditch around the base of the tree prior to the onset of freezing weather & fill it with a water gel & cover it with soil. As the ground freezes so does the water gel. Insulate the top by piling snow on it & keep the sun off the snow via some form of light colored covering. $\endgroup$
    – Fred
    Commented Dec 31, 2017 at 1:52
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    $\begingroup$ In area where I grew up, morning frost was very common followed by sun which killed vegetable crops. The farmers would water the plant before the sun come up thus melting the frost off the plants. You might want the try this technique. I understand fruit trees are taller than vegetable crops. but just a thought. $\endgroup$ Commented Jan 2, 2018 at 3:35

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Delaying flowering would definitely be a better solution than attempting to save the flowers after the fact. Its definitely going to take some experimentation and hypothesis testing to determine what works for that tree variety.

I have done a bit of plant research for an indoor aeroponic system that I operate with strawberries. Putting them in the refrigerator allows me to trick them into a winter cycle. I have read that most plants also have a process that measures the length of darkness that tells them when to grow and when to flower. Not sure if this occurs in the tree stems/buds or not; but regardless it is very hard to control the ambient lighting of a tree.

As far as your technique for keeping roots cold; I think piling snow is a great idea. You can then cover the snow with straw, sawdust, cardboard, blankets, or other to help insulate it and make it last longer. You could also use a low flow rate sprinkler or mist nozzle at the base of the tree on a cold night to make some additional ice.

Note that the ice will not keep the ground "frozen" because soil has salts that make it freeze at a temperature below that of clean water. However it will certainly keep the soil colder for longer.

If possible, it would be good to test it on one tree while leaving a the same variety of tree alone as a control. This way you will be able to roughly see how many days you were able to delay flowering.

Perhaps something additional to test is cooling the whole tree. A large amount of uninsulated ice in the surrounding area will create a temporary micro climate that will keep the whole tree cooler for longer. Shading the tree from direct sunlight, and blocking from wind would increase this effect.

Good Luck! Please share a link to your results, I'd be interested to see how it turned out.

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One thing that comes to my mind are ice rink coolers. You could design similar pipe system that would keep surrounding ground cool. Few problems that could occur: - pipes are filled with antifreeze and leaks could cause ecological disaster and forbiden substances would penetrate fruit so you should find ecofriendly antifreeze - you need strong cooler unit which will have high energy consumption so using insulation like styrofoam or mineral wool would be advisable -together with building adjustable shutters or sun blockers, price of such system would be high and financial study should be done -using energy from sun to power such system would also increase initial investment maybe other alternative energy sources would be better

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    $\begingroup$ Some people also paint fruit tree white using lime because white color is not absorbing as much sunlight thus preventing early vegetation and because lime (calx) kils vermin. $\endgroup$
    – Katarina
    Commented Jan 4, 2018 at 17:38
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Options:

Site location:

NE slopes get spring sun at a lower angle of attack, and so warm up more slowly.

In addition NOT being at the bottom of the slope may save you from pools of cold air.

Sprinklers.

Set up sprinklers on tall poles. I suggest using Senniger wobble head sprinkers as they have very large droplets, which take longer to freeze. Turn them on when temps are about +4.

You need a water supply sufficient to run them for all the trees/shrubs at once.

Deep freezing + insulation

In the winter pack the snow down with skis or snowshoes. This will allow the ground to freeze deeper.

As soon as the snow is gone, spread insulated tarps (about $20 used for a 10x20 tarp). Try to get white ones.

In addition at the end of winter when the cold is nearly done, use your snowblower to bury the trees 2-3 feet.

Roots generally require a tempurature of 5-7 C before they get active.

Shading.

South and west of your plants erect poles that are tall enough to hang shade cloth. Shade cloth comes in widths up to 15 feet and densities up to blocking 70% of light. I would suggests stringing high tensile strength wire on the tops, and use common curtain rings to hang the cloth. Set a grommet about every 16" in the edge of the cloth. Available at anyplace that sells greenhouse supplies.

Pruning

One of the problems with prunus is that pollenizers may not bloom at the same time as the fruit. One way around this is to deliberately leave some branches on pollenizers low to the ground, so that you can bury them in snow to delay their bud break. Sandcherry is used this way for hybrid plums.

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