Temperature required to hold dormancy--is 37 degrees enough?

[rockm]

Besides the temperature, the number of sunlight hours to which the tree receives daily also affects to the dormancy.
Bonhe

Light only "affects" dormancy in late summer into fall as leaves drop. Shortening days drive that preparation process. Once a tree has no leaves, it CANNOT SENSE LIGHT. Lengthening daylight in late winter and early spring don't have much, if any, affect on dormancy.

Emergence from dormancy relies pretty completely on soil temperatures. Once the soil temp rises above 40 or so, roots become active.

Soil temp is a much more reliable "sensor" for trees than day length hours in the early spring. Relying on soil temp helps trees avoid being suckered into pushing new leaves if winter temps are still around.

 

[GrimLore]

The bonsai nursery where I overwinter my live oak maintains their cold greenhouse at 35 F after letting it fall into the high 20s in December for a week or so.

You spiked my curiosity. I am "guessing" they also control humidity and airflow to prevent problems or no?

Grimmy

 

[rockm]

You spiked my curiosity. I am "guessing" they also control humidity and airflow to prevent problems or no?

Grimmy

The greenhouse has fans to vent heat build up in the winter. Gravel floor is wetted down occasionally to prevent it from being too dry. On the warmest days, they also have to move trees outside. It's a pain in the butt with the rollercoaster winter temps in Central Va.

 

[GrimLore]

The greenhouse has fans to vent heat build up in the winter. Gravel floor is wetted down occasionally to prevent it from being too dry. On the warmest days, they also have to move trees outside. It's a pain in the butt with the rollercoaster winter temps in Central Va.

Thank you and as I suspected it is no more or less work then a good Tropical setup with the only real difference is light as the temperature control is a no-brainer. Having the room here this year for Tropicals has been a good experience so far and we are adding 40 plus feet of shelf and light this Summer while the room is empty. The whole picture in both situations is important and others should know it is not just temperature.

Grimmy

 

[rockm]

It's important to understand that dormant temperate zone plants respond to SOIL TEMPERATURE not light, even in a cold greenhouse.
Dormant plants are not responding to light levels in a greenhouse, they are responding to heat at their roots.
Light is energy. Trapped in a glass-enclosed space, light is converted to heat.

 

[crust]

Cheap alternative - a blueberry grower I know chills fruit from the field to 34 F using a big window airconditioner. He simple wired a new thermostat into a standard window air conditioner. He is able to get a 20 x 15 x 10 ft room down to 34 F in July in Michigan. Uses a air condition with a rating for 3 rooms. All off the shelf part. I'll try to find info on the Thermostat he installed.

Oh! please do get me the technical info--this sounds very encouraging. The Coolbot unit is just a little mysterious electronic box that you hook up to an AC that allows lower temps somehow--I think its like $350.00.

 

[crust]

FWIW, this can be dependent on species. Rule of thumb is to maintain a root zone temperature of BELOW 40 F to maintain dormancy in most species. I would think that 40 is a bit too high for some stuff. I've got amur maples for instance that are pushing buds with temps consistently in the low 30s here. Sustained at 40, they would leaf out in a week or so.

The bonsai nursery where I overwinter my live oak maintains their cold greenhouse at 35 F after letting it fall into the high 20s in December for a week or so.

Interesting--my honeysuckles do similar things( budding below 40) but in the craziness of spring I have never really noted the exact temps.

 

[JudyB]

Oh! please do get me the technical info--this sounds very encouraging. The Coolbot unit is just a little mysterious electronic box that you hook up to an AC that allows lower temps somehow--I think its like $350.00.

I just wonder if a regular room aircon unit would be able to handle working and not break in the winter cold temps. Most of these applications seem like they are based on making cold during summer.

 

[0soyoung]

It's important to understand that dormant temperate zone plants respond to SOIL TEMPERATURE not light, even in a cold greenhouse.

Bud break is controlled by the temperature of the bud. The leaf/flower emerges because of cell enlargement by osmosis of water. Cooling the roots reduces the rate of water loading (into the xylem) which, in turn, prolongs the emergence of foliage/flowers.

 

[rockm]

Bud break is controlled by the temperature of the bud. The leaf/flower emerges because of cell enlargement by osmosis of water. Cooling the roots reduces the rate of water loading (into the xylem) which, in turn, prolongs the emergence of foliage/flowers.

so it kind of goes back to soil temperature and root activity.

 

[0soyoung]

so it kind of goes back to soil temperature and root activity.

It isn't a subtle point. Buds do not respond to spring soil temperatures. Buds respond to the air temperature they are in. Prove it to yourself:
Take a dormant wood cutting and bring it inside. There are no roots, but buds will crack and open.

 

[crust]

I just wonder if a regular room aircon unit would be able to handle working and not break in the winter cold temps. Most of these applications seem like they are based on making cold during summer.

It will only be used for cooling during warming weather in the spring so it should be OK--during the depth of winter I never have problems keeping the storage cool enough.

 

[JudyB]

It will only be used for cooling during warming weather in the spring so it should be OK--during the depth of winter I never have problems keeping the storage cool enough.

I do understand this, it is what I too am searching for. But will the mechanical parts of the unit work at 42º after being out in 05º temps a few hours before? Such are the swings of temps we have here,(as probably do you) and the rigors of a machine built for summer use, would be required to make the plan work. Just trying to find the answer to this, as I am planning to perfect my system with the addition of such a unit this year.

 

[MinnesotaKirk]

I feel ya Crust. That burst of warm weather we on Saturday had buds swelling in my cold frame, especially the tridents. I keep it between 35 and 38 all winter when it is cold out and everyone stays dormant. Things should be quiet this week but are going to heat up again this weekend.

 

[bonhe]

Light only "affects" dormancy in late summer into fall as leaves drop. Shortening days drive that preparation process. Once a tree has no leaves, it CANNOT SENSE LIGHT. Lengthening daylight in late winter and early spring don't have much, if any, affect on dormancy.

Emergence from dormancy relies pretty completely on soil temperatures. Once the soil temp rises above 40 or so, roots become active.

Soil temp is a much more reliable "sensor" for trees than day length hours in the early spring. Relying on soil temp helps trees avoid being suckered into pushing new leaves if winter temps are still around.

I don't think so! Day length and temperature are critical environmental factors that affect the plants.
With those leafless trees which I placed in the same direction of the garden, the trees received more indirect sunlight will wake up at least 2 weeks before the trees received less indirect sunlight! I already documented this observation for more than 3 years!
You can do this simple experiment: put two leafless tree pots in the place without direct sunlight in the winter time. One is covered with dark paper bag, one is not covered. Let see which one will wake up first.
Bonhe

 

[rockm]

I don't think so! Day length and temperature are critical environmental factors that affect the plants.
With those leafless trees which I placed in the same direction of the garden, the trees received more indirect sunlight will wake up at least 2 weeks before the trees received less indirect sunlight! I already documented this observation for more than 3 years!
You can do this simple experiment: put two leafless tree pots in the place without direct sunlight in the winter time. One is covered with dark paper bag, one is not covered. Let see which one will wake up first.
Bonhe

Um, OK. How often does the soil freeze in Riverside, Cal.?

I could compare notes with trees in indirect and direct sunlight in my backyard here in Va. I've been observing my trees for about 25 years now. Trees that stay in the shade remain colder than trees in sun. The trees in partial to full sun break buds much MUCH earlier than those in shade. That's because the soil and mulch around the trees in the shade stay much colder than the soil and mulch around trees in sunlight.

Lagging the soil temp is one of the tricks bonsaiists that have real winter have learned to use to prolong dormancy to avoid early bud break.

 

[Leo in N E Illinois]

I know plenty of people who leave standard window air conditioners in their windows all winter. Blow the leaves out of them in spring and then they fire up just fine. Some units are "reversible" in that they can be used to heat the room too. They all function fine, even though their working parts are hanging outside in our frigid northern winters. In Crust's application, he would not be turning on the unit when it is -25 F outside, it will be turned on when its over 35 F. I doubt he'd have any trouble. Window air conditioners are designed to have the bulk of the unit exposed to the weather.

When trying to use a room air conditioner to cool a space to near 32 F or cooler, the only issue might be if the space being cooled is humidity, ice might build up on the coils inside the space being cooled. But if the thermostat is set above 32 F, say at 35 or 37 F, when the unit cycles off for a spell, say at night, the ice will melt and drip off. Doubt it would be a big issue.

I do feel some alpine and northern species, the zone 4 & 3 hardy species, it is likely that 40 F is not quite cold enough to hold dormancy. I have seen larch, & some alpine plants start to wake up and grow at temps in the 38 to 42 F range. But growth at 40 F and lower is quite slow, The purpose for chilling the cold storage room is to stall the breaking of dormancy. Something that is only needed during the warm spells in late winter before the normal "last hard freeze date" for the area.

 

[JudyB]

My problem isn't trying to keep it cold longer in spring, mostly my problems are when we have prolonged too warm temps in the middle of winter. I go ahead and let them wake early in spring if they want, and just keep the temps up and fire up the lighting. I'd love to have more info on what your berry grower uses @Leo in N E Illinois
Thanks ahead.

 

[crust]

I do understand this, it is what I too am searching for. But will the mechanical parts of the unit work at 42º after being out in 05º temps a few hours before? Such are the swings of temps we have here,(as probably do you) and the rigors of a machine built for summer use, would be required to make the plan work. Just trying to find the answer to this, as I am planning to perfect my system with the addition of such a unit this year.

You have slightly different weather and facility. My facility stays pretty cold even with winter fluctuations because I let it freeze solid in there and because it has R 32+ wall and R 47 ceiling insulation and only one small window and a gasketed insulated steel entry door so it holds cold like a icebox but once there is sustained 50's its starts creeping up. During colder evenings I use a ventilation fan to draw in the colder night air. I definitely am using it to delay early spring wake up--but never have mid-winter problems. I would contact Coolbot and give them your skinny and they could advise you on your concerns, I would be tempted to say putting in a 2 Ton mini split unit with a Coolbot controller may be a premium way to go but perhaps a regular refrigeration unit would be necessary--my research has not gotten to that point yet. So far most every example of working cooling units I come across are ones in rooms built as coolers with heavy foam insulation and no windows--and smaller than my set-up. I will keep you posted if I get more intell.

 

[bonhe]

rockm, Um, OK. How often does the soil freeze in Riverside, Cal.?
Well, tricky question!

Trees that stay in the shade remain colder than trees in sun.
Agree.

The trees in partial to full sun break buds much MUCH earlier than those in shade.
Agree.

That's because the soil and mulch around the trees in the shade stay much colder than the soil and mulch around trees in sunlight.
Well, this conclusion makes me feel uneasy! If I was you, I would be more careful in design the method for comparison!
Bonhe