length of nightly cool down and plants

[flplantguy]

Like the title says, Im wondering about the length of time a given location reaches its low (high) temp for the night (day) and the effect on plants in the garden. If the time is only minutes (like a light freeze) does it have any effect at all?  If a plant likes cool nights and its warmer before a small drop at sunrise can you even count it?  We average out numbers for plants in heating and cooling degree days for this purpose but its more for farming and less for plant microclimate.  In Florida inland locations cool more than the coast and warmer nights are an issue on the water, but what about the adjacent lands that hit lower temps for short periods?  How does that effect the ability of a plant to metabolize effectively, or is there a way to offset the negative impact?  Im also thinking hot concrete in the city in summer making the 74 at dawn last 5 minutes instead of 5 hours.  We talk benefit in winter but what about summer and excess heat?

[kinzyjr]

The duration matters significantly, although an absolute low can impact the duration as well.  As an example, it it is 27F in one location and 30F in another, it will almost always take longer for the colder location to climb out of the hole and get above freezing.  On the topic of record freezes, a lot of microclimate factors go out the window and you just get what you get.  Along the I-4 corridor and the record lows tend to stay between 15F-20F.

9 hours ago, flplantguy said:

If the time is only minutes (like a light freeze) does it have any effect at all?

A few considerations in this case:

• How quickly did the temperature rise? 
• If we toss out the minimum due to duration, what should we do to provide a more accurate mark?

For my own use: Should you count the record low? - Yes, and you should also consider other metrics like hours below 32F, 28F, 26F, ... others may have differing valid opinions on the matter.

9 hours ago, flplantguy said:

In Florida inland locations cool more than the coast and warmer nights are an issue on the water, but what about the adjacent lands that hit lower temps for short periods?  How does that effect the ability of a plant to metabolize effectively, or is there a way to offset the negative impact?

It usually shows up as increased foliar damage, especially with frost.  Some of the weaker plants kick the bucket, but they were on borrowed time regardless.  It also depends on exactly how low it gets.  If the coast is 34F and it is 28F four miles inland, then you'll see a very drastic difference in the amount of damage and death.

9 hours ago, flplantguy said:

Im also thinking hot concrete in the city in summer making the 74 at dawn last 5 minutes instead of 5 hours.  We talk benefit in winter but what about summer and excess heat?

The street plantings here have to be irrigated more to establish because of this.  Inside of the city limits, different PWS have recorded somewhere between 10 and 15 days in June over 95F.  The airport outside of town has a total of 3.  A DVP2 nearby had 11 and other weather stations in the area that were in the same range.  Going on the low end, that's still 7 more days above 95F.

How does it impact the plants?  Some don't mind it, but Veitchia species in particular do not like when it is hot, windy, and dry with low humidity - they'll show some sunburn here.  Coconuts love the additional heat and push spears like they were gene-spliced with bananas.  Everything is situational.  We've had nearly 13 inches of rain, so the negative effects have been somewhat muted and the positive effects amplified.

A few metrics of interest in regard to cold duration:

• Freezing Degree Hours: https://www.palmtalk.org/forum/topic/9124-freezing-degree-hours/
• Days Below 40F: A potential metric for tropical gardening suggested by @RedRabbit and incorporated into the almanacs posted on the Florida Freeze and Weather Station Data.

NOAA has ongoing studies concerning UHI, but not sure how much of this is helpful in calculating benefit/cost of the additional heat: https://www.heat.gov/pages/mapping-campaigns

[flplantguy]

I think the most important aspect of this going into the future is how to mitigate hotter nights.  As urban areas and concrete expand it will only get worse and if plants cant cool off at night they will suffer like we do.  The benefit in winter has a ceiling in summer also, but unlike all our cold weather protection strategies we dont have much we can do for hot nights to keep our plants happy. Shade, water, and careful fertilizing can only go so far.  What else can be done to summer "zone push" for night temps, or overcome urban heat island imapcts? Not much that i know of especially for plants that also need long days as well as cool nights. Anyone know of any tips or tricks?

[flplantguy]

"If we toss out the minimum due to duration, what should we do to provide a more accurate mark?"

This question is a good one for summer cooling and plants that like it cooler at night during their growing season also.  Plants that grow cool can survive, to varying degrees, temps above their prefered range, but we usually mean daytime not hot nights.  An 80 degree night here can be followed by a 88 degree afternoon, so which one is more damaging?  That diurnal range is key for so many species; what can a gardener do to offset the negative impact of it being less than ideal?  Here its a warm and humid night, in the desert warm (or hot even) and dry nights happen.  A metric for tolerance of minimal diurnal range and duration perhaps, meaning a plant can handle 4 months of nights that are only 10 degrees (F) cooler than daytime or less. A constant hot tropical island would kill this plant but a summer in florida would not.  We have species like this in the subtropics, but i lack the plant physiological knowledge as to what allows for some tolerance as opposed to none. 

[Silas_Sancona]

13 minutes ago, flplantguy said:

"If we toss out the minimum due to duration, what should we do to provide a more accurate mark?"

This question is a good one for summer cooling and plants that like it cooler at night during their growing season also.  Plants that grow cool can survive, to varying degrees, temps above their prefered range, but we usually mean daytime not hot nights.  An 80 degree night here can be followed by a 88 degree afternoon, so which one is more damaging?  That diurnal range is key for so many species; what can a gardener do to offset the negative impact of it being less than ideal?  Here its a warm and humid night, in the desert warm (or hot even) and dry nights happen.  A metric for tolerance of minimal diurnal range and duration perhaps, meaning a plant can handle 4 months of nights that are only 10 degrees (F) cooler than daytime or less. A constant hot tropical island would kill this plant but a summer in florida would not.  We have species like this in the subtropics, but i lack the plant physiological knowledge as to what allows for some tolerance as opposed to none. 

This should be easy to answer .. = Evolution. 

Plants have spent many thousands / tens / hundreds of thousands ...etc... of years being put through evolutionary filters which influence every environmental stressor they can endure.. 

That said, there are two opposite areas of the globe where many of the plants within them exhibit similar levels of non-flexibility.  Plants from the true arctic and true tropics, where climate-related conditions don't vary much over the span of a year / years, etc. are often the least adaptable outside of where they originate. Why? ..circles back to how they evolved ..or, ...didn't evolve ..beyond the basics.

This is why, for example, if X plant evolved in a cool evening cloud forest climate, it will likely melt if subjected to numerous nights above X threshold temperature ..say the 80F you mention..  Unless you have tons of money, very difficult to give that plant the nights it would prefer through the summers there.

Humidity is another factor.. Plenty of plants here and/ or in CA that can tolerate hot nights,  ..if the air is fairly dry..  Flip it and there are numerous plants that ..take away the humid nights, or humidity for days on end and they may struggle ( Palms from an island near the equator in the south pacific, growing in California, for example ) .. simply give up after a given amount of time, or just grow at a slowed down pace.  Again, evolutionary factors at play.

Yes you can adjust the environment in a yard to extend the adaptive capabilities of certain things that may struggle w/ out some deg. of tinkering, but ..there's a limit, imposed by ..Evolution. 

Take a Kentia.. Yes, they can be grown in FL.. but, the likely outcome is once they reach above a protective canopy, into full FL sun, chances are they are they will decline and croak. That's how it goes.. No amount of babying will change how that Kentia responds to the limits imposed upon it by evolution, not in one -or a dozen- 125 year human lifespans anyway. 



 

[flplantguy]

One example i was thinking of is the plant using chemistry to accomplish what we are discussing. The chemistry of the plant and its metabolism dictate what temps it grows the best in.  Then add in dormancy adaptations and there you go. If a plant lacks one or both it dies quickly or slowly depending on the process that is disrupted. A good example being brugmansia sanguinea.  Once temperatures go above 75 it stops flowering but doesnt necessarily die at that temp.  it has some process that cannot continue after that. 

As we enter times with hotter weather i can picture companies advertising products to take advantage of people and having this information here for people to consider may help.  Here in florida it happens all the time that plants are not sold that will thrive and people lose their drive for planting.  or they grow fine as a winter annual and die in summer, harboring diseases and spreading problems.  

On the flip side there may be some kind of chemical treatment to help a plant deal with a slight but constant issue it cannot handle well that disrupts its processes.  Hot nights bursting ribosomes in the cell or something like that.  Combine all of it and i see a lot of problems in the future with false hope and bad info leading people astray.  I may plant a plant i know is iffy here as an experiment but i am well aware if those issues, if someone isnt and they buy a plant it can be very disheartening for them.  Then add in a neighbor who CAN do it for one reason or another (soils and majesty palms here come to mind) and people blame themselves and give up completely.  This understanding of the process per individual plant cant be simplified since they are all different, which is why we tend to guess at a plant until we know.  But we cant say for certain: "plant X melts in nights above 75 due to chemical Y being unable to form during process A so process B cannot occur and the plant cannot divide cells and dies"-its different in every plant. 

Last night i wrote this with the idea of a clear path to encourage heat tolerance in plants for the future, but it was late enough i think my brain wanted a "key" to it all, and just like hardiness zones there really isnt one.  It would still be nice to know why certain plants are so dramatic about it though lol.

[flplantguy]

Found the answer to my own question in a study looking for what the plant pathways are for heat tolerance and its a doozy.  The takaway is they have a basic map of a highly complex process, but no useful data yet.  Its needed for food production in the future and any finding should apply to ornamentals also.  Food insecurity could lead to ways of keeping endangered species alive in less favorable locations but its a long way off. 

[kinzyjr]

On 6/26/2023 at 10:46 PM, flplantguy said:

I think the most important aspect of this going into the future is how to mitigate hotter nights.  As urban areas and concrete expand it will only get worse and if plants cant cool off at night they will suffer like we do.  The benefit in winter has a ceiling in summer also, but unlike all our cold weather protection strategies we dont have much we can do for hot nights to keep our plants happy. Shade, water, and careful fertilizing can only go so far.  What else can be done to summer "zone push" for night temps, or overcome urban heat island imapcts? Not much that i know of especially for plants that also need long days as well as cool nights. Anyone know of any tips or tricks?

These are good questions that I'll offer my suggestions.  My garden has a lot of subtropical 9b/10a species like Veitchia, Archontophoenix, Adonidia, and coconuts, while also having over a dozen Trachycarpus fortunei and and 5 Howea forsteriana.  The requirements for these palms are night and day, and you can't keep everything perfectly happy.  The 9b/10a stuff appreciates the heat island effect, stone landscaping, sheltered siting and elevated planting that tends to mute radiational cooling. 

So how do you make oceanic and temperate palms grown in an area where 95F is an average daytime high?

The short answer is you're never going to meet all of their desired conditions.  @Silas_Sancona gave a great explanation about how thousands of years of adaptation have selected the strongest growers for a given set of conditions that might be very different than ours.  While I have luck with Howea forsteriana and Trachycarpus fortunei, my work has not rewarded me with a formula that will work for Jubaea chilensis.  For those species where it has worked, you mentioned shade, water, and fertilizer as conditions you could control that would assist in survival. 

These are some other considerations:

• Soil conditions:
  • What does the palm prefer in its native habitat in regard to pH and soil makeup?
  • Sandy soil tends to cool before other more moisture-retentive soils - that may fall to your advantage.
• Airflow:
  • Is it possible to get good airflow through the area in the summer without making advective freezes too bad?
  • Does your property have grade that could assist in funneling cooler air to a low spot?
  • Are there areas where you can space out plantings to filter light while also allowing the appropriate cooling to take place when the wind blows?
  • Is there a natural "cold spot" in your yard that lends itself to these types of plantings?
• Pests:
  • Since the plant is out of its preferred conditions, it tends to become more attractive to pests.
  • Nematodes can be especially devastating to out-of-area plants.  I've heard concrete is a good nematode repellent - but you have to keep it from absorbing light and storing heat if you use it in this manner.
• Follow up on heat island mitigation factors (https://www.heat.gov/pages/urban-heat-islands)
  • Summary:
    • Use less concrete/asphalt/brick since they have a relatively high heat capacity
    • Keep the colors light to lower albedo
    • Block the light from reaching those surfaces with vegetation so less is absorbed

It's an uphill climb and no two experiences are the same.  If anyone else knows some good ways to reverse zone push, please share.

[D. Morrowii]

29 minutes ago, kinzyjr said:

It's an uphill climb and no two experiences are the same.  If anyone else knows some good ways to reverse zone push, please share.

I was too lazy to pull the twin wall polycarbonate panels off the top of the back pergola (winter time protection). It started getting super hot under there last week even with airflow on 3 sides. On Sunday I put a fan on a thermo switch that goes on when temps reach 90F and turns off when it gets down to 87F. So far its working for the container farm. There are also misters under there that turn on for 7 minutes twice a day.
I’ve thought about finding some solar powered mini fans to put under the more rain foresty type palms while they are small. Not sure it would work but it might be a fun project to try. 

[flplantguy]

I have a ton of WNW wind flow in the yard from the open spot on that side but not on the east or south, so only cold fronts can get in and seabreeze.  I wouldn't try to grow an oceanic palm here lol, even of they had a spray or treatment for heat tolerance.  I think it would be for extreme events only, or to help a plant that is marginal for a short time in summer.  With the crazy heatwaves lately even native plants may get a benefit from that kind of care especially in the urban heat islands.  Magnolia is one example here that may get pushed out over time in the cities.