Cooling the plant on hot days

HedgedAndLevered · Aug 28, 2023

I have a plant growing on my roof, full sun all day. Looks like its going to get above 85 degrees (reaching up to 100 degrees at the peak) from around 11AM to 9PM a few days next week. I work from home and so I can actively manage cooling.
What's the best way to do this? I was thinking ~ a quart of 40 degree water every hour? Maybe 2 quarts?
Should I mist the plant too? kinda worried about the magnifying-glass effect from water droplets if I go that route though...?
Should I shade the plant for an hour or so near peak sun? It is in the early stages of flowering so I want to avoid that if necessary.
In general its a pretty big plant so it should be pretty robust. Still, we had a couple days of 100 + humidity last week and the leaves just finished getting un-curled yesterday.
Unfort last week I was out of town so I just had a friend do 2 gallons of room temp water in the morning, didn't really cut it.
it reached 112 real-feel considering the humidity though so that was quite extreme.

imgrowing · Aug 28, 2023

1st thought is, why on the roof?
2nd thought is, cold water every hour might shock the plant into hermy
3rd thought is, why on the roof?

HedgedAndLevered · Aug 28, 2023

1. Because I have zero space inside my penthouse to put up a grow tent and I have no legal place to grow it outside
2. This is what I'm asking about essentially. What is too extreme of a temperature gradient? Cannabis is fine down to 40 degrees for quite a while, but is it a problem to *rapidly* cool the soil from ~80 degrees to ~50 degrees?
3. Because I have zero space inside my penthouse to put up a grow tent and I have no legal place to grow it outside

GanjaJack · Aug 28, 2023

So I hear they're developing a new strain... it's called

Cacttabis...

Trash_2002 · Aug 28, 2023

You could use a greenhouse shade, there's many levels of shade screens/nets 15%/30%/50% etc to cut down sun intensity and also temperature.

imgrowing · Aug 28, 2023

only thing i can think of is keep it watered with normal temp water, and lots of it.
maybe next round try a strain that is from the tropics and can handle hot temps.
wish i could help more but i know nothing.
i'm just one of those guys that spout shit off on the internet because it makes me feel smart

Oldchucky · Aug 28, 2023

Let’s see some pictures of the plants. And the size of the pots.

Zill · Aug 29, 2023

Hedge,

Never, ever use cold water to water your plants. It completely shuts down the plant until the root temperature returns to normal. I learned that years ago working in commercial flower greenhouses.

Zill.

HedgedAndLevered · Aug 29, 2023

thanks!
ok so what temperature is "cold" though? I'm seeing optimal range at 65-85 degrees, so considering this will be done in the 90s, 50 degree water should bring the soil to ~65 degrees. Might cool the plant a bit more for an hour or so but would get back up to temp pretty quickly. Good plan?

its only a couple days for a few hours I'll need to do this so if it needs to delay the development a day or so, its probably better to be from cool roots than from heat stress...? looked pretty bad for 4-5 days after the hot day last week, started to look good from the bottom up at a rate of about 1 node per day... could have been worse though, that day ended up killing my neighbor's plant...

Zill · Aug 29, 2023

Hedge,

Maybe I missed something - Are you watering the plant with cold water to reduce the temperature of the plant? That doesn’t work.

The one thing that could transiently reduce your plant’s temperature might be placing a fan directed at the canopy and misting water into the air stream. Water evaporating uses energy and evaporative cooling might accomplish what you’re trying to do. I know, wires, fans, water source on a roof.

Zill.

HedgedAndLevered · Aug 29, 2023

Water evaporating at higher rates from the plant leaves should create a pressure gradient and promote greater water movement from the roots than normal temps, no? certainly would cool down the bottom stem, and so similarly there should be a change in temperature from simple temperature conduction up the plant...?

Besides, just by nature of the fact that having cold, dense matter below the plant should cool it off generally; as a thought experiment, surrounding the plant with an ice-water moat that doesn't even touch the soil should, for instance, still cool down the area a bit.

If you worked in a commercial greenhouse you're the expert though, is this speculation all garbage? are we still going to be talking about 95 degree+ leaves, and so it would still end up with heat stress?

If so, would misting with ice water be fine then?
even in the air it would probably warm to 40 degrees before touching the plant

and how much do I need to worry about the magnifying glass effect with droplets on leaves?

HedgedAndLevered · Aug 29, 2023

Also I have an umbrella on my roof next to the plant. Would it be worth completely shading the plant, assuming I'm already doing these other mitigation tactics?

GoldenDragondnw · Aug 29, 2023

As one who has actually dealt with the heat give em some sort of shade...that umbrella will only work so much Also...keep em watered as well as you can. Man that's a hard grow. Thats the best I can help you with. Good luck on your grow man.

Zill · Aug 29, 2023

Hedge,
Water transpiring through leaf tissue, stomates, is not going to cool the leaf. I grew a sativa last year in my veg garden. Last year in the NY Hudson valley it was flippin hot all season. I mean 95F was the norm. The plant did fine.

Think about your questions ( all good by the way). Will water droplets magnify the suns rays, you ask. Who cares. I’ve never diagnosed a plant ailment that was shown to be due to a magnifying effect. These things grow outside.

I like the umbrella idea. When I worked on tobacco farms in CT the tobacco was known as shade tobacco. It was grown under fine textured nylon netting. Stretched for hundreds of acres. Maybe get four buckets, four 4x4s and some sand. Place the buckets at the four corners, stick the wood stakes in the bucket and hold in place with sand. Then stretch the netting over the plants. Don’t worry about reducing sunlight. It will be fine. That will drop the temperature.

Zill · Aug 29, 2023

Golden’s got great point. Whatever you do don’t let the plants suffer from lack of water. That will take them out quickly.

I noted my plant all last year was being baked by the sun. But like Golden recommends I kept pouring the water on the plant.

Cophiljay · Aug 29, 2023

I'm in the high (6000 ft) desert. I grow outdoors in 90+ degree weather. Above 87-88, I use misters. Even a couple hours of 90-100 degree sun will damage these ladies. The "magnifying glass effect" isn't a thing. Never had a burn while misting and leaves have hundreds of water droplets on them.

GNick55 · Aug 29, 2023

for the water droplets being able to burn plants the plant leaves need hairs for the droplet to be able to sit above the leaf surface and than form a focal point with the sun..
so meaning no water droplets do but magnify and burn a marijuana plant..

HedgedAndLevered · Sep 1, 2023

GNick55 said:
for the water droplets being able to burn plants the plant leaves need hairs for the droplet to be able to sit above the leaf surface and than form a focal point with the sun..
so meaning no water droplets do but magnify and burn a marijuana plant..

eh not exactly
assume parallel rays of light (from the Sun) hitting the suspended droplets. a suspended sphere can't focus light outside itself, which should make sense, since every refracted ray is bent back at the same angle when the ray exits the sphere. (and in fact, for a perfect sphere, the focal point is exactly in the center of the sphere
conversely for a half-sphere on the leaf surface, the idea is that focal point would be closer to the center of the half sphere, e.g. on the leaf surface

this is a myth though apparently! https://www.newscientist.com/articl...ur-plants-at-midday-wont-damage-their-leaves/
good to know

Zill said:
Hedge,
Water transpiring through leaf tissue, stomates, is not going to cool the leaf. I grew a sativa last year in my veg garden. Last year in the NY Hudson valley it was flippin hot all season. I mean 95F was the norm. The plant did fine.

Think about your questions ( all good by the way). Will water droplets magnify the suns rays, you ask. Who cares. I’ve never diagnosed a plant ailment that was shown to be due to a magnifying effect. These things grow outside.

I like the umbrella idea. When I worked on tobacco farms in CT the tobacco was known as shade tobacco. It was grown under fine textured nylon netting. Stretched for hundreds of acres. Maybe get four buckets, four 4x4s and some sand. Place the buckets at the four corners, stick the wood stakes in the bucket and hold in place with sand. Then stretch the netting over the plants. Don’t worry about reducing sunlight. It will be fine. That will drop the temperature.

transpiration doesn't cool the leaf directly? water going from liquid form inside the leaf to gaseous form in the air has to take thermal energy from the plant for the phase change.

good idea, actually my plant is already in a 1 inch thick PVC pipe box, with netting I'm using (for LST) which is about halfway up the width of the plant at this point (which I would strongly recommend always doing, works great and was like $20 at home depot and super easy to disassemble and store every season).
I can just add another 6 pipes with connectors, extending the box up one level, and put shading above it, when necessary.

thanks!

Trash_2002 · Sep 1, 2023

Trash_2002 said:
You could use a greenhouse shade, there's many levels of shade screens/nets 15%/30%/50% etc to cut down sun intensity and also temperature.

Greenhouse shading net, pretty common at any greenhouse. Can be tied almost anywhere and pretty easy to stretch and unstrech when needed for extreme heat days.

Not sure if you got this msg tho, the easiest cheapest fix available

cheers

Tzg420 · Sep 1, 2023

Watering regularly should be helpful and watering with regular temp water would lower the temp of roots, I would not recommend watering with ice or ice cold water, misting I think may be helpful as well..what I have done is set up a fan and in front of the fan and surround the plants; containers of frozen water…

Cooling the plant on hot days

Cooling the plant on hot days

HedgedAndLevered

imgrowing

HedgedAndLevered

GanjaJack

Trash_2002

imgrowing

Oldchucky

Zill

HedgedAndLevered

Zill

HedgedAndLevered

HedgedAndLevered

GoldenDragondnw

Zill

Zill

Cophiljay

GNick55

HedgedAndLevered

Trash_2002

Tzg420

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