How hot is too hot when it comes to substrate?

[cornfed]

It's starting to get hot here. I'm looking at 97 on Thursday.



I went out just now with a digital meat thermometer to check the substrate temperatures after reading a little on how roots will not grow right if it is too hot.

Everything is in the grow phase and was watered for the first time today about two hours ago. I have two benches. One gets sun from sunrise to 11:30ish AM. The other gets sun from around 9am to 3:30ish PM.



I inserted the thermometer about 2-3 inches deep.

Shade bench - 83 degrees
Sun bench - 94 degrees
Seedling tray in pop-up greenhouse (door wide open) - 94 degrees.

I'm thinking of painting my pots white... Over the weekend I watered twice a day to try to lower the temperature.

What do you think? Am I in trouble?

 

[just.wing.it]

Nursery trees sit on asphalt parking lots all summer in black buckets....They're fine.
White paint couldn't hurt though....
Could place something in front of the pots, blocking them from direct sunlight.....like a little wall on the front of the bench.

 

[cornfed]

That's a good point. I'll breathe.

Although if it is within my power to maintain an "optimal" temperature, I for sure want to try. I want them to be healthy and grow as fast / much as possible.

Especially with the seedlings. They're so small!

I thought about a little wall too! Or wrapping them in aluminum foil?

I think the white paint will be the most attractive option though.

This graph is from a temperature study on a black & a silver car. The difference is greater the hotter is is outside, and the difference is substantial.

 

[Tums]

Nursery trees sit on asphalt parking lots all summer in black buckets....They're fine.
White paint couldn't hurt though....
Could place something in front of the pots, blocking them from direct sunlight.....like a little wall on the front of the bench.

Black containers and asphalt certainly isn't the best thing for the plants though. Here are some sample research papers from the nursery industry about how excess heat is bad for roots:

Container Type and Substrate Affect Root Zone Temperature and Growth of ‘Green Giant’ Arborvitae

Root zone temperature (RZT) in nursery containers commonly exceeds ambient temperature during the growing season, negatively impacting crop growth and quality. Black nursery containers absorb radiant heat resulting in excessive RZT, yet other types of containers and different substrates can...

www.mdpi.com

(PDF) Effect of heat stress on container-grown plants

PDF | On Jan 1, 1989, D.L. Ingram and others published Effect of heat stress on container-grown plants | Find, read and cite all the research you need on ResearchGate

www.researchgate.net

Some of the methods mentioned to reduce root temperature:

• Limiting sun exposure to sides of containers through spacing, shading, or container shape (apparently that's where most of the heat comes from)
• Using lighter colored, more reflective containers
• Using porous/breathable containers vs. plastic, although this uses more water
• Watering EARLY in the day is better than watering in the afternoon
• Burying or nesting pots in larger containers or the ground

 

[0soyoung]

It is very simple. More than 95F/35C is too hot.

Get a simple meat thermometer probe and stick it in the pot, among the roots as see how hot they actually are.

Water has a huge heat capacity, so the roots tend to stay cooler until the substrate/soil dries.


btw, roots are dying at temperatures above 115F/40C.

 

[PA_Penjing]

I wrap old t-shirts around the pots im worried about when the temp goes above 90F. I keeps the soil damp and I assume much cooler, but I suppose I should test it with my thermopop. I do make sure to take them off the pots at night so the soil can breathe and dry out a little. It's not a long term game, don't want to create anaerobic conditions

 

[cornfed]

Thank you!

Based on my skim of the resources you shared, I am going to generalize that 38C (100F) seems to be where problems can really occur and 30C (86F) is optimal, although this will obviously vary by species. Length of exposure over the growing season is a factor.

I'll have to look at these more closely. Good stuff!

 

[cornfed]

Get a simple meat thermometer probe and stick it in the pot, among the roots as see how hot they actually are.

That's exactly what got this thread started. The thermometer said 94 on a couple of pots which got me worried enough to ask the question. Thank you!

 

[Warlock]

LOL>. its like 97f in the mornin when we wake up in Texas

However, we couldn't tolerate your winters.. lol

 

[cornfed]

LOL>. its like 97f in the mornin when we wake up in Texas

However, we couldn't tolerate your winters.. lol

Haha. As evidenced earlier this year when they shut off our power for a couple hours so they could send it your way when you Longhorns were shivering.

Our summers are pretty darn hot & humid too. It'll get to 100F for a period typically.

I'm just talking about the substrate here though. I think I have a plan now.

 

[Emanon]

I think I have a plan now.

re that plan... I got some great advice from others on here when I decided to deal with the same issue. Here's the thread: https://www.bonsainut.com/threads/r...hen-using-a-black-plastic-training-pot.43373/

 

[Cadillactaste]

My front deck sensor in the sun can and has read well over 100F. I water twice a day when that happens. Small pots get New Zealand spaghnum moss as top dressing. Two smaller pots are also inside a pot with Akadama...to offer more moisture on those hot days.

 

[Pitoon]

Nursery trees sit on asphalt parking lots all summer in black buckets....They're fine.
White paint couldn't hurt though....
Could place something in front of the pots, blocking them from direct sunlight.....like a little wall on the front of the bench.

Some sit under the sun for years.

 

[cornfed]

re that plan... I got some great advice from others on here when I decided to deal with the same issue. Here's the thread: https://www.bonsainut.com/threads/r...hen-using-a-black-plastic-training-pot.43373/

That thread is a gold mine. Thank you!

 

[Slippery Pete]

Funny to see you asking about this -- Dr. Carl Whitcomb, the man who created that Rootmaker II pot on your bench there, has studied this topic of root zone temps extensively and writes about his findings in chapter 13 of Plant Production in Containers II. As expected, direct exposure of the side of the container will inevitably raise temps inside the pot, and he has found it to be fatal to root growth of most tested plants, though some far more than others. One thing to keep in mind is *where* you are taking your soil temperature. Whitcomb cites one example where the temperature on the inside walls (which we often find roots circling around...) was 120°F whereas measurement of center of container was only 100°F. My point being, if temp is something you're concerned about, you should be measuring temp on the inside walls that have been exposed to sun. That is not only going to be your upper limit, but also your focus as this is where the roots will grow in a standard container. Things are different when air-root-pruning container are implemented. Can't remember how... i think something to do with air exchange regulating temp... don't quote me on that though. I'm a zombie right now. Bedtime for me, lemme know if you want a bit more info and i'll get back to ya tomorrow. I highly recommend getting the book for yourself in any case

 

[cornfed]

Thank you. I've read a lot from Dr. Whitcomb this spring. I think he is on to something. The nursery I got my trees from uses his system and all of my pots are Rootmaker.

I actually have heard of that book and want a copy. I see it on his site, but I was hesitant about spending the money. It's pricey.

The thread that @Emanon linked has a couple research papers in it. They claim that air pruning pots reduce substrate temps, but color I think made a larger difference.

I took my temps about an inch from the sunny wall, and I do rotate the trees every so often so the roots and branches will grow in all directions.

 

[JackHammer]

LOL>. its like 97f in the mornin when we wake up in Texas

However, we couldn't tolerate your winters.. lol

I am also asking myself some questions about this... I have a greenhouse and it is 100 in the morning when I open the door. Afternoon heat is easily 115+. My tropicals love it and have been exploding. Unexpectedly, my other plants seem to really like it too. Basil, tomatoes, even shade cabbage, is doing well without any drooping or sun burns. It is way to humid for the succulents though. I water about twice a day.

I want to put more things in the greenhouse but I am not sure when hot is too hot. So far I haven't gotten to "too hot" but I might lose some plants when I do.

 

[Firstflush]

The shiny reflective tape I wrapped my 1 gal nursery pots in has worked well if you are concerned.

 

[cornfed]

Plant Production in Containers II.

That's a hard book to find. University Library has it. Only used copies on Amazon. What is it about this hobby and out-of-print, expensive books?

Lots of free information from him here though, on the right sidebar.

 

[Brian Van Fleet]

Nebraska is a long way from the equator. Mist everything down when it’s blazing hot and it will be ok.

Cheers, From several hundred miles south of you…