What temperatures stop production?

[dfollett]

Heat is not a problem here, so I know nothing from experience of tomatoes not producing because of heat. Biggest problem here is cold soils and late and early frosts. I know from reading that when heat gets too intense, tomatoes stop producing new blossoms. I am experimenting with tomatoes outdoors in Earthtainers with a make-shift greenhouse around the top.

So far, it is working great. They've survived 8 nights below 25⁰ over the last month (as low as 15⁰) and are growing very well and setting fruit. My concern now is not cold nights, but daytime temperatures. I’m relatively high elevation (5280’) and things cool off significantly when the sun starts to set. However, during the middle of the day (12:00 – 3:00 pm) temperatures inside where the tomatoes are climb to between 90⁰ and 95⁰ - and I’m sure that over the next month they will hit 100⁰ to 105⁰ or so. It is only that hot for about three hours then quickly drops to 70’s then gradually cools off to 45⁰ to 60⁰ each night (on the sub-20⁰ nights, they get down to the high 30’s).

My question is: Do I have to worry about them not producing because of those few hours on the sunny days where the temperatures spike?

[Al@NC]

I got this off a site:

http://www.omafra.gov.on.ca/english/...fo_tomtemp.htm

Table 1: The effect of temperature during flowering and fruit set of tomato Temperature
Effect on flowering, pollination, fruit set

Greater than 95° F --> Reduced fruit set
65-80° F --> Optimum for fruit set
Less than 55° F --> Misshapen or catfaced fruit may result
Less than 50° F --> Poor fruit set

Over 95 and you'll probably see blossom drop! If you could improve air flow and are able to try and keep the temp below 90 then you should have better results but all of this depends on the varieties you grow also...

Al

[tomatoguy]

Our high temperatures are probably a bit lower than yours, unless you count last year, when we had over a week of highs between 110 and 112. However, I have noticed a big difference in my plants that get full sun and those that don't. My main garden is a raised round bed, 16' across and 24" deep. It is shaded, after around 2:30 PM, in the summer. My other bed is just a narrow strip, beside the driveway. It gets full sun, all day. The bed that gets full sun starts out earlier, produces well until mid-June, then drops off quickly. The main raised bed starts more slowly but produces more per plant and lasts up until the first frost. Once I figured this out, I started planting early varieties and peppers, in the full sun bed. The peppers don't seem to mind the heat and the early tomatoes generally play out eventually, anyway, especially the determinates. Have you considered shading your plants from the afternoon sun? If it works here, it should work in Utah.

mater

[Stvrob]

I'm thinking if the heat is brief, the humidity is low, and the plants have sufficient water then they will be able to keep themselves cool thru the hottest part of the day thru transpiration. I think it's the hot humid weather that gets the pollen to sticky. I grew tomatoes when I lived in salt lake years ago.

[Dewayne mater]

I've come to believe the night time temps are as critical as the day time temps. I think if you get good cooling at night, say at least down to 65 then tomatoes can take the day's heat and keep on going. This is especially true if you live in a dry place because humidity is definitely critical. Warm to sorta hot temps plus high humidity and the production drops. Add in night time temps like we get in DFW in the summer of 76-78 for a low and production drops greatly. I definitely see drop off in production when the temps hit 92 or so and stay there. That said, even if it gets hot, if you get a few days of cooler temps with a cool front and you can get another waive of fruit set. A last factor seems to direct sunshine or shade in the hottest parts of the day. By adding sunshade cloth in late May, the southern exposure sun is filtered 50% from noon on. This shade has clearly extended my season year after year. I understand what is at work in all of these variables is with too high of temps and humidity, the pollen clumps and will not propagate. This is all anecdotal of course, but you asked.

Dewayne Mater

[dfollett]

Thanks all for the info.

It makes sense and is what I was hoping to hear. They definitely have an opportunity to cool off at night and the daytime humidity when the temperatures spike inside the plastic-wrapped cages is quite low. As soon as we get past the danger of frost, I'll unwrap the plastic and heat will no longer be an issue.

[Stvrob]

Originally Posted by dfollett

Thanks all for the info.

It makes sense and is what I was hoping to hear. They definitely have an opportunity to cool off at night and the daytime humidity when the temperatures spike inside the plastic-wrapped cages is quite low. As soon as we get past the danger of frost, I'll unwrap the plastic and heat will no longer be an issue.

Whoa there... This is the first you mention of plastic wrapped cages! I thought I was providing an answer about high deserty kind of summer heat. I didnt realize your plants were wrapped in plastic. You might should review the responses above, I certainly dont want to see you cook your plants while trying to keep them warm in the spring.

[dfollett]

Feldon30 - I like the 47-74 rule. They are temps that I can easily maintain. When I take the plastic covers off the first of June, they will experience cooler nights than they will be used to. I pulled historical records for a weather station 3 miles from my home and the highest nighttime low we have experienced since 1950 was 68 (That was on a 7/13 and a 8/2).

Stvrob - Sorry I wasn't more clear. It is way too cold here to plant outside without protection, so I am experimenting with several Earthtainers outside with the cage above wrapped in plastic. I've learned how to keep them plenty warm at night, but on bright sunny days, the temperatures inside the cages spike about 20 degrees above the ambient temperatures. I wanted to make sure I don't go through this exercise only to have them not set fruit because of a few warm days. Given the comments above, I'm comfortable that it won't be a serious problem. Even if it does diminish fruit-set slightly, it is vastly more than I can get with nothing planted.

[Stvrob]

You may be OK, but be aware that the humidity situation is much different with the plastic. With the plant inside transpiring to keep cool, and limited exchange of outside air, the humidity is liable to be a great deal higher than compared to a hot summer day. That probably WILL affect fruit set.
I hope you can remove the plastic soon!

[feldon30]

I don't know that there is any scientific basis to it, but I've heard the rule of 47-74 for tomatoes. That is, overnight temperatures between 47° and 74° are optimal for tomato fruitset.

I know fruitset pretty much stops in May in Houston. Temperatures no longer dip below 80°F at night and stay warm and muggy (humidity being another huge factor in fruitset).

[kilroyscarnival]

Thanks for this confirmation of what I thought I had absorbed. It's been getting hotter here in Orlando, but night-time temperatures are still in the mid 60s so things still seem to be going okay. I finally have some unripe fruit on my Wins All, and am getting some new ones setting on the frost-damaged Big Ray's. I have a later few Big Ray's I started in containers; might be disappointed there. Wonder what would happen if I brought them in at night to cool down? Kind of ridiculous, but might be a fun experiment.

[kwselke]

Feldon's numbers of 47 - 74 are so close to my numbers of 50 - 75 that it just does not matter. In Houston the issue is not how hot it gets during the day but how cool it gets. This time last year the lows were over 75 and I knew my crop was dombed. I looked at a long term forecast today and supposedly I should get three more weeks of lows below 75. I'm hoping for a good harvest this year.

[dfollett]

I hope it stays cool for you (21 here again this morning).

[Cole_Robbie]

For my late fall plants, when the nighttime temps start to dip into the 40's, the fruit will turn mushy and not taste as good.

[dfollett]

By the time they ripen temperatures should be out of the 40's (I hope).