What is considered 'frost'

archerb(8)February 9, 2009

I know that frost is bad for plants. I also know that a frost is different from a freeze. At what temperature does "frost" happen? I'm starting to keep my plants outside and even plan on putting a couple in the ground (with WOW's). The lowest temp I see for the next 10 days is 41 and the high is 80.

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daylilyfanatic4(Zone 6 SE NY)

Hi frost is basicaly frozen dew the temperature has to get below freezing for it to form however it won't all melt if the temp creeps back up to 33* if the temp is below freezing and the dew point temp = the air temperature then frost will form. also tomatoes don't like cool temps so I'd wait a week or two until it's warmed up. Also make sure your passed your avg last frost date for your area.

    Bookmark   February 9, 2009 at 10:30AM
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Here in the Central Basin in TN, zone 6b/7 the avg last frost date is around April 15.

    Bookmark   February 9, 2009 at 11:01AM
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petzold6596(8b southern NM)

google . They have 'year at a glance' and a 'daily/monthly' historical weather data base including high/low temps for most mid to large cities.

    Bookmark   February 9, 2009 at 11:41AM
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I see you are in zone 8, but you don't say where you are located. I'm also zone 8 and our avg last frost date is March 17th. It has been beautiful here also, but the temperature can drop quickly. To give my plants more light before I can transplant in the garden, I take them out side for a few hours (after hardening them off) and then bring them back inside before the sun sets. You can even leave them out all day if you check the weather report and it is not supposed to be at or below 32. I wouldn't advise planting anything until closer to your last frost date. If you have WOW you might be able to plant out as much as 2 weeks earlier, but to be honest I've never used them so I can't say for sure. Good luck!

    Bookmark   February 9, 2009 at 11:44AM
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I'm located just south of Austin. I've checked and the last expected frost date is March 7-15th. However, I'm putting more stock in the 10-day weather forecast to determine a good time to plant.

Right now, as soon as I can prep the beds, I plan on putting at least three in the ground. I have three WOW's and I'm anxious to see how well they work. Any more than the three will have to get protected by half milk jugs. I would have planted sooner with the WOW's, but getting everything set up is difficult due to a child with the sniffles and wife who needs inside housework done :-). The reason I'm asking is I see that the coldest it is going to get over the next 10 days is 41 degrees and was curious if this was too cold for the plants. I plan on having them all out and in the ground by March 7th, but wanted to check the 10-day at that time before committing and wanted to know what was considered too cold for transplants (frost).

    Bookmark   February 9, 2009 at 12:05PM
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32 degrees is when frost will form. Anything higher will not hurt the plants. Happy gardening!

    Bookmark   February 9, 2009 at 12:52PM
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In addition to avoiding frost, some rec soil temps above 50F some even 60. Here are just a few references.

"The safest way is to get a soil thermometer and take the temperature several inches down in the bed you intend to plant. Tomatoes want their roots in soil at least 50 degrees Fahrenheit, but it is better to wait until it warms to 55. When gardeners become impatient and plant early, tomatoes can become dwarfed and stunted, and may not recover."


"When it comes to transplanting Tomatoes, you want to ignore your calendar. Outdoor temperature is the most important factor in transplanting Tomatoes; far more Tomato plants are killed by being put into soil that's too cold than from any pest or disease. Both the soil and the air temperatures needs to stay above 55 degrees Fahrenheit, even at night, before your plants are ready for transplant. When in doubt, just keep them inside a bit longer."


"Tomatoes like warm soil and don't tolerate frost, so wait until warm spring days arrive and soil temperatures reach above 60°F to plant."


    Bookmark   February 9, 2009 at 1:57PM
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azruss(8b Tucson)

Frost is 32 but freezing doesn't happen instantly. I put out my first 3 plants with wow on Jan 1 and they're doing great. Now that they have started peeking out over the tops of the wows I gently cover them when it gets really cold. 40s is NO problem at all with wows.

    Bookmark   February 9, 2009 at 3:51PM
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wvtomatoman(z6 WV)

"What is considered 'frost'"
Frost is a covering of ice crystals on a surface. It is produced by the depositing of water vapor onto a surface cooler than 32°F.

"I know that frost is bad for plants."
Yes it is. Frost damages plants because the water inside the cells of a plant freezes and breaks the cells' walls. That part of the plant will die. BTW, black is not a good color when it comes to tomato plants. :)

"At what temperature does "frost" happen?"
See above. You spoke of a temp prediction of 41. Keep in mind that is the ambient surface temperature prediction. Surface temperatures are usually measured 4 to 6 feet above the ground. The coldest air is closest to the ground. So, to present a hypothetical case, let's say the weather person predicted 34 degrees for your area. Let's also say that the prediction was spot on. You still might get frost on your seedlings if the temperature close to the ground (where your seedlings are) was let's say 3 degrees cooler than the "surface" temperature.

"I'm starting to keep my plants outside and even plan on putting a couple in the ground (with WOW's)."
You should be okay with WOWs.

What I always recommend when asked about when to set out tomato (actually any frost tender) plants is:
Determine the average last frost date for your area. Add 2 to 3 weeks to that and you should be fine.

The reason is that the average last frost date is just that an average. So, depending on the year the last frost will be before, on, or AFTER the average last frost date. By waiting a couple of weeks you avoid disasters and you allow the soil to warm which makes it better for the plants anyhow.

Good luck.


    Bookmark   February 9, 2009 at 5:03PM
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Also pick up your own digital thermometer and have it calibrated.
Our 'official' weather monitoring station is less than two miles from our house. In the parking lot at the fire station and snugged up against a south inside corner of the building.
The 'official' low temp is always at least 2-4 degrees higher than what I record in our yard.

(that would also account for about 99% of recorded global warming LOL)


    Bookmark   February 9, 2009 at 7:58PM
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jll0306(9/ Sunset 18/High Desert)

Our weather station is less than a level half mile from us and yet it can also be quite a few degrees colder in our yard than the reported temperature, especially in the early am. (I know this because I've prowled the yard at 3:00 a.m. with a flashlight and a thermometer, looking for microclimates.)

    Bookmark   February 9, 2009 at 8:28PM
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