Adjusting last frost date for warmer conditions
 

I am in zone 8a and for the last several years based on all the information I could find I have used March 25th as my average last frost date. It occurred to me that we have been hearing about warming climate conditions for for the last ten years or so and I wondered if my last frost date had changed. I looked in several places and March 25th was still about the average answer. I decided to try to calculate it for myself. Weather underground has a station less than 5 miles from my place and has daily records going back several years. I used the last 10 years with a temperature of 32 degrees or less and got an average date of March 18. That is a full week earlier than conventionally is quoted and as much as 3 weeks earlier than than the latest of those quotes. I calculated again for the last 5 years and got an average last frost date of March 11th.
This may indicate a trend or it may be a result of small sample size there is just no way to be sure. Either way what we are being told about last frost dates may be incorrect.
Has anyone else found this to be the case?

What county are you in?

Worth

Worth
I am in western Gillespie County

Last Frost Date (LFD) is based on historical data with a probability of 10% chance. They also 5% chance. That is why it wont change from year to year.
With today's weather forecasting science you can beat that historical LFD. Here is what I would do:

Monitor forecast starting 10 days before my LFD. If the trend is toward warming (nothing below 34F) and the lows are near 40F or higher, I can go ahead and plant out as early as possible. My bench mark is for 38F(+) low and 57F(+) for highs. In my PNW climate this will continue from late March til early May and there won't be a big change.
But in zone 8a in TX your weather should warm up much quicker. So by mid April your high can go to mid 60s . But also you might get that 5% chance of freeze warning. So you have to keep monitoring weather data until mid April. I say this, because I have gardened in Atlanta, GA (zone 8) and the spring weather down there is very similar to Dallas TX, for example.

Gardeneer

Daily temperature fluctuations can get pretty wild here. The historical average temperature on April 15th is 78 degrees, and a 5% chance of a freeze still exists until that day.

An average based on historical data. inaccurate easily by 2 weeks either way. For myself it isn't worth pushing the envelope with much to risk and really nothing to gain because I find that it is warmth, not lack of frost that tomatoes need. Waiting until it is reliably warm before planting out results in the least risk and the best start for my plants. In my experience, plants put out early seeking some head start advantage to weather wind, cold soil and near freezing temperatures just sit there shivering and purple waiting for it to get warm before taking off and growing/setting fruit anyway. might as well start them a week or two later and not have their growth checked by adverse conditions.
Using cover such as a low tunnel is needed imo to plant out early with any advantage of earlier fruits in a market garden situation for example, where being first is an advantage for sales. For the home gardener, I would mainly think there is no real advantage and plenty of risk putting tender plants out too early.
KarenO

Your asking a good question that deserves a good answer.
I am trying to figure out a way to word it.




Worth

[QUOTE=ABlindHog;524334]Daily temperature fluctuations can get pretty wild here. [B]The historical average temperature on April 15th is 78 degrees, and a 5% chance of a freeze still exists until that day.[/B][/QUOTE]

That was also what I thought.
So you have to venture abit and be prepared for just in case that 5% happens.
It is not difficult a task. An empty bucket or a 3, 4 gallon pot inverted on a plant can provided enough protection for short lived over night near freezing temps. I have done it few times down in GA. It is often the cold winds that do the damage by removing the heat at much faster rate.

Gardeneer

Don't use 32 degrees for calculating Last Frost Date. That is the temperature for freezing. Frost can occur at 37 or 38 degrees (or higher with perfect conditions). Last Frost Date is based on Frost, not Freeze.

Lee

I dont even want to get into how utterly ridiculous some of the charts and maps can be.
Where you live you are just barely in zone 8A the only thing that puts you there is the Edwards plateau and the Balcones escarpment.

That little zone boundary runs almost right along with it.
Your county and mine are at about the same latitude but I am below the escarpment.
When I hear of freeze warnings on the news this is what I hear year after year.
Mason Llano Lampasas northern Travis northern Gillespie Burnet and Williamson.
Sound familiar?

Well anyway regardless of what people think, we do have to take chances down here if not we may lose the little window we may or may not have to get bloom set.

Without these chances we can lose almost if not a whole month of growing only because we were scared to take a chance only to have the heat set in and ruin our crop.

Even if the soil is a little cool and the plants are growing a little slow those roots are growing.
When the soil gets to 60 or so the plants explode because they have the root structure to do it and are established.
This doesn't take long in Central Texas.

Here is what I do I look for the red buds to bloom.
When that happens I can be pretty sure it wont freeze again.
It has only bit me once in many years.
I think that was three years ago I lost my crop I was out of town big deal plant over.
The other thing I do is I plant just about twice as many seeds and therefore plants as I need.
I select the best ones and plant them normally the first week of March one or two times in February.
If after the hay buckets or what ever doesn't protect them then I have the back up crew to take over.
I have never had to use the back up crew yet and they have been in a cold spot or two.

I cant begin to say how many times someone says to me, your planting a little too soon aren't you.
That same amount of times I have tomatoes and they say I dont know how you do it I cant seem to get any you must have a green thumb.
Or you cant grow tomatoes in Texas I am from Utah or Ohio and I never had any problems Texas sucks.
'

Worth

I tend to pay attention to average last frost date, actual soil temperature and historical last frost date. Somewhere in all those numbers is a date when tomatoes and peppers get planted. Mostly it is what the weather is like when I finally get around to feeling like it is time to plant without losing everything and then wait a few more days just to be sure.

Pretty dang scientific, huh.

[QUOTE=PaulF;524441]I tend to pay attention to average last frost date, actual soil temperature and historical last frost date. Somewhere in all those numbers is a date when tomatoes and peppers get planted. Mostly it is what the weather is like when I finally get around to feeling like it is time to plant without losing everything and then wait a few more days just to be sure.

Pretty dang scientific, huh.[/QUOTE]
Yep! I agree and do it much the same way.
Intuition based on based on data.
Dutch

[QUOTE=PaulF;524441]I tend to pay attention to average last frost date, actual soil temperature and historical last frost date. Somewhere in all those numbers is a date when tomatoes and peppers get planted. Mostly it is what the weather is like when I finally get around to feeling like it is time to plant without losing everything and then wait a few more days just to be sure.

Pretty dang scientific, huh.[/QUOTE]
That is a system.

Soil temperature, IMO, is very important. I would wait until it gets to 57-60F. The foliage can tolerate cool weather fine but the root system needs a minimum temperature to uptake the nutrients. So with 60F soil temperature and deep planting under black plastic, the night lows are not going to bother tomatoes. For years, my tomatoes plants have been exposed down to 36F at nights , many times after plant out. Some say they would get stunted. But not so.My plant get real big in the season and bear good amounts of fruits.

After 23 years of using my tulip poplar's buds opening as my guide I am still batting 1000. I generally wait til I see the little bits of green all over the tree opening up and then plant.

Bill

I use the mesquite trees- If they are budding out, then the danger of frost for this yewar is done.