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Reefmonkey

Houston USDA Hardiness Zone and Microclimates

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I know that Houston falls into USDA Hardiness Zone 9a, which means our average annual low is between 30F and 20F. I was wondering, though - I imagine this low is for average lows through the whole area, but doesn't take into account microclimates, such as urban heat islands, etc. We all are familiar with local TV weathermen regularly telling us that a certain cold front or freeze advisory is only for those viewers living north of I-10, as well as Galveston generally being a little warmer in the winter than, say, out by Willowbrook Mall.

Does anyone know of any data compiled that shows that inner loop Houston and nearby areas, and Galveston Island may actually be 9b, or even 10a?

Edited by Reefmonkey

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i don't know of any. but any gardener can tell you about microclimates. you can often grow what would normally be considered an annual for a particular area right up against your house, when the same plant won't overwinter 2 feet away from the house. on the same note, my first montrose commons neighbors and i have no problem growing plumeria without bringing them inside over the winter, when they're not supposed to be able to overwinter in the long run here. and daffodils in washington dc bloom a week or two before the same species in the suburbs. both eight foot high fences and proximity to a lived-in house create microclimates, as do cities. is there some specific reason you want to know or just general curiosity?

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is there some specific reason you want to know or just general curiosity?

Well, I happened to stop by Flamingo Gardens, the huge palm tree nursery on BW8 and Hammerly, and noticed they were selling a lot of trees that were 10 to 11 hardiness zones, like the Christmas palm. judging from the size of the trees, I can't imagine they are all meant for indoors planting, some just could not be moved indoors in winter, and even while they are there at the nursery, there is no way they could be protected from a frost. just got me thinking about the whole thing.

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I can say that, working at night, I would often see the coldest temperature of the night coming home. Several times when the weathermen reported a possible freeze for Northern Harris County or Montgomery County, while IAH may report a temp around freezing, the Heights never got within 5 or 6 degrees of that. I'm not sure that I've seen an actual freezing temp in the Heights since I've lived here (5 1/2 years). It might be somewhat of a gamble to plant those trees, but it is a reasonable gamble.

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mapping climate zones in more detail is definitely intriguing to me. especially for a place like the big island of hawaii which supposedly has 10 or so different climate zones, but is always one shade of red for zone 10. yet, i've never found a map of this. i've thought about trying to do it on my own--you'd need elevation, precipitation, average lows, and (for the dry regions) evapotranspiration rates to do it--but i've never seen all of this info available, never mind at the same scale. so it's obviously very difficult to do. for microclimates, it would be even more difficult because of the scale--but it would be interesting to see it mapped.

Edited by rgr

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I rarely disagree with Red, and I don't want to poke the "climate-change" issue with a stick, but I'm not sure I'd gamble on those. I have seen hard freezes in the last ten years or so, and a spectacular hard freeze/ice storm around Christmas in (I think) the late 80s. I thought last spring was unseasonably cool and it's sure cooler now than any late August/early September I remember. I'm thinking if we're barely on the cusp by a few degrees or so based on microclimates, I wouldn't risk it. I have a big palm tree in my patio area, but it's protected on three sides by the house and garage.

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There are no mirco-climate conditions in Houston.

Regarding the weatherman and I-10... and cold fronts... they say this sometimes because that is where the cold front stalls from time to time.

Micro-climates tend to be constant pockets of weather/environmental conditions that are unlike the constant weather/environmental conditions they are contained within. A stalled cold front is a transient condition.

For example, along the coast of CA - with the Pacific ocean - your nightime lows could be in the 60's - in August. But go inland just 5 miles... and inferno. San Francisco has microclimates...

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my parents have a satsuma tree in the backyard of their house, they do a few things to protect it in the winter.

they put a pile of fallen leaves around the base, as the bio-process of that will generate heat.

they have some old sheets that have been sewn together to form one large massive sheet. if the temp is going to get within a few degrees of freezing, they will toss the large sheet over the top.

if it is going to get really cold, they put a light under there as well (incandescent, not fluorescent, I think they may have switched to halogen though, as that generates even more heat) to generate a bit of heat.

It has worked well for them, but it wouldn't be practical to make a sheet for some of the palms out there, but piling leaves at the base, and wrapping the base may make an impact.

they also just put their plumeria against the house, of course they haven't seen a flower in years, but that could be more related to the amount of water and light it receives through the season (I bring mine inside though).

Edited by samagon

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Looking at actual temperatures of the last 5 or 6 years, as opposed to guessing, I see that Hobby Airport tends to be a couple of degrees warmer than IAH. That is to be expected. However, the coldest temperature recorded at either airport in the last 6 years (didn't look any further back) is 26 degrees. Going by the Hardiness Zone definition of "average minimum temperature", I think it is safe to consider Houston proper a 9b zone. Galveston has had only one day hit 32 degrees in the last 6 years. I would consider it 10a.

Obviously, the USDA creates its averages over a much longer period of time. However, realistically, temperatures from 1910, when Houston had a population of around 100,000, are not really useful. In the last 30 years, we've only had a couple of really hard freezes (below 20), and the average year doesn't drop below 25 degrees...and then only for several hours.

*Note that 'hard freeze' has its own definition in weather circles, which is different than the way I used it.

Edited by RedScare

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It makes me want to get a weather station in my backyard and start collecting data from now on.

You might want to get a weather station that "communicates" with 2 or 3 remote sensors. That way you can put one near your house, and one on the back fence or away from the house. Then you can see the difference. My weather station has 1 sensor that I have mounted at the back patio -- but I am pretty sure it registers temps that aren't as cold as if it were mounted 20 feet behind the house. The low temp recorded by it for last Friday night was 31.3° (Saturday morning). I'm sure it was colder than that.

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The most commonly accepted USDA record length is 30 years. By those measures:

Houston Intercontinental is a borderline 9a/9b

Houston Hobby is a solid 9b

Galveston is a 10a

San Antonio is a 9a

Conroe, College Station, and Dallas are 8b

The funny thing, is temps during this event are forecast to be a mere 4 or 5 degrees below "normal" winter lows. The reason it seems so cold is the climatological norms are biased by the terrible 1980s.

And there are thousands of micro-climates all over greater Houston. If you've got a canopy of pines over your yard for instance, the reduced radiational cooling on clear night will keep tender plants from frying nearly as fast. Too many more examples to note. The differences you see between the coastal areas of Cali and the valleys is just a plain climate change, not a micro-climate... not that they don't have gobs of them.

Jason

Edited by JasonDFW

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The coldest ever temperature measured in Houston (IAH) was 7 deg. That was over Christmas in 1989. I remember my parents returning to town to find water streaming down their driveway. The pipes had burst in the attic.

The new complex on the rail line in the Museum District were wrapping their newly planted palms today. They are pretty tall. The place is under construction, so they had the means to get jacked up that high to do it.

My theory as to why so many tropicals flourish in parts of Montrose is that it's so densely built that the plants are more protected than in wide open spaces, with less brick and mortar.

I always remember all of the (early) cold fronts that chicken out at Conroe. I call that the Pine-Line. It's quite evident - notice next time you make the drive. I'd say that's where the zones change.

I've used the light bulb method for providing warmth as well. If you're going to wrap a plant and the wrapping is going to touch the plant, use fabric. Plastic will cause damage. Plastic is effective for a tenting method though.

Flashback - that 1989 freeze killed many palms around the city, like Highland Village and River Oaks Shopping Center. Let's hope it doesn't get that bad.

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My neighbor and several other had pipes burst in the Heights over the weekend. It froze the ground (the first 1 1/2"). It seems as tho the orange tree I planted last year came through it ok with hay around the base. Hopefully we will start seeing warmer temps.

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