Hi/Low Thermometers are the same. If they are meant to be used outside. Being in the greenhouse does not make a difference. Look at the weather forecast in your area or compare to another thermometer to see if it reading the temp. correctly outside.
It could be that in your area of Calif. the temps do not fluxuate much. And the greenhouse is staying the same as the outside temperature.

I have a digital one in my indoor grow-room-I put the sensor right smack-dab in the middle of the plants. With indoor conditions and a thermostat at 71-under a 1000 watt metal halide lamp, I usually get daytime temps 75-77 and nights around 63-65 (it's in the basement). Got it on ebay for $29.

I'd suggest you purchase more than one thermometer (and probably a few thermostats if you will have an exhaust fan and variable heat source. I hang a cheap, but large dial thermometer just inside my greenhouse doors so I can walk by and see the general situation. The real test of temperature is down at the plant level (4" above the ground) so carry a hand held pocket thermometer to check inside (plus outside) temps. Set your cooling thermostat to kick your fans on when your hand thermometer indicates 85-90F+/= at the plant height. My heater kicks into full heat when the temp falls below 60F. Never rely on just one thermometer. They can malfunction and your plants usually suffer when they do.

Go to radio shack and get one of their Long-Range Wireless Thermo-Hygrometer. It's a 2 piece system, the sensor goes into the greenhouse and the base stays in your house. It will sound an alarm if temps get too hot or too cold or even too dry or wet in the greenhouse.

I bought mine last year and it has saved my plants countless times. You can even buy extra sensors if you wish.

X

Here is a link that might be useful: Long-Range Wireless Thermo-Hygrometer

Thanks for the link to the product at Radio Shack.

If I keep reading all your helpful hints, I'll be way ahead when the GH really gets built.

Thanks again...you folks are great!

I want to thank Xera for the link too. I rushed right out on my lunch hour and picked one up. I have it up and running here at the office for some testing. I will put the sensor outside to see how it reads through walls.

I also have the HFGH and would love to join the club. Enyardreams' photos were so incredibly helpful when I put it together back in the fall. Thanks!

It's hard to think about greenhouse gardeing, when indoor hydro gardening is so much easier. No bunnies, deer, slugs, etc...My baby girl loves picking lettuce from my ebb&flo system. No habeneros for her yet!

Wish I could post pics, but I am at work and the pics are on the home comp.

Love to all passionate gardeners, indoors or out.-Jason

I'm real interested in the Radio Shack product linked above, but a search just brings up an owners' manual, I can't find the product. It's quite frustrating.

Anybody know where I can find such a product? We need an alarm to notify us when we're 100 feet away that it's gotten hot and we've forgotten open up the ventalation. If you can help us I will kiss you, at least mentally.

Thank you!
Laura