Yes, that gives a VPD of 0.16 which is low enough to promote disease.

Ventilate. The outside air holds much less water vapor than the air inside your GH right now. Heat if necessary.

Here is a link that might be useful: VPD calculator

Thanks stressbaby. I am going out there now to turn the heater up. Thanks for the help. Barb

Rose: Last winter the humdity in my Rion was way out of control and several plants became diseased. Ventiliation wasn't great because we can't cafford to heat the great outdoors. This winter our electic is going up 20% so I wont be using the smaller HFGH until spring. I'll use it as a seedling/cutting GH. I gave away many plants on Freecycle this summer. I got rid of those especially liked by spider mites. This fall I already have a white-fly invasion in the GH. :-(

Hi Greenhouser,
I did what Stressbaby told me too. This junk crap heater (brand new) Is not cutting it. I start a new job next week my first paycheck I am buying a new heater. I put a fan in front of the heater to throw the heat and it is still 53.9 temp and 89% humidity. I am waiting for the sun to come out and will open it up and ventilate it. I have a angel trumpet sitting in front of teh door getting ready to bloom. I didnt want it to freeze to death. I bought it not knowing what color it is. I was waiting for it to bloom and than cut it up. Tahnks for the advice. I remember last winter what you went thru. Here I go...LOL Barb

I have 3 heaters in the Rion. Two are on thermostats and one is only for really bitter cold nights and as a backup if one of the others die. Even with 3 running and the 6 fans... the humidity remained high. On warm days I would open the doors and even that didn't help much. I never got rid of the spider mites. They spread like wildfire once the plants went outdoors and wiped out most of my peppers and tomatoes and did severe damage to the eggplants. We did managed to freeze some peppers and get enough tomatoes to can for winter. I would say 95% of the peppers were wiped out and the tomato crop reduced by at least 50%. All the squash were killed by both whitefly and the mites. This past summer was a disaster.

Rosepedal,

Were you able to get the humidity under control in your greenhouse?

I'm curious, what is a good percentage of humidity to have in a greenhouse? I have quite a few tropical fruit trees and then mostly annuals which I brought in to propagate more from. Mine generally stays at 50 to 60% humidity. It's cold and rainy right now with no sunlight so my humidity has been much higher the past couple days. But as soon as the sun comes out again it should be back in the 50/60% range.

Just curious what everyone's thoughts are regarding a good % of humidity?

~Tj~

I never managed to get it in the 50 to 60% range. It stayed in the 75 to 90% range a good part of the time. It varied by the outside weather to some extent. Cutting back on watering and being careful not to spill water helped a little.

Here's my question!!!
Everyone says that spidermites hate humidity right???
Then are you having so many of them if the humidity is so high. You should be mite free right? I am begining to think that spidermites have become resistant to humidty and now the only thing that can rid them is cold temps...Like freezing ones!!!
SNOW......
So whether or not we have humidity or not based on your experience, we are still going to get them.....:-(
I am gegining to think misting does nothing for them either. If anything it only gives them a drink and a bath!

meyermike: At one time everyone said the earth was flat. ;-) Spider mites thrive as well in high humidity as they do in dry air. It makes no difference to them. They even thrive when drenched in insecticide or covered with 5% Sevin dust. Misting is useless. It's an old wives tale that they hate humidity.

I dont have any spidermites yet. I am waiting for winter and will deal with them than. I cant get the humidity down but the plants are going gangbusters. gH 55.9 Humidity 77% Outside 36 brrrr

It seems they attack in the dead of winter when it is dry. I am not looking forward to January and febuary. Boo hoo Good luck everybody we need it....