Impatiens being a shade plant will not flourish in high temperatures. Even if you got them to germinate it is unlikely they will do much outside in the heat. You have so many tropical plants that others can't grow pick one and grow that. I would dearly like to grow some plants that just won't tolerate my climate. That is the nature of plants and we gardeners have to adapt

Reed, I love Tom Clothier's data base. It helps take a lot of the guesswork out of knowing when to plant seeds. At his website there's also a similar set of data for other plants like annual and perennial flowers. I first really came to understand the relationship between actual soil temperatures and seed germination/plant growth and performance from Dr. Sam Cotner's incredible book "The Vegetable Book: A Texan's Guide to Gardening". I also learned so much from him about the way air temperatures impact growth and performance of our vegetable plants. While there are many gardening books that I love, love, love and find useful, Dr. Cotner's book is the only book I consider absolutely essential and responsible for whatever success I have had in raising vegetables. I grew up in a gardening family so understood all about when we planted each vegetable, but reading Dr. Cotner's book in the mid- to late-1980s helped me understand it all in terms of, not dates, but rather soil temps and air temps. After that, vegetable gardening became so much more easy to understand and manage. I've completely worn out my first copy of Dr. Cotner's book, and my second copy is starting to look pathetically well-used. And for anyone who hasn't seen the soil temperature part of the OK Mesonet, I'll link the three-day average map below. You can increase your own soil temps by mulching the soil heavily in the fall to help hold in the heat, or by covering it with black plastic several weeks before you want to plant. You also can use clear plastic placed over PVC hoops to form low tunnels over beds you want to heat up. The same greenhouse effect that keeps plants warm in our greenhouses will heat up soil in a low tunnel in no time at all. I usually won't plant anything until my soil temps have been in the right range for good germination for 3-5 days and the forecast looks favorable for the next 10 days too. I take my own soil temps using a thermometer with a metal probe, but also check the Mesonet maps to follow general trends. If you have raised beds or sandy soil, you may find that they warm up more quickly than grade-level soil and more quickly than dense clay soil. Dawn Here is a link that might be useful: 3-Day Ave Temp Bare Soil at 2

The greenhouse should be okay. Keep the humidity up. Alex is a rainforest plant so likes the humidity without extremes of temperature. If you get some of the savanna/woodland species, like a lot of the Livistonas, they'd really appreciate the 35's right through and not be worried by lower humidity.

jostob: what have you tried? smoke is worth trying if you are having trouble.