Hi Will,

You are right - a juniper will not make a happy house plant. They require a winter dormancy so to grow one indoors, you will have to mimic its outdoor environment: for 3 - 4 months during the winter, low light, low temps, low water.

That will hold true for any plant that like cool (alpine) conditions. Likewise, if you want to grow a plant that, in nature, has high temperatures and humidity, you will need a greenhouse to mimic their natural environment. The plants we normally consider houseplants are houseplants because our indoor environment mimics theiroutdoor environment.

But you knew all that.

Daisy

I learned with Bonsai that it is still whatever kind of tree, just smaller. We would never think about planting a Juniper, Japanese Black Pine, Maple or any other tree indoors. I think in terms of what the tree would require if it were grown as a normal tree.

Yes, heat and humidity requirements would be very helpful. I presume it took decades to collect the information for winter hardiness; I think collecting it for summer would be more challenging. Any nursery could collect the winter numbers, however the point of most nurseries is to get a plant to "growing size" and get it sold. They can collect info both of their own experience in survival as well as feedback of customers who lost a plant over winter. However the individual gardener would generally be the one who knows if it made it through summer. That info would be hard to discern though, since the group collecting the info would not know if the plant died from neglect, heat, humidity, too much rain/watering; there would be many unknown factors.

I can see where it would be difficult for nurseries to incorporate heat zone information. If I put a "full sun" plant in full sun and another in a less sunny site they would both be in the same zone; if the less sunny were morning or afternoon it could make a huge difference. The same is true of course for winter conditions - how close to a house, rocks, snow cover, windbreaks, etc. I guess when it comes to final analysis we try a plant and see what happens. Gardening is at more art than science. Finding others who are successful and asking questions is one of my most important tools - I could never possibly know everything I want in a lifetime.

Thank you Tiffany! If I understand correctly, I can use the hardiness zones to determine minimum required by taking the highest listed zone number and using the top end of that range. Thus, if a plant is listed as Zone 4-9, then it would probably need a minimum of 30 degrees F.

However, at some point that seems to break down. Many commonly used indoor houseplants are listed as Zone 10 with a 30-40 degree minimum temp range. Yet, we know they do just fine all year round in 70 degree indoor temps.

Again, Will with the zone designations you're talking "survival" not plants actually thriving and being beautiful. I always think when I see "zones 4 to 10" on a plant label that this plant will most likely do best in zone 6 or 7 but it doesn't tell you that.

So lots of things that grow outdoors here (I'm almost in zone 10 for what it's worth) do experience temps down into the 30's occasionally - that would be maybe 3 or 4 winters of the 15 we've lived here. The plants "survive" but were significantly slowed down for a year or two afterwards so there was damage they wouldn't experience in an indoor controlled environment.

Just a thought on the juniper as a houseplant idea - couldn't you just put the juniper outside for the winter to give it the cold dormancy it wants? Maybe a bonsai in a very small container would be more frost sensitive, but in a protected spot? It might be worth a try, if you really wanted to grow such a plant indoors.

Speaking as a dedicated pusher of zones here . . . can you tell?

There's something magical about going from Z9 to 10. It's just as powerful as the difference between Z7 (ground freezes a little,) and Z8, where it never would. Any plant only hardy up to Z9 is not suitable to be inside permanently. Once one gets to the warmer parts of Z9, there's no opportunity for any kind of reliable (maybe some winters, but not others) cold dormancy.

In addition to considering outside, an unheated room, garage, basement, greenhouse can be an option for trying to make potted hardy plants happy. Some are easy, like putting tulip and Hyacinth bulbs in the 'frige for a month.

For indoors, the easiest thing is to stick to tropical, opportunistic growers, plants that grow whenever conditions don't prevent it, and are likely to never experience a slump if indoor conditions are stable. Generally, hardiness and house plants are mutually exclusive since most plants traditionally kept in pots inside are of tropical origin. Those from tropical regions are perfect in regard to temps, with other issues possible, like plants that are divas about low humidity, or that need more sun than what is possible for the few hours per day the sun might shine directly into an E/S/W window.