Could you be a bit more specific as to where in Colorado? The elevations vary widely in this state. As far as the Denver metro area is concerned, I would have to say with a resounding "no" that they are not necessarily smaller. Iris size can be impacted by any number of factors: sunlight, location, water, soil conditions. But in general, elevation is not a factor in the Denver area. Now, in the mountains, perhaps.

So, it would potentially be late cold snaps, lack of water.. etc that would contribute to Iris being smaller instead of elevation? Makes sense, thank you

Mind you, I've never seen the iris in Oregon.

I did just think of something. I think there are iris that have specifically adapted to alpine climate and they are smaller like a dwarf variety.

There is a poster here named Gardenchloe. She lives in Westminster and she has the largest bearded iris I have ever seen here. Even the leaves on her plants are huge. I don't know what she does to them, but they are obviously very happy.

Elevation does indeed have an effect on plant growth. However, not necessarily directly. Smaller size is a result of having to cope with the harsher environment that accompanies the elevation rather than merely the elevation in and of itself. The shorter season found in the high country for example forces plants that live there to grow, flower, and set seed in a much shorter timeframe than pplant at lower elevations with longer growing seasons.

The same can be said of plants growing in the Denver metro area, although maybe not exactly the same factors at play as they are in say, Leadville at 10,000 ft. But the arid shortgrass prairie/steppe climate that is found along the Front Range poses its own challenges. Nutrient poor, mineral rich soils, high winds and a scant annual rainfall of only 15" prevent plants from reaching large sizes that may be found elsewhere. While we do share many native species with other Great Plains states, ones such as big bluestem grass are a mere shadow of the same plant growing in places like Illinois.

Some of this stuff we can control in our gardens of course, but I think the fact of the matter is that in general, many of our favorite garden plants growing here are not going to preform on the same level as they would in a place like Oregon.