How do you distinguish Azalea from Rhododendron? Another shrub I wanted to identify has fairly similar flowers to that Azalea and has leaves that look more like rhododendron (in my terribly non expert option). While the shrubs I have that I know are rhododendron have flowers that are very different in shape (and flower later in the spring).

Rhodys are usually much larger and have very large leaves in comparison.

Regarding size: That last one is one my wife bought from a nursery three years ago. About half the shrubs she has purchased over the years died (usually sometime in the spring after surviving their first winter). The rest tend not to grow much even when surviving multiple years. So I assumed this one is small, not because it is natural to be small, but because it never fully recovered from being transplanted. The earlier photo was a shrub that was already there when we bought the house 29 years ago. The leaves on this small shrub are larger than the leaves on the big Azalea of the first photo. Anyway, did your answer mean the small shrub is Azalea? or unidentified, or what? (My wife never recorded what shrubs she was purchasing).

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How do you distinguish Azalea from Rhododendron?"

All azaleas are rhododendrons but not all rhododendrons are azaleas :-) 'Azalea' is just the common name for certain species of rhododendrons. They can be both evergreen or deciduous and usually (but not always) hold their flowers individually rather than in clustered trusses (and not all rhodies produce clustered trusses!!)

Leaf size is not always a good indicator as many rhodies have quite tiny leaves (lepidotes) - smaller than even most azaleas! Botanically, the difference is determined by the number of stamens - rhododendrons have 10 stamens in each blossom; azaleas have only 5.

Thanks for giving me an important detail to check. But that led to a bit more confusion. What would six or seven stamen mean?

First red flowered shrub in my other recent thread:

http://forums2.gardenweb.com/discussions/3918053/shrub-with-red-flowers-in-may-in-eastern-massachusetts?n=4

That has 11 tiny stalks in the typical blossom. The (usually) longest one is clearly different (I assume style) and slightly longer than most. The other ten randomly vary in length. In some blossoms the longest stamen is longer than the style.

Next, the white one top of this thread. Most blossoms have 7 such things: Longest is (always) style, then five stamen equal in length to each other and slightly shorter than the style, then one more thing that looks just like a stamen but much shorter (is it some other flower part?). Then a few blossoms have eight total: Style longest, four stamen slightly shorter than the style, plus 3 more things that look like stamen but much shorter.

3rd, the see through white in my second picture of this thread. It flowered earlier and most flowers are damaged by now. But each undamaged flower clearly has ten stamen of exactly equal length plus one style over twice as long as the stamen,

So you are saying the red flowered "azalea" from my other thread isn't an azalea, nor is the transparent white flowered "azalea" (that I found a perfect match for being sold online by a nursery named a "pleasant white azalea"). You sound like you know what you're talking about.

Does the clustering mean much, or just health and/or maturity of the shrub? The giant bushes that I'm confident are rhododendron (with purple or white flowers) haven't flowered yet, but I can see (and recall) they have tight clusters of many small blossoms, such that what I recall as their "flower" was not a blossom but a cluster. The one at top of this thread has looser clusters of up to six blossoms, so I was seeing each blossom as a flower. The ones with red and clear white flowers have mostly single blossom flowers and some small clustering.

BTW, all three have five petals per blossom.

I ought to remember which of these are evergreen, but on smaller shrubs I tend not to notice. Looking through some old photos: The shrub foreground left in this photo is a giant old rhododendron, purple flowers, definitely evergreen (though you can't tell from this photo.) The shrub foreground right in this photo is the one discussed above with red flowers. Can't recall if it is evergreen.

Nothing is really absolute in the gardening world but in general, rhodies will have twice as many stamens as will azaleas. Azaleas can have more than 5 stamens but I would think that would indicate some true rhododendron parentage in the hybrid. Species azaleas should always have only 5.

Clustering or flower formation is variable as well. In general, azalea flowers are produced individually on stems (terminal flowering) but often these stems are so tightly clustered together that they appear as a single mass of flowers or a truss like rhododendrons produce. This link may help: Classifying Rhododendrons and Azaleas. And this one that really highlights the parts of an azalea flower up close and great detail!

I tend to just refer to them all as Rhododendrons since that is the genus, and not worry too much about whether it is an azalea or rhododendron as far as the common name. Makes life simpler. ;>)

When I started the two threads, I hadn't even guessed the genus. So I'm glad I asked. Given the contradictions in the online info as applied to these three examples, maybe that is as much as I'll get. Online it said the two different leaf types on one shrub are "spring" and "summer" leaves distinctive of evergreen azalea. The older leaves match the description of "summer" leaves meaning they must have lasted all the way through the winter, while the new leaves are the spring leaves. I'll watch to see how that tracks to confirm they are really summer and spring leaves as described. So the one with red flowers takes opposite sides of two classification rules (two leaf types on one shrub is Azalea. Ten stamen is rhodie.) The spring leaves on the red flowered one are such a perfect match for the spring leaves on the white flowered one, which feels like it must prove a close relationship. But ten stamen vs. six implies otherwise, and the summer leaves of each don't match the other. (The only leaf type on the pale flowered shrub look like a miniature version of the leaves on the full size rhodie, and it has ten stamen, so that one seems to consistently fit online rules for not an azalea).