Ask Mr. Smarty Plants

Most likely the white iris are the most vigorous and they have taken over the spot under your tree. I suspect the additional non-white iris planted throughout the years have not survived their first winter and have died out, leaving the strong white ones to flourish.

Here are some thoughts from David Troyer about bearded iris (although non-native) changing colors from a search online. 

Why did my bearded iris change color?

That is the question I get asked quite often. In fact a couple of years ago while I was in my iris garden a couple came by and asked me that question. Now my first answer is that they do not change color – more on that in a minute. But back to the couple visiting the garden. When we began to talk and I explained that bearded iris do not really change color the man got really mad and finally yelled at me, “I know what happened, don’t tell me they don’t change colors because they did!” It was time to stop talking. After all someone convinced against his or her will is of the same opinion still. In case you are open to an explanation about what happens to make you think bearded iris change color, I have 10 explanations for you. I think this will cover all the bases unless you don’t want to hear. Most often it seems those people say that purple iris change to white or vice versa. So I will use that as an example.

So here goes -- 10 reasons in no particular order:

1.  Certain bearded iris cultivars are more vigorous than others and can choke out the less vigorous. So let’s say you plant purple and white iris but in a year or two they have all changed to white. In fact if you were to dig up every single rhizome and replant them under ideal conditions you might find a few of the purple bearded iris you thought were dead are still there. Or perhaps the purple ones were completely choked out.

2.  Let’s say you dug your original irises from a bed where only purple irises were blooming, perhaps you assumed that you dug only purple irises. However, there may have been a number of white bearded irises in the original bed that weren’t blooming at the time you dug but that did bloom later in your own garden.

3. Another possibility is that you dug a tiny piece of a white bearded iris along with other purple iris and in a year or two the white iris which is vigorous grows and begins to bloom. As it spreads and crowds out the purple it appears that the purple iris are changing to white iris.

4. It is even possible that someone or something could have thrown or dropped a piece of another color iris in your bed. Like a child, neighbor, lawn mower, or animal tossing or dropping it. It is amazing how hardy an iris can be. I know I have tossed away pieces of iris rhizomes that I thought had no life in them or were rotting and found in a year or two they have established themselves. So it is very possible for this type of thing to happen.

5. Another possibility is that a flower was pollinated and the seed developed and fell to the ground where it grew. It might have taken a couple of years and then it blooms and grows and slowly takes over the iris bed. Bees do pollinated bearded iris and the seeds will develop and fall if you don’t collect them. And they can be a different color than the parent.

6. A chemical herbicide could have drifted onto your bearded iris and can cause a temporary (usually)  change in an iris pigmentation. I know it can do some weird things to iris. Once I had Bermuda grass in my iris so bad that I decided to take a few out and spray the rest with roundup. It made them bloom with parts missing and different colors.

7. The soil, sunlight, and chemicals in the soil can make some pigmentation variations, although not usually great changes.

8. Strong sunlight, heat, or shade and the temperature can also vary the color. But not to the degree of changing colors completely.

9. Bearded iris often do not bloom every year. So it is possible that a dark iris like a purple one did not bloom in a particular year. So, you could dig what you thought were all purple iris one year and planted them. The next year you are surprised that they are all white or part white when you only planted purple iris. But really the purple just did not bloom the next year. Very often I have noticed that some years a certain variety will fail entirely to bloom even when I had hundreds of plants.

Now the last reason why bearded iris appear to change colors:

10. This is a little more complicated. On rare occasions and iris may produce a “sport” which means the iris may produce and increase the differs greatly in appearance from the original plant. These sports do not change the original plant but they could grow and be vigorous as in the first example. There is an iris named HONORABLE that is known to have produced several sports over the years.

I understand from others that there does seem to be a rare bluish purple sport of the old hardy white iris commonly growing in the south and that the purple sport is quite vigorous.  However the purple sport sends off shoots that produce only white flowers and the purple sport could them seem to disappear. I suppose this would come the closest to a real change in color.

All I can really tell you is that people and companies who have been growing bearded iris for over 100 years all say that they don’t change colors. If I ever see it happen I will be happy to agree that it does. But in 50 years of observations I have not seen a bearded iris change colors.