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The Trouble with Iris
Q. I have irises that are supposed to bloom twice a year, but they have only bloomed once—and that was in the spring two years ago. Light isn't the problem (they receive morning sun), so I obviously need something to give the poor things a boost. But I don't know what to use. I have been giving them Holly-Tone, but it doesn't seem to be helping. I heard you talk about Calcium Phosphate on the show; would that help? And where can I get some? I've called three places and they don't know what it is!
    ---Berta in Wayne, PA
A. There must be something about the combination of those two words, because Berta is not the first listener to trans-combobulate and mis-remember the substance ROCK PHOSPHATE, a naturally occurring mined mineral that functions as a super-powered plant bloom booster. There is a CALCIUM PHOSPHATE, but it's no garden amendment. In fact, Berta might be the first person to ask garden centers for the calcium component of cow's milk.

Anyway, Irises do best when fed lightly, and I suspect that these 'poor things' are already on the overfed side as opposed to 'needing a boost.' At least the Holly-Tone they've been getting is a natural fertilizer. But it's also a famous soil acidifier; and like other fertilizers that lower soil pH as they feed, it's designed for plants that require an acidic soil, like azaleas, rhododendrons, blueberries and, of course, the namesake hollies, while most iris thrive in soil with a neutral pH. (Japanese iris—bog plants that thrive in and around water—are the only Iris family members that crave acidity.)

And since our listener has specifically discounted lack of sunlight as the problem, I will seize upon it as the answer. Irises are notoriously light hungry; the more hours of sunlight they get, the more likely that you'll see lots of flowers. Try transplanting the roots into an area that gets morning and afternoon sun—and have a light hand with any fertilizer—and I suspect the blooms will return—at least in the Spring.

...Because iris that "flowers twice a year" is not very common in the North. Now some tall bearded iris may seem to be repeat bloomers, as they've been bred to have as many as ten buds per stem, and in ideal situations, that sequence of flowers opening up in succession will cover a nice stretch of time. But true reblooming irises produce flowers reliably in the Spring (again, providing they have the correct conditions, especially enough light), take a distinct rest, and then have the capacity to flower again in the fall.

But that second run of flowers is far from guaranteed in any situation, and rebloom becomes less and less likely the further North you garden. And you have to plant a variety designed to rebloom in your specific area; a West Coast rebloomer won't do a double gainer on the East Coast.

Q. I am new to your show and gardening. Perhaps you have already answered this question, but I have a number of bearded iris plants in my garden that send up full leaves each year but no flowers. Are the bulbs just done or is there a way to get them to bloom again? They are in full sun and we have very sandy soil. I live in northwest lower Michigan (the pinky of the mitten). Thank you very much.
    ---Wendy in Empire, MI
A. You may be a rookie, but your 'bearded' identification is very helpful, as there are many types of plants in the huge iris family, from true bulbs like the tiny and beautiful Iris reticulata that poke up in the Spring to your 'bearded' plants, which despite being in the same genus, grow from an underground rhizome (a fleshy root-like thing) and can tower close to four feet in height. Some bulbs do peter out over time, but that's not generally the case with rhizomes—and lush leaves are a sign that the plants are basically healthy; pooped-out bulbs simply fail to reappear at all.

I will accept your report that these plants have lots of sun and excellent drainage, both of which are essential for getting blooms on bearded iris (which get their name from the fuzzy frills on their lower leaf petals; iris without such chin hair are called 'beardless'). And when sun is strong and soil sandy, the best guess regarding lack of blooms is that our listener has simply treated these things like other plants and buried what is obviously the root area well below ground. This will serve you well with 99.9% of the plant world's denizens, but not with bearded iris.

While some clean-shaven iris can do well with their rhizomes completely covered, tall bearded iris need to have at least the top portion of that rhizome ABOVE the soil line. Yes, the actual root—above the soil line and exposed. No, this makes no obvious horticultural sense, other than the fact that these plants adore good drainage, which being above ground certainly helps achieve. No matter the reason, many iris growers have told me that they had to cast away all planting cowardice before they saw really ridiculously lush flower clusters on their plants.

It is an unusual trait, but a few other plants share it. Think of those giant amaryllis flowers that brighten up our holidays. Growers who have gotten them to reliably rebloom have learned (often the hard way) that at least half the bulb needs to be above the soil line for the plant to be happy.

So when it comes time to plant (or replant) the rhizomes of bearded iris, don't be deep. Be shallow.