I read a recent comment about re-bloomers not doing well in hot climates. I guess zone 9b would be considered a hot climate. The first irises I bought from a nearby iris farm (no longer in business) were re-bloomers that not only didn't re-bloom, but they all died in the first year.

Still the idea appeals to me, so I bought a couple of pkgs. of re-blooming irises at Costco this past fall and planted them in place of some non-rebloomers that I moved elsewhere. Is there anything I can do to make it more likely that these new irises will re-bloom? The ones I have planted are Beverly Sills (hasn't bloomed for the first time yet); Ozark Rebounder (both bloomed with several irises); Blatant (one out of two bloomed); October Splendor (in bloom now); His Royal Highness (two out of four have bloomed)' ; Concertina (one out of two have bloomed); and Victoria Falls (both blooming).

I grow a lot of "re's", and I'll tell you my "secrets"....but don't spread them around ! First off, mine grow in zone 5a...so I'm WAY colder than you, but, that just means getting rebloom should be easier for you !
I'll start with a few "generalities". You will find that some re's need to be divided quite often, to get rebloom, while others really "resent" getting disturbed.....you will figure out which is which, as you go along. Also, you need plants that are "zoned" correctly (or , at least "close to correctly") for your zone....and that really won't matter too much, to you, as you are 'zoned pretty high", so almost ALL the re's should preform , for you.
I fertilize , in spring, as soon as new growth begins (all my irises)...use a "balanced" fertilizer. "Common knowledge" says don't use anything high in nitrogen (that's the first "number" in the ratio: 10-10-10, for example) I believe in soil tests!! Your extension office can fix you up with a commercial "tester", and the info you'll need to collect, and submit, the soil samples, or, there are "home" test kits, available. No sense putting something on that you don't need ! I've found that , in order to get rebloom, they need "extra", or "more" fertilizer, than the "spring only "irises, so , just before spring bloom...when you se the first stalks starting, I give the re's another "small" dose of fertilizer, and ANOTHER dose, when spring bloom is done...and by "small", I mean , maybe, 2 tablespoon per clump, scratched into the soil ,around the base of the plant. If re's are going to rebloom, they can NOT be allowed to "go dormant" ! All irises WILL go dormant, when spring bloom is over...they are "resting" before they start making increase...so, how do you keep them from going dormant ? Water, and food. When the soil gets dry, to a depth of about 1"-2"...water them well....no mulch around the plants, though, because with the extra water, and richer soil, soft rot will be a problem if the soil is mulched. If you keep them actively growing through the summer dormant period (for most irises, that's about a 4 week period, following spring bloom). I know I'll hear lots of ", Not too much water...not too much fertilizer", but, all I can say, is it works for me.
I grow most of my re's in "dedicated" re beds...that makes it much easier to give THEM the extra rations, without "overdoing it" on the spring only irises. If yours are "mixed" with "spring only" irises, just be "careful " with both the water , and the food. If I was smart enough with the computer , I'd make a graph showing "when" I feed, but, 1st dose (all iris), just as new growth starts in the spring. 2nd dose (re's) just as stalks are starting in the spring. 3rd dose (all irises) when spring bloom has ended, and you have removed all the spent stalks. Subsequent doses for the re's, about every 2-3 weeks, until the season ends (for me , that a "freeze"....for you...maybe you can get bloom all year ?!?)
Try this on a few "test plants", and see what results you get ! As I said, the extra water, and "food" sort of "goes against" all the "iris lore"....but it works for me !

I have never had a reblooming iris rebloom for me apart from the common Crimson King iris. Thanks for the tips Arlyn.

Thank you, Arlyn for the detailed reply. Do you most often use a 10-10-10 fertilizer with them?

5-10=10 is better the first number for nitrogen is y our key. Our any rebloomers which you own hybridized in your zone? I would start with those. If you are an AIS member also join the Reblooming iris Society. they have yearly reports.

I "generally" use whatever fertilizer "mix" that my soil tests say I need. I also use a "higher than normal" amount of nitrogen, than is usually "suggested" for iris. Over the years I've found that what "works well for me", might be the" WORST thing YOU could do. Climates, soil types, etc. are so variable across the country , that what works for one, may not work for all. The old adage is that the first "#" in the ratio...nitrogen...is what "grows the leaves", the second "#"...phosphate...is what "makes the flower", and the third "#"....potash..is what "makes the root". That is basically true, and many of the "bloom buster" type fertilizers have a "high" second number, to promote more flowers....but, phosphate use by the plant is tied directly to nitrogen use, by the plant i.e. , without the proper amount of nitrogen, the plant can't utilize the phosphates ( in our area, at least, the high use of phosphates are the big driver of a lot of the "water pollution"....the plants can't use it all, and it leaches into the water shed).
If you do have a "commercial" soil test...or ask for some insight from your local Extension agent, they will point out several things about "fertilizer ratios", as well as how soil ph (acidity / alkalinity) is tied to the plants use of nutrients, that will explain it much better than I can ! In my years of gardening.. most have been a "learning experience"!!! Sometimes, you "do", and regret it ! . One final bit of advice.....anything that's IN the soil..or that you PUT in the soil, is REALLY hard to get OUT of the soil ! So, ONLY add what a soil test tells you you need !

Don't soil tests usually focus on pH, and suggestions for nutrients go there? And if the pH is 6.5-7.0, all nutrients the flowering plants need (in general) will be available, that is none of the minerals will be locked up since the soil is not too acidic or alkaline? So then any basic fertilizer will do (although I like to use organic rather than chemical fertilizers for the most part). My soil is 7.0 pH on my last soil test an area I grow roses and irises. In the past, it test as low as 6.5, which is still good for essential nutrients to be available.

You are correct in that ph needs to be correct..or close to correct...for whichever type of plant you are growing. A good soil test will also show how much "N", how much "P", how much "K"...what the humus level is, as well as both ph counts. You CAN get results on the trace elements, for extra $$, if you want. There are tons of folks who garden, and never have their soil tested...tons of folks who supplement their soil "by the seat of their pants"...and tons of folks who have wonderful luck doing it. My thoughts are, if the guys who "do it as a business (farmers, large scale nurseries, etc.) test their soils, and add 'accordingly', it's probably not a bad idea !

Here's a listing from the Reblooming Iris Recorder on your irises.
Beverly Sills..rebloom reported in zones 5, 6, 8, 9 in CA, KY, MA, OR, WA, Australia, The Netherlands

Blatant .. Zones 5, 6, 7, 8, 9 , 10 in AZ, CA, FL, IN, KY, OH, SC, TN, TX, VA, Belgium, Netherlands

Concertina zones 4, 8, 9 in CA, KS, OR, Ontario, Australia

Ozark Rebounder zones 6, 7, 8, 9 in AL, CA, MO, OR, WA, Netherlands

October Splendor zones 5, 6, 7, 8, 9, in CA, ID,IA, IN, MS, KY, MD, MS,OR, PA, TN, TX

His Royal Highness Zones 4, 5, 6, 7, 8, 9 inAZ, CA,ID, KY, MO, NC, NE, TN, TX, VA, WA, Netherlands

Victoria Falls 4, 5, 6, 7, 8, 9, 10 in AR, AZ, CA, GA, ID, KY, MA, MD, MI, MO, MT, NE, NM, NY, OK, OR, SC, TX, VA, WA, WV Australia, Netherlands, Switzerland

Thank you, Arlyn. So I should expect reblooming in my zone. Perplexingly, a few have robust growth, but have not yet bloomed for the first time, including one each Beverly Sills, Blatant, and Concertina, and two of His Royal Highness, while others of the same cultivar bloomed in April. Do you think these might still possibly sent up blooming stalks? Is there any way I can help them do you think with additional fertilizer, although they all got bloom fertilizer when I planted them, and a foliar bloom booster (organic) a month ago. Otherwise, they look very healthy.

add fertilizer after first bloom & before the 2nd.

Thank you, Lucy. What fertilizer do you use?

The summer before last I put in a bunch of new irises including Champaign Elegance. They all bloomed last spring except for CE. So I thought maybe it will bloom in the fall...Nope. But this spring it suddenly has 16 fans and at least 6 bloom stalks. It was saving itself for an amazing show!

I think many of the newly planted TB's are going to bloom this spring, maybe due to the mild winter. Often only about half bloom that first year.

I put in a lot of new iris last year, and the vast majority have blooms, buds or pregnant fans this spring!

When we had horses we u sed aged manure. Have not used fertilizer in some years as have not been able to spread it. Use a low nitrogen 5-10-10 if using commercial fertilizer.

tveguy3 said:I think many of the newly planted TB's are going to bloom this spring, maybe due to the mild winter. Often only about half bloom that first year.

So Tom, Irises need cold to bloom in the following spring, usually, at least TB"s?

I unpotted a crowded 12" pot of irises in Feb., which are kind of short...I don't know what kind, but they look like regular irises only they are short (and white), and didn't get some re-planted until weeks later, and I just put them into 4" pots, and surprisingly one sent a stalk with a bloom up. Is that unusual?

So do most irises need some frost?

By mild winter I mean only lows of minus 15 degrees F. Most years we have several days of minus 20 or more degrees. We had a lot of days above freezing this winter as well. As I understand it, most MDB's require a chill period to do well. Not really sure about the others.