I would say no fertilizer or compost for zone 5 right now. Save it for spring. Winter's coming.

Compost should never cover the tops of the rhizomes (bearded). Sounds like you have the gardening itch. Perfect time to dig a new bed with compost and let it age over the winter. I try to divide right after spring bloom and dig in compost at that time.

way to late in my z5 MI ....

fert MIGHT cause lush vegetative growth ... when the plants are supposed to be slowing down and hardening off for winter ....

killed about 25 hybrid T roses with thoughts like these... in another lifetime ...

after learning said rule.. i never fert after about 8/15 ... giving the stuff a good month to do its thing ... and then hoping it is all used up by about right now ...

and that goes for any plants.. not just iris.. in my garden ...

let me state it backwards ... if you put so little on.. that you remove the risk .. then it probably isnt worthwhile ....

ken

I don't think I would fertilize now, either. But I am not too afraid to recommend lightly mulching, depending on what you use, for the ones that you just planted. Maybe not for the ones that you planted earlier and are growing well. I think it depends on how wet it is where you are, how deeply you planted your rhizomes, and some other factors. The idea is that it can help keep your rhizomes from getting frost heave. This is when the ground freezes and thaws over and over and the expansion pushes the rhizomes out of the ground. Mulching keeps the ground from changing temp so widely and often. If you do mulch, be sure to remove it in the very early spring, which is a good time to clean up all the dead leaves and such around the plants before they can contribute to rot.

aseedisapromise ~

I've always been curious about winter mulch in wet-winter climates. Does it encourage rot? Or are the temps so cold it's not an issue? TIA

It has to be a very, very cold place before mulch is reliably frozen the entire winter. I've heard rumors of such happenings in zone 3, but that's about where you have to be. Here, even thinking about winter mulch can rot iris.

I have lost rhizomes when they got pushed out of the ground by the freeze/thaw cycles that we get around here. It is really detrimental when it occurs in a place where the wind is actively drying things out all winter long. There is no dry like the dry that goes along with sub zero temps. My city holds the record for the quickest and biggest temperature change ever recorded. (From Wikipedia: "Spearfish holds the world record for the fastest temperature change. On January 22, 1943 at about 7:30 a.m. MST, the temperature in Spearfish was -4F (-20C). The Chinook wind picked up speed rapidly, and two minutes later (7:32 a.m.) the temperature was +45F (+7C) above zero. The 49F (27C) rise in two minutes set a world record that still holds. By 9:00 a.m., the temperature had risen to 54F (12C). Suddenly, the Chinook died down and the temperature tumbled back to -4F (-20C). The 58F (32C) drop took only 27 minutes." I know that this example isn't really going to show a change in the temperature of the ground much, but we do get thaws for days in Jan, and then very cold again. Often my spring bulbs all start to grow in January. I can't plant anything that can have only one season at a time!

So around here some people rely on mulching to help. A lot depends on when you mulch, and when you take it off. I would say that I am in a place that is between the very dry west, and the wet east, so a lot depends on the kind of winter we get. Sometimes the ground will stay frozen all winter, but not too often. Sometimes it is so dry in January that the frozen ground turns to dust as it sublimates. Sometimes we get three feet of snow in December, and then it sticks around all winter insulating the ground so that it never freezes. So you have to look around and decide what to do based on what you see happening. Mulching is something you can do after the ground freezes, to keep it frozen in the years that we get 60 degrees in January. But you do have to be careful to take the mulch off before the ground generally starts to thaw for good. In the snowy years I have to go around and remove it from each place that gets clear of snow, as it gets clear. Then I remove the dead leaves from the rhizomes so they are nice and clean and rot free. Then I go in and try to thaw out my hands. The wet year that I didn't get right out there and clean things up, I did have some rot.

So maybe I shouldn't have said anything really, since I have this extreme climate. It is zone 5, but really squirrely. But I do pile leaves up around my iris on the side of the garden that gets the wind, and then take them off in early spring, and I have had better success that way.

For years, every october, i've given my iris a shot of 0-10-10. The extra potassium hardens their cell walls, and seems to increase hardiness. Here in nevada, the soil +K levels seems adequate for the most part, but a little extra doesn't hurt. My soil is very sandy/gravelly, so believe it or not, i have to water all winter long, so I generally plant iris a bit deeper than what would be recommended in other regions. As a consequence, my iris don't get to heave much due to the low moisture content of the soil, even tho we'll have 40 degree swings in temperature daily. Mulch simply blows away here, so there isn't much benefit. I'll mulch first year plants with fine straw or pine boughs, corralled in cages around each plant or row, but thats about it. We'll get 50 - 70 mph winds frequently during the winter and spring months so mulching is difficult at best

timmba, in general, when it comes to phosphorous, a little extra can hurt. Extra phosphorous is mobile in the soil, and is a major cause of the large dead zones at river mouths across the globe. I try to check my soil for what it needs, and give it only what is necessary.

I don't know about its effect on iris in particular, but there is research into phosphorous that shows it does increase hardiness, but I don't think that in zone six that is really an issue for bearded iris anyway. Also, you would have to know exactly when your plants were doing their hardening off for the winter, and then time the phosphorous appropriately for it to do any good.

I am really happy that I am not alone in my drying winds and big temperature swings. I can relate to mulch blowing away! I think it is good to garden by looking at what is happening today, and not operating always by a set of rules.

And isn't it a tribute to iris that they can survive in so many different places! I just love them.

aseedisapromise --
I can assure you after 30 years of growing iris and a career in horticulture, that i not only stand my my practice of adding exta phosphate and potassium -- i recommend it for all rhizomatous plants and bulbs in the fall. I mix my own formula to approximate a 0-10-10 mixture of rock phosphate and slowly available potassium.
To equate my practice with the irresponsible practice of applying readily available phosphorus by the ton to miles upon square mile of lawns in this country hardly seems justified. i don't have a lawn, and my landscape in mostly natives. Thats is the greatest culprit in the phosphatization of our waterways.
I can further assure you that i know exactly when and what my iris are taking up in any given time of the year. In the spring bearded iris take up all NPK ratios at a 1-1-1 ratio. After blooming, due to hormonal changes within the plant the process reverses to approximately a 1-2-2 ratio, to facilitate the development of anchor roots and rhizomes for next year. In the fall when little photosynthesis is going on, the process drops to a 0-1-1 ratio, with the greatest concentration of phosphate and potassium in the rhizome and root structures that facilitates hardiness.
Since phosphate in its natural form breaks down slowly and doesn't travel far in the soil, the little that i use doesn't in any way contribute to the environmently damaging use of superphosphate in lawn fertilizers, and commercial crop fertilizers.
I'm not a novice at this, and its not correct for you to assume so

Well, sorry if you took this personally. I just think about all those folks everywhere who search for info on fertilization of iris, and I wanted them to have more things to think about than just plopping on the 0-10-10 in October. I don't think there is even a waterway in NV that makes it to the ocean! But I noticed the OP was zone 5, and for my zone 5 October is too late. But I also perform "fall" fertilization of bulbs and iris which for me is before fall as it exists on the calendar. We have a short growing season here. I wasn't trying to say you are irresponsible. I just wanted to try to increase responsibility in general. And I think we can't just always assume that it's the other person who is responsible for problems that exist, and what we do makes no difference because it is done on such a small scale. The rewards for a practice always accrue to the person performing them, but if there is harm it accrues to everyone. I guess you know this. I'm happy you have been thinking about these things, too, and acting accordingly. Anyway, I can see I need to work on my writing skills to get the points across that I am trying to get across. And it wasn't that timmba is an ignorant, irresponsible gardener, but that one has to use the correct fertilizer, and time the application correctly for it to do any good, and to not, in fact, do harm.