japanese iris soil requirements and options

I've been reading up a lot on the Japanese iris, and need some help separating truth from rumor and old news.

First I read that Japanese iris had to be dug up and the soil replenished every three years. Then I read that one person had an undisturbed clump for 20 years, but they had occasional flooding that deposited new soil in that spot. I also read that dumping compost over/near them could also take care it. Some people were saying that planting daylilies with them satisfied some soil need, and to me it sounded like occasionally using superthrive would satisfy the trace nutrient problem, also. Does anyone have an opinion or update on any of these ideas?

Thanks, LJ

I grow over 130 varieties of JIs, and have been growing them for about 16 years.

I can tell you they are not nearly as fussy as people say about their growing requirements.

Under OPTIMAL conditions, JIs reach the point where they need to be divided every 3-4 years. Most people don't grow them under optimal conditions, but they still do excellent. I have some in dry areas, that I have not divided since I planted them 15-16 years ago. You want to divide them before they get so large you will be throwing your back out lifting them (Pirl), or when they start to die out in the center (it's noticeable, they leave an empty ring in the center).

So, many can stay in place for 5-6 years, at least. It's hard to give an exact, as cultivars vary in their growth amount. Some are very slow growing (both Pirl and I found Butterflies In Flight to be). Others grow very quickly, and would need division more often.

When you divide them, new info says to move them to a new site, but I have not found that to be needed. JIs are very heavy feeders, so when you divide them amend the soil very well with compost, or composted manure. It also never hurts to put compost or WELL composted manure on top of the crowns in the spring and fall. The reason that would help, is that JIs grow their new rhizomes on top of the old rhizomes, so when the rhizomes reach the soil surface that starts the decline, as they can no longer get the nutrients they need, and can not get as much water. That's also why it's recommended to plant the JIs deeper than other irises, to a depth of about 2 inches.

I can't imagine what daylilies would supply that the JIs need. I would be interested to know if there was something daylilies imparted in the soil that would help JIs, but nothing I know of.

If you plant JIs in normal soil, with the addition of some peat for acidity, and compost or well rotted manure to feed them, and enough water to make them happy, they are good to go.

If you want to give them OPTIMAL conditions, also give them some Miracid every month or so, I alternate every two weeks with Miracle Gro, and extra water, but this is for OPTIMAL growth, and I can't stress enough they don't need it to grow and flower well.