Growing Japanese Iris

I was lurking in the Iris forum yesterday and picked up on a problem with JI. I didn't know about this and thought I'd share in case there are others out there with JI who also didn't know. I know I saw pictures of JI in Steve's garden.

"At the time of division, the iris should be moved to a new location where japanese iris have not grown before. It appears that JI excrete something into the soil that causes the iris to lose their vigor over time and decline. It is recommended that something other than iris be planted in the soil since other types of iris will grow there but not vigorously. The one exception to this is soil that has a lot of water leaching through it such as soil under a downspout or by the side of a stream. Whatever is causing the iris to lose vigor appears to be removed by water leaching. Research is ongoing in Japan to determine what is causing this effect. Japanese iris in Japan are commonly grown in pots and it is easier to replace the soil the iris grow in."

They then go on to suggest that JI be divided every 3 to 4 years. When you start to see roots coming out of the soil, that's the time to divide. The gal who brought this up says that she grows her JI in pots and then buries them to overwinter.

Hi beaker- I saw your post on the iris forum but didn't get around to responding. Here is what I understand goes on with JIs:

1. JIs are very heavy feeders and depletion of soil nutrients (micronutrients) affects enzymatic reactions that slow the iris's growth.

2. ph needs to be acidic, with 5.8 the best to aim for for optimum uptake of nutrients.

3. Enzyme activity is also affected by temperature with warmer temps. speeding growth but too warm slowing it.

4. Other factors resulting from overgrown clumps (that's why they need to be divided frequently) are detrimental to the surrounding soil. Their rhizomes are rather woody, making it difficult to get nutrients to the roots.

5. Modern Japanese iris cultivars are all descended from one common ancestor, Iris ensata. Unlike our beloved lilies which are a combination of a number of lily species, the hybridizers of JIs have nothing different to bring into the gene pool to improve hardiness.

I have the specie Iris enstata growing in the same place for the last 15 years, the only plant from the Michigan Bulb Company that has persisted in my garden. It doesn't even get water in the summer!! (but it hasn't bloomed in years either, but isn't dead!). Also an older variety that I just shovel prune every couple of years but I have never moved it. So I think there is something about the modern cultivars and their lack of vigour.

I don't know if there is any real "toxin" that they excrete into the soil; it just seems like it due to the factors that affect their vigour. That is why growing in pots and dividing often may provide more satisfaction and success. I have been growing and occasionally hybridizing them for the last 10 years or so with mediocre success, mainly due to lack of time to give them the attention they need. Two rows of seedlings, about 20 plants, dwindled and disappeared over the course of about 5 years. I should have wised up and payed attention to the last one or two standing and use for hybridizing!!

If you want to try them go for the cheapest, Aitkens Gardens prices start at $5. I bought a bunch of new ones from them last year. Or look for them potted in a local nursery. They need lots of water all summer but they don't like sitting in water in the winter. I lost a few that way because my backyard floods in the winter. This year I'm using composted steer manure and compost right on them. I hope they like it and don't croak on me.

Thanks so much for the response, Pard. I have four on order and didn't pay much more than the 5.00 you suggested. I was going to put them directly in the garden, but am now leaning towards pots. What's four more along with all the others? Any suggestions on fertilizer? I don't have access to compost, so I'll probably use a good peat based mix. Right now I use the MG sprinkle stuff in the general garden and then I do use regular rose fertilizer on my roses and clematis.

Whether they are leaching something into the soil or not, if they are that heavy a feeder, I would be concerned with them having an impact on other plants, like the lilies.

Maybe you can try one or two in the garden and the rest in pots. That way you can compare how they do. Get a bag of composted steer manure, its cheap and well worth it for JIs. Work it in well. You can also top dress established JIs with it. If you haven't used it before and think it is gross it really isn't. If it stinks bad it hasn't been sufficiently composted! You should get a complete culture sheet with your JIs if you ordered from a reputable grower. I think the instructions I got with my last order said to NOT fertilize when you first plant them (bare root). When established I use whatever I have like Miracle Grow or better Miracid. I have also used rhody/azalea fertilizer. My new JIs are just inches from my lilies so I will see how that works out. They are separated by a divider of those scalloped concrete edger thingies you see all over, and the lilies are a little higher also. Gardening is just one big experiment for me!

Probably the most important thing I can tell you is when you get your order, open it immediately, clean off the plants and plant ASAP! They can not lay around like bearded irises. If the roots dry out you will have problems from the get-go.

Pardalinum: I've been dividing JI's by using a knife right through the soil since they're all planted in the garden (four gardens) and for the vacancy, where a chunk was removed, I've been adding cow manure (the only one I can get) and our own compost. Then I water very well at least twice but generally more often to settle it in and I do add mulch of long needled pine. Am I going about this the right way?

Hi Pirl-- That sounds ok if the cow manure is very well composted. I usually do the same thing if I don't want to remove the whole clump but I just mix some compost with the native soil. My concern here is these plants will be going into dormancy and too much nitrogen may not be a good thing.

Thanks for the response. My thought was manure is generally 1-1-1 so it wouldn't be too much nitrogen. Maybe I'll mix the manure and compost for the vacant spots. I do appreciate your help.