Soil preparation/amendments for Irises...

Our soil is a clay/loam type and I would like to amend with material to improve drainage for a new iris bed. Some have suggested amending with 'course sand', however, at the garden centers around here they seem to have only fine 'play sand' (not recommended) and they don't know anything about amending with 'course sand'. Others have amended with 'crushed gravel'. Is anyone familiar with exactly what that is or where to get it (if it is useful)? Others recommend a 'clay chemical additive' which seems very expensive. We have already added some compost. If you have added 'course sand' or some other amendments or additives to improve drainage for irises, please tell us about it and also what your recommended proportions are. Thanks. T.

My native soil is extremely heavy clay, and I add anything and everything I can find to loosen it up for the irises. Among the amendments I have added are: sand (any type), perlite, gypsum, sphagnum peat, alfalfa meal (horse feed), and lots of well-aged (10+ yrs) horse manure/pine sawdust cleanings from my barn. I don't compost, but if I did I'd be using huge amounts of that, too, along with leaf mold (or just ground up leaves) and any other organic material I could get my hands on. Clay tends to gobble up organic amendments fairly quickly, so they need to be added every time the bed is reworked. Sand is a permanent addition, but if it is added without sufficient organic amendments, it can combine with clay to create concrete.

My greatest success in growing irises has been by raking the soil up into windrows with gutters between the windrows. The windrows are only 4-6" higher than the gutters, and the irises are planted on top of the windrows. This arrangement provides exceptional drainage around the rhizomes, even during periods of heavy rainfall, and keeps the soil in the windrows light and uncompacted for easy root and rhizome growth.

Light soil and excellent drainage are the keys to success.

Laurie

Well! Thanks! Your amendment strategy sounds like a 'can't miss'! I guess underlying my anxiety about the amendments is getting too much nitrogen in the mix (and then, of course, adding the wrong sand and turning the soil into concrete!) I suppose I will go 33% soil, 33%compost/manure and 33% course sand unless someone tells me that's inadvisable... About the raised beds--ours are raised--some about a foot--with low stone wall edges. They have sufficient drainage gutters I think. This will do for the mixed border if I focus on drought tolerant perennials and herb-y plants and do hand watering, I hope. For winter mulch I read something about putting down an inch of course sand and an inch of sphagnum moss...That sound like it might be a good thing to do. Thanks again for the insights. T.

T.,

Sand+clay=concrete is only a problem if you don't add sufficient organic material to lighten the mix. Sand has been my most useful and long-term amendment, but I do also add plenty of organics.

Nitrogen can be a problem for bearded irises in clay soils if added in excess. My horse manure is mixed with so much wood product and it is so well-aged that its nitrogen content is probably very low. It would be advisable to stick with low-nitrogen organic amendments.

I don't mulch my irises because I don't care to coddle plants that can't handle my very harsh climate on their own. Mulch can, however, be helpful for newly planted irises during their first winter. I know some folks mulch with sand, but if it is applied too early or left on too late, it may promote rot and/or a lack of bloom in bearded irises (bearded irises may not bloom if buried too deeply). I suggest you wait until after the ground freezes, then mulch your irises with evergreen boughs. The boughs won't compact around the rhizomes, and they'll allow good air circulation while maintaining a steady soil temp. They're also easy to find right after Christmas if you cruise the curbs around your neighborhood. They have the added benefit of being a whole lot easier to remove in the spring than an inch or two of sand/moss spread over your irises.

Laurie

Laurie, I want to be the first to say that you're brilliant. What you said made so much sense and it is exactly the advise that I need. I was wondering why my irises have rotted. Now I know I haven't raised my windows enough nor have I dug deep enough gutters. I can just see me explaining this to my husband this weekend. lol

pins2006,

Well I'll tell ya, my "brilliance" has come at a hefty price. I've lost hundreds of iris cultivars to rot over the last several years. This windrow/gutter planting arrangement was new for me last summer, and it's working beautifully so far.

As I mentioned, my native soil is extremely heavy clay. Before the windrow/gutter system, my main beds were planted flat on a very slight grade over my septic drainfield. The soil drained well in spite of its heavy texture, but I kept losing large numbers of irises every year, anyway.

We had a couple consecutive winters with severe cold (as we always have here in northern MN) and virtually no snow cover. I lost about 300 iris cvs over those two years, but I don't think the losses were exclusively attributable to the frigid, snowless winters. In fact the more I observed my iris beds, the more convinced I became that many of the losses were caused by suffocation in my compacted concrete clay (probably exacerbated by the stressed condition in which the irises came out of winter). Those poor irises simply couldn't grow or even breathe in that miserable clay ... but they sure could rot!

Last summer I dug and renovated most of my iris beds. I amended heavily and tilled repeatedly to lighten the soil and then raked it up into planting windrows. To my great delight, we also FINALLY had decent snowfall last winter, so all my newly divided and replanted irises were well insulated through their first winter. I had very few losses this year for the first time in years!

One other very important benefit of the windrows/gutters for me is that the gutters naturally attract foot/hoof/paw traffic. We not only have a rather large 4-footed family who loves to help me in the garden, but also a heavy local deer population who loves to wander through my iris beds. When I was planting flat, the deer and other animals (not to mention my own big feet) would quickly compact the clay into concrete all around the rhizomes. Now that the rzs are planted on top of the windrows with these convenient "walking paths" (gutters) running between rows, I'm finding very few paw or hoofprints on top of the windrows, and the soil is remaining light and uncompacted for the irises.

Oh, one more thing - the gutters are extremely easy to keep weeded. I just run my little Mantis tiller through them once a month or so to cultivate and till the weeds under. When I hand weed the windrows, I just toss the weeds into the gutters and till them under, too!

Since this windrow/gutter arrangement is only one year old, it's still too early to tell how it'll work over a longer term, but I'm sure pleased with it so far.

Laurie

Great explantion, Laurie. What size window do you find works best for you? Also, same question for the gutters. I really do want to try that with an area in my garden that used to be a stream before the county moved it into a retention pond. (Development went in behind my house.) Anyway, the water is gone, except for when it rains and the whole area retains water for the longest time. It really needs a lift.

Carmen

Carmen,

That's where my windrow/gutter system falls short at this point. Because I have so many irises packed into my main beds and because I haven't yet figured out where to create a new bed (or had the energy to do so), my windrows and gutters are squeezed too closely - about 9" wide each. That means I'll have to divide my clumps every couple of years as they reach the edges of the windrows. It also requires pretty doggone good balance to get down the gutters!

If I had more room, I'd definitely make both the windrows and gutters wider. I'd be careful not to make the windrows wide enough to accommodate hoof/paw traffic, though.

I don't know if even the windrows/gutters would provide enough drainage to keep bearded irises happy in an area that retains water every time it rains. Have you considered Louisiana irises, instead?

Laurie

Yep, I've got Louisiana irises in the worst sections. I've been considering Japanese irises, too. Do they like water? Fortunately, our soil isn't hard clay, so, I'm hoping I can build it up enough in areas to make it work. If not, I'll be looking for more bog plants.

Japanese irises love water. If they can handle your heat (and I'm not familiar enough with JIs to know), they should do fine in a damp-wet area. I do know they require acid soil, though.

Laurie

Well, I've got all the acid soil they could ever want! I'll check on the heat requirements. They are so beautiful!