Irises are known for being hardy, but they're not THAT hardy. From the very beginning, it's important to make sure your iris plants get off to a solid start. Here are some tips for planting your tall bearded irises-
1.) I suggest dipping your iris before planting in a 1:2 bleach water mix. Most nurseries (including ours), before shipping. However, I've found dipping before planting further prevents any mold in the rhizome. With heat, shipping, etc., this is a good preventative measure. Be sure to let them dry in a cool place before planting.
2.) Don't bury the rhizome. Unlike bulbs, rhizomes should be planted so that the surface of the rhizome is exposed. Create a small dirt mound, place the rhizome on top, and cover the roots, not the rhizome.
3.) Once your rhizome has been planted, it may fall over. You can use a wooden craft stick, dowel, or a garden marker to prop the plant up so that it can take root.
4.) I've found that it takes a couple of weeks (sometimes three) for an iris to get rooted. After that time you should be able to lightly tug on the iris leaves and feel a pull. You always want to make certain that your iris has enough time to get rooted before the first fall frost. This is why I prefer to plant my irises during July/August.
5. ) A few days after your iris is planted, the leaves may become brown or yellow. This is due in part to the fact that transplanting an iris is a shock to the iris. Wait until the iris is rooted to pull off any dead leaves. I also suggest watering the iris twice a week, with at least one watering being a mix of water/Scott's Super Bloom.
6.) Check for rot/mold. Iris rot is one of the biggest problems I encounter in the gardens, particularly with clay soil where the water absorption is poor. Once your rhizome has been planted, check it weekly for rot/mold. The rhizome shouldn't be crumbly or mushy. Consider dipping the rhizome again and replanting it where there is better drainage.
7.) Don't forget to write down the name of the iris variety on a garden marker. I typed the name of my varieties and printed them off. Then I cut the tags to size and taped them to a metal garden marker. I used clear packing tape to seal the tag. They have held up well in the weather and the ink hasn't smudged.