These iris care directions were written with the northern Colorado and southern Wyoming areas in mind. If you have longer, shorter or wetter growing conditions, adapt to your area and particular growing conditions.
The most important rule to remember when you get your irises is to plant them immediately when you receive them. If you have to wait a few days, keep them spread out in your house in a cool, dry place. Never put the rhizomes in a refrigerator. If rhizomes are kept out of the ground 7 days or more, the chances of survival are dramatically reduced.
Time to Plant
In our zone 5 here, irises need to planted by the end of July ( August 10 at the latest), to get optimum root development before a hard frost, (around Sept. 25 here in Colorado). Planting on time increases the chance of blooming next spring.
Full sun is best, but irises will still bloom with half days of direct sunlight. Do not plant irises in waterlogged areas. Rebloomers can be planted as a group, make sure they are same variety in each group.
Irises need to be planted in well-drained soils. If your soil has a lot of clay content, till in sand until your iris bed comprises of a 50/50 clay sand mix. We feel that using mulches, compost, or manure of any kind contribute rot and weed seeds and is very detrimental to the iris plant. We have a neutral PH in our soil here which is okay for iris.
Depth, Spacing and Watering
Cover rhizomes with one inch of soil where the white-green division line on the rhizome is. Plant about 24 inches apart and give them an inch of water the first time, then 1/2 to 3/4 inches of water per week until new growth appears. After the iris are established, apply 1/2 inch of water per week till mid to late September. Deep watering at larger intervals is better than frequent shallow moisture.
We have used and recommend rabbit feed (pellet form) for fertilizer. Rabbit feed has the phosphate, micronutrients, and protein that the iris plant needs. Spread the fertilizer around each plant (never directly on the plant), and immediately water it in. Fertilize in mid to late March and again in late July for rebloomers and newly divided plants. Iris plants need very little if any nitrogen, and if too much is used rot will start and you will have very few blooms. Phosphorus is the main ingredient iris use to promote blooms and good root growth. If you use a garden fertilizer, use a 5-10-10 or similar proportions. Never, never use manure as fertilizer.
Do not ever cut leaves back in the fall. Brown leaves will help protect rhizomes in winter and they also catch much needed moisture. Clear out all dead leaves and debris from iris beds in early spring and dispose of it. Completely disposing of leaf trash will reduce rot and leaf spot substantially.
Remove bloom stalks after blooming to prevent color distortions in your blooms the following year. If you want to keep the color of the iris the same as originally purchased, bloom stalks must be removed soon after blooming.
Dividing or Moving
Iris clumps need to be divided about every 4 years. The best time to divide them is at the end of July or first week of August. Trim the leaves to about 6 inches, fan shaped, on the rhizomes you wish to keep, air dry them overnight, and plant them as quickly as you can.
Hope these have been helpful!
Happy and healthy gardening,
Charlette and Tim Felte