by Kathy
Roth
Colorado State University Extension Master
Gardener
Larimer County
Q: I’m new to Colorado and heard bearded
irises are drought tolerant and a good xeric choice. Can you explain?
A: Best Perennials for the Rocky Mountains and High Plains published
by CSU Extension states, “Conditions in Colorado are excellent for iris
(due to) our alkaline soils, intense sunlight and low humidity,” but they
do need regular watering after planting and throughout their first growing season.
Plus, in prolonged periods of drought, they also need extra water.
Q: My Iris have finished flowering; do I cut them down now?
A: No. Only cut or snap off the flower stalk at the base, leaving the leaves
as they are storing energy for next year’s showy blooms. In the fall,
remove the dead leaves to avoid a hiding place for insects and disease.
Q: I have a neighbor who wants to give me some
of her iris, but we don’t know how or when to divide them.
A: Now is a great time (4-6 weeks after bloom), before they have their second
growth period in late summer. Use a garden/spading fork to lift clumps. Shake
off or use a hose to remove excess soil; that enables you to see the thickened
roots (rhizomes) clearly to separate each iris more easily. Toss any rhizomes
that appear soft, old or diseased. Cut leaves to approximately six inches –
they will look like a fan.
Q: How should they be planted?
A: After preparing a sunny site with good drainage, amend the soil and plant
each rhizome about 24” apart to give them ample room to form large clumps.
William Shear, in his book The Gardener’s Iris Book, describes
a space-efficient method of marking out a series of “reverse” triangles
(20”-24” long on each side), planting each rhizome at the point
of each triangle. The planting hole should be 8-10” deep. Mound the soil
in the center of each hole to place each rhizome. Spread roots outward and down
the slope of the mound. Barely cover the top of rhizomes with dirt (they need
air). Water thoroughly and mulch with organic materials like wood chips or pine
needles. Water well after planting and continue to water a couple of times a
week as the plants establish.
Q: When do I fertilize them?
A: Fertilize in spring when growth starts and after flowering, using 5-10-10
or 10-20-20, Iris prefers high phosphate fertilizer compared to high nitrogen
like many other plants. Alfalfa pellets (rabbit food) are a good choice; sprinkle
a handful per plant and water to help pellets break down.
For best growth, divide and replant bearded irises every three to four years,
or you will get fewer flowers as the clumps get crowded. This is also a great
way to get to know your neighbors!
Gardening and Insect Fact Sheets are available on-line by clicking HERE.
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