FREQUENTLY ASKED QUESTIONSWhy do my irises grow fine and not bloom?
Can irises be made to bloom longer?
My tall bearded irises sometimes fall over. What can I do?
How often should irises be divided?
What time of the year should irises be divided?
How do I divide them?
How do I replant them?
Are soil additives, such as alfalfa, helpful?
What do I do about iris borers?
What is the smelly yellow ooze in the rhizome and at the leaf bases?
What are the small rust/brown spots on the leaves?
Should irises be cut back in the fall?
Why do my irises grow fine and not bloom?The most common cause of decreased bloom is an overcrowded clump in need of division. Bloom will have decreased steadily for a few years, probably most clearly in the middle of the clump. Another possibility is insufficient sunlight. Finally, the nutrient balance may be wrong--lawn fertilizers, for example, can cause heavy foliage growth but do little to promote bloom.
If a rhizome has only been planted for a year or two and not bloomed, especially if it had been stressed (maybe by overcrowding) beforehand, it may take that much time to be established before blooming again. Time is the best remedy for that, but be sure the plant gets adequate nutrients.
One thing that hybridizers strive for are stalks that will stand up. This isn't always successful--there is one recent top award winner, Victoria Falls, which is a fantastic iris in many respects, but when you mention its name to an experienced grower, you are likely to hear "Yes, she does" in response. These irises, and maybe yours, are best grown in protected areas or by people willing to stake them. Or cut the fallen ones quickly before anyone sees.
I like to grow some of the shorter TBs and other bearded irises of intermediate height (16-27"): Intermediate Bearded (IB), which bloom just before the TBs, and Border Bearded (BB), which bloom with the TBs. And I especially like the Miniature Tall Bearded (MTB), or Table Irises, which have slender, flexuous stalks and small flowers. These shorter ones are good for even moderately windy locations.
There are lots of moderate height TBs that will generally stand up well, though. Even a few of the big, large-flowered ones are built with strong stalks--my favorite in that category is Dusky Challenger. Many of the beardless irises stand up real well too.
How often should irises be divided?They should be divided when the clump begins to get crowded. Often a clump in need of division will bloom poorly, especially in the central portions. For most modern bearded iris hybrids, three or four years of good growth can be expected, although some varieties are exceptions on both sides. Japanese Irises will last about the same amount, while many Siberian Irises will go a year or two more. Louisiana Irises tend to spread so extensively that containment and tracking are more an issue than crowding. Many Spurias can go a decade or more without need for division.
Siberian Irises and Japanese Irises often work well replanted a month or so later, since they are planted deeper. Dividing these beardless varieties soon after bloom season works well in areas where heat and drought is not expected over the following month or two. Spring division can also work well in areas where the soil is not too wet to work during that period. They also require regular watering for the remainder of their establishment year.
Dig a hole with a hill in the middle. The object is to plant the rhizome on a firm base of earth with no air space underneath and with the roots spread out and extending down and out. Fill in the soil, pack it down well, and water very well. If you are planting bearded irises, their tops should be barely covered with good soil. (If your soil tends toward clay, leave the tops of the rhizomes exposed.) No extra watering should be necessary, except in prolonged dry spells immediately after planting. If you are planting beardless irises such as Siberian Irises or Japanese Irises, they should be two or three inches deeper. Keep beardless irises well watered for the remainder of the growing season.
Soil should be in the proper pH range for the type of irises you are growing. Add limestone to raise the pH (making the soil more alkaline); add elemental sulfur or ferrous sulfate to lower the pH (making the soil more acid). Keep in mind that other factors will also influence soil pH--rainwater, proximity to concrete, peat moss, pine needles or oak leaves.
Food is often thought of in terms of the big three--nitrogen, phosphorus, and potassium--with proportions expressed numerically on fertilizer labels, such as 5-10-10, 27-3-6, or 30-15-30. Nitrogen promotes growth, especially leaf growth, and can in some irises encourage rot if present in too large amounts. Small amounts of 5-10-10 or 6-24-24 are good choices in these situations. Most new irises need little or no food their first year if planted in good soil. One plan that works in many areas is a fall feeding of a slow release fertilizer, and a light feeding of a chemical fertilizer like Miracle-Gro a month before bloom time. It is best to feed around the rhizomes, not on them. Do consult your local growers on what works best in your soil and climate.
Alfalfa has shown some promising preliminary results as a growth stimulant. It contains several trace vitamins and minerals and triacontanol, thought to stimulate growth. It contains some of the big three, maybe about 5-1-4, but should be used with a well-fed soil, not instead of it. It has been used in the forms of pellets, meal, and as a prepared "tea" mixture, all with good results, both mixed into the planting soil and sprinkled/spread on top of it. Do get plain alfalfa, not feed that contains additives such as corn or salt.
What do I do about iris borers?Borers are a problem especially in the northeastern quadrant of the USA and nearby points. The single most important step to take in avoiding borer problems is to do a very thorough cleanup well before the expected last frost, removing all withered foliage and debris. These were likely where eggs were laid last fall and overwintered, and taking it away before they hatch goes a long way toward eliminating problems.
Beneficial nematodes have been suggested as a biological control mechanism. They need to be applied and active when the eggs hatch and the larvae are making their way to target leaves, triggered by the day that the temperature first goes above 70-72 degrees, often soon after the last expected frost date.
The most effective active measure that can be taken is to spray with a systemic insecticide like Cygon-2E, a powerful chemical. Two or three sprayings should be done, the first at or right before hatching and repeated after 10 days or two weeks or until the beginning of the tall bearded bloom season. After that point, the borers are so large that no reasonable biological or chemical control will be effective. In many areas, such chemicals are no longer available. Merit (imidacloprid) has been suggested instead, although its toxicity to bees and other lifeforms can be problematic. Sometimes the borers can be seen in the bottoms of the leaves and killed by squeezing there.
Prevention is much easier than dealing with problems in progress. Plant bearded iris rhizomes so that their top is at or just under the soil level, even showing above it if your soil tends toward clay. Insure good drainage. Do not mulch them or fertilize them heavily. Control borers.
Beardless varieties like Siberian Irises and Japanese Irises regularly turn brown in the fall, some time between the first frost and six weeks later, and they should be cut back to an inch or two at that point.
Another time irises are cut back is during the division process. New roots can only support a reduced leaf mass, and smaller fans are less likely to blow over in winds.
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