Cultural Information - Month by Month
by John Weiler - California
Remember to remove dead leaves and tip cut any spotted foliage from bearded irises to prevent large outbreaks of iris leaf spot disease now that cooler, wetter, weather has arrived. Leaf spot disease is caused by a fungus, Didymellina marcrospora. Although the disease is rarely fatal, it disfigures foliage and may debilitate the plant so that flowers are not as large or colorful next spring. Once well established, this malady is difficult to control. One of the most effective methods to prevent a massive outbreak is to clean up the iris patch and keep checking it periodically. Don’t forget that all such debris should be put in the trash, not in the compost pile. This effectively eliminates the vast majority of the dormant fungus before it becomes active to infect the irises. There will still be small amounts of the pathogen remaining on tiny fragments of dead iris leaves in the soil. To prevent these from causing a major problem, one of two approaches may be used.
Vigilance in watching iris plants after the first rain, heavy dew, or sprinkle irrigation in the cool weather will reveal early stages of infection appearing as round, dark green spots on leaves, generally in the upper portion of the leaf. If allowed to remain a few days, these greasy looking spots begin to turn yellow, then brown. At this stage, the fungus is capable of being spread very rapidly by drops of rain splashing, by the gardener walking along the row carrying microscopic pieces of the fungus on clothing to other iris plants, by breezes, and by pets. The control technique is to remove the spots by tip cutting any leaf showing the spots in early phase before the disease is wide spread. Almost any fungicide will give some control but those available tend to wash off the plant with each rain or sprinkle irrigation and must then be reapplied. Adding a sticker-spreader, available at most garden centers will increase effectiveness. There may still be an occasional spot appear on a few leaves and, again, these should be cut off the plant promptly to prevent maturation and rapid spread of the disease.
For irises, winter in the valley is nearly over and growth will be starting rapidly about the time you read this. If you didn't do so earlier, clean up all dead and diseased leaves and then spray with a fungicide to control leaf spot. Products available for controlling rose mildew will work also on irises. Weeds should be eliminated by pulling or hoeing, being careful not to stir the soil too deeply since some of the iris roots will be close to the surface. Weeds have an amazing ability to pull nutrients out of the soil and may prevent irises from getting nutrients they need during this rapid growth. If you have grown irises in the same soil for more than two or three years, their performance can be improved by fertilizing. It is best to use any complete lawn fertilizer that does not contain weed killers (often labeled “weed and feed”). Apply at no more than half the rate recommended for lawns. In most cases this amounts to no more than 1/4 cup of the granules sprinkled in an eighteen inch square around the clump. Such fertilizers must be watered into the soil before they can benefit the irises. This means that if you are also spraying for leaf spot, fertilizer should be watered in before using the fungicide. Soil tilith can also be improved by top dressing with a light layer of humus being careful not to apply it directly to the base of the plant or rhizomes.
Since most of you grow bearded irises, there is little else to recommend if you have followed suggestions of leaf clean up, spray for leaf spot and fertilizing recommended in the past two newsletters. If you have not done those things, time’s a-waste-in. One problem that may crop up is the appearance of aphids, and plant lice, during cool weather. These should either be sprayed with non-toxic insecticadal soap or with malathion as soon as noted. As weather warms, emergence of natural predators will control all but heavy infestations. Once the predators, lady beetles, lacewings and a tiny parasitic wasp appear, all insecticides should be avoided since they will damage the predators as well as the aphids. For those growing beardless irises such as Louisiana, Siberian or Japanese types growth is beginning. These type of irises are rather heavy feeders. All of them will produce better results this year if they are fertilized, particularly with an acid fertilizer like that sold for rhododendrons, camellias and azaleas, sold in almost all garden centers.
Preparation for the showing of iris stalks should be nearly complete by now. Foliage should be free of leaf spot, aphids, and white flies and weeds should be non-existent. If you have not taken care of these matters you can still tip cut leaves and spray with a fungicide. If there are aphids or white files, a forceful spray of water will help especially if followed by spraying with one of the environmentally friendly insecticidal soaps. Weeds can still be removed by old fashioned elbow grease.
With warmer weather slugs and snails are very active and have voracious appetites for iris foliage and flowers. Some control can be had by walking plantings after dark with a flashlight. The gliding critters can be picked by hand and put in a can with a small amount of oil. If you are squeamish, small flat containers can be sunken into the ground and filled with beer. Snails and slugs seek it out and drown in a stupor. Slug and snail baits are available but must be used with caution around pets, birds, and small children since they are poisonous.
If you are growing beardless irises, like Spurias, Japanese, or Louisianas, make note of any yellowish foliage with green veins. Such a condition indicates that the plants lack iron. This can be corrected by dusting soil with iron sulfate or using the product Ironite in granular form. both must be watered in and will correct the deficiency rapidly. For a longer term correction, a sprinkling of soil sulfur at one pound per one hundred square feet will gradually make the soil less alkaline releasing iron for the plants to absorb.
When the spring iris bloom is nearly gone, it will be time to focus on grooming for summer care necessary for spectacular bloom next year or again this year if you grow reblooming irises. Spent bloom stalks give a messy appearance to the garden. Bloom stalks may be removed by cutting near the base with a sharp knife or pruning shears or they may be snapped off by a quick jerk sideways. This is done in the morning when stems are turgid and will break off more easily. Once old stalks are removed it is time to fertilize again.
Use any complete fertilizer without herbicides in it at one half the recommended strength for lawns. Remember to keep irises watered but never soggy during the summer. Rebloomers especially must be kept growing if they are expected to bloom again this summer, autumn, and perhaps, winter. It is during the next eight to ten weeks that the rhizomes do most of their growing to store up food for the next bloom. With that in mind, a bit of advice on one practice that should be discouraged. For some reason, many gardeners think iris foliage should be cropped back to the stubs during the summer. This is a mistake. Removal of much of the green leaves robs the plant of its ability to manufacture well thereby decreasing growth of the rhizome and making top quality bloom unlikely next season.
All bearded and all but the latest beardless irises have now finished blooming. The Japanese irises are yet to reach peak bloom and some reblooming irises are producing a second flush of bloom only two or three weeks following the spring bloom in April. Keep all irises growing actively to promote increases of size in the rhizomes for better bloom next spring or autumn if you grow rebloomers. This means keeping plants watered and fertilized. One pest that may attack irises is the white fly. This is a tiny insect no bigger than a gnat that feed upon juices in the leaves of plant. Some years, particularly following mild winters, they can be especially bad. They appear as adults on iris foliage as white flecks which fly when disturbed. As the season progresses they lay eggs on iris leaves. The eggs hatch and the larvae attach themselves to the leaf with piercing mouth parts and secrete a shield-like scale over themselves where they are protected. They now look like tiny tan or cream colored freckles and continue to feed on juices of the leaf robbing it of much food reserves. Best control may be had when numbers are still small. Spray with insecticidel soap or one of the insecticides like orthene. Also effective for adults is the use of bright yellow sticky paper attached to a stick and placed in the ground amongst irises. White fly adults are attracted to the yellow color and are trapped in the sticky surface of the paper.
The time is ripe for transplanting bearded irises. The warm (hot) months are best to transplant these irises. In the San Joaquin Valley they transplant with best next-spring performance anytime from late June through mid October. Any later date shows gradually diminishing height of bloom stalks, size, and quantity of flowers next spring. If you grow reblooming iris, and expect bloom next autumn, “time’s-a-wastin.” The earlier rebloomers are divided and transplanted, the more likelihood of fall rebloom this year with “typical” autumn stalks. These reblooming irises will bloom normally next spring even if planted late summer or autumn but, if fall bloom is desired this year, get off your duff!
If you have grown beds of irises in the same ground for a number of years and they just don’t grow as they used to, produce fewer flowers of lesser quality, you may be rewarded by taking them all out of the ground, storing them in a shady area that is dry and renovating the damp soil by digging in some compost and a complete fertilizer without weed killers. Then, cover the soil with a clear plastic sheet for two to six weeks, with margins of the plastic tucked into the ground at the edges. The sun heats the soil and air beneath the plastic to the point that it kills most organisms including those preventing good bloom and growth of irises. After two to six weeks remove the plastic and replant the stored iris rhizomes. They will grow almost explosively and produce flowers like you haven’t seen in several years.
Prime transplanting time for bearded irises is now. If you have not reset irises for two or more years, thinning and transplanting will help you grow better quality irises next year. It is best to dig the entire clump, wash it clean and then break apart the separate fans with attached rhizomes individually. Select the largest rhizomes for replanting and share the rest with friends or donate the extras to the Fresno Iris Society Annual Rhizome Fund Raiser. Store in a cool spot out of the sun for 24 to 48-hours to heel over broken or cut end before replanting. If possible, replant in an area that has not grown irises for two or three years. If irises must be replanted in the same soil, cover the area with 2-4 inches of well decomposed compost or humus, sprinkle superphosphate over the soil at the rate of one pound per 100 square feet and dig and mix humus and superphosphate into the soil a full spade’s depth before replanting irises.
Irises should have foliage trimmed down to 6 inches when transplanting and any damaged, knobby, or diseased roots trimmed off before replanting. Plant deeply enough to just cover the rhizome and then water it well. Care must be taken not to over-water during hot weather or rhizome rot may get started. The plants will establish quickest if the soil is kept damp but never water-logged. If you are one who enjoys growing things other than irises and who likes vegetables, try planting some of the smaller vegetables between your rows of irises or in mixed flower borders. Now is the time to seed directly such things as loose-leaf lettuce, a number of kohl crops such as turnips, radishes and kohlrabi and such things as beets, chard and carrots. Lettuce makes an attractive border or good color contrast in a mixed border and can be had in yellow-green, deep green, or bronze foliage types. Swiss Chard is also very attractive amongst irises or in a mixed border having handsome foliage and colorful leaf stalks in either white or rhubarb red. The ferny foliage of carrots is a deep green and an attractive contrast in texture to the rigid foliage of irises. Started now, these will all mature by or before Thanksgiving and can be harvested throughout winter.
For those interested in reblooming irises, the best time for replanting with the hope of fall rebloom this year is rapidly coming to an end. All bearded irises, whether or not they rebloom can be successfully transplanted July, August and September with good expectation of next spring bloom.
If you have not yet transplanted or thinned out old clumps of irises, there is still time this month. If irises are growing well and have husky rhizomes, they transplant well even through October, but smaller rhizomes will not be satisfactory for next springs bloom when planted that late. If you have already transplanted older clumps of irises, watch for the beginning of new growth, the emergence of new leaves from the tip of the foliage fan. Once new growth has started, you may fertilize with any lawn fertilizer that doesn’t have weed killer included. Apply at half the rate recommended for lawns and water in.
With the coming of cooler weather and the prospect of rainy season, heavy dew and fog, irises are likely to be infected by leaf spot disease caused by a fungus, Didymellina macrospora. Although the disease is rarely fatal, it disfigures foliage and may debilitate the plant so that flowers are not as large or colorful next spring. Once well established, this malady is difficult to control. One of the most effective methods to prevent a massive outbreak is to clean up the iris patch now before damp, cool weather arrives.
Remove all dead iris leaves and any portion of each green leaf that has a brown tip or any brown spots on it. All such debris should be put in the trash, not on the compost pile. This effectively eliminates the vast majority of the dormant fungus before it becomes active to infect the irises. There will still be small amounts of of the pathogen remaining on tiny fragments of dead iris leaves in the soil. To prevent these from causing a major problem, one of two approaches may be used. Vigilance in watching iris plants after the first rain, heavy dew, or sprinkle irrigation in cool weather will reveal early stages of infection appearing as round, dark green spots on leaves, generally in the upper portion of the leaf. If allowed to remain a few days, these greasy looking spots begin to turn yellow, then brown. At this stage, the fungus is capable of being spread very rapidly by drops of rain splashing, by the gardener walking along the row carrying microscopic pieces of the fungus on clothing to other iris plants, by breezes, and by pets. The control technique is to remove the spots by tip cutting any leaf showing the spots in early phase before the disease is wide spread.
A second approach is to use a chemical control, a fungicide sprayed on both leaf surfaces before rains begin. Almost any fungicide will give some control but those available tend to wash off the plant with each rain or sprinkle irrigation and must then be reapplied. Adding a sticker-spreader available at most garden centers will increase effectiveness. There may still be an occasional spot appear on a few leaves and, again, these should be cut off the plant promptly to prevent maturation and rapid spread of the disease.
Remember to remove dead leaves and tip cut any spotted foliage from bearded irises to prevent large outbreaks of iris leaf spot disease now that cooler, wetter weather is arriving. For those growing beardless irises like Siberian, Japanese, Spuria, Louisiana or Californicae, now is the time to divide and transplant old clumps. Remember that beardless irises grow best in soil enriched with humus.