Growing Irises

There are numerous web sites and books available for the routine planting, cultural growing, and disease and pest control of the Iris Flower. Two recommendations suggested include:
Region 18 Web Site
Abbey Gardens

Ginny and Don Spoon have graciously given permission to utilize information from their WINTERBERRY GARDENS Catalogue. Many thanks go to the Spoon family for their contribution to AIS and to Reblooming Irises. Many of their suggestions follow.

Rebloomers - RE Varieties that produce more than once crop of bloom stalks in a single growing season.
Remontants - RE Another term for Rebloomers
CYRE - Cycle Rebloomers Varieties that complete two distinct cycles of growth, blossoming and increase in any one growing season. The second new increase does not require vernalization (chilling period) to produce bloom stalks. Has a predictable and dependable second period of flowering, usually in late summer and (or) fall.
RE r - Repeaters Produce additional bloomstalks on old growth immediately following the initial production of spring bloom. It is not unusual for these varieties to extend the spring bloom season for four to eight weeks.
ASRE - All Season Rebloomers Varieties that can send up bloom stalks throughout the growing season.
RE c - Rebloom Carriers While it does not rebloom, they have produced reblooming seedlings when crossed with a rebloomer, which is valuable information for hybridizers. They carry genes eliminating the need for vernalization.

Reblooming irises produce more than one growth of bloom stalks in a single growing season. Rebloom cultivars having just a sporadic tendency to rebloom are called Occasional Rebloomers.

The Cycle Rebloomers complete two distinct cycles of growth, blossoming and increase in one growing season and produce the second cycle of bloom stalks from maturing new increases. The second growth and bloom cycle, usually beginning in later summer or early fall, does not need vernalization (a chilling period), although cool nights seem to promote more rebloom.

Rebloomers are found in bearded iris (MDB, SDB, IB, MTB, BB, and TB) as well as beardless varieties. Beardless varieties which rebloom are called Repeaters. They produce additional bloom stalks from old growth. Repeaters rebloom immediately following initial production of spring, though sporadically from year to year. Repeaters can extend the regular spring bloom season from four to eight weeks.

There are even a small number of rebloomers that are Continuous Rebloomers or All Season Rebloomers that send up bloom stalks throughout the growing season whenever a rhizome or its new growth has sufficiently matured. Their bloom time is not controlled by day length, but by soil temperatures.

Spring only bloomers that produce reblooming seedling if crossed to a rebloomer are called Rebloom Carriers.

USDA Hardiness Zone Map

Rebloom could depend on Zones and upon your cultural conditions.

Zone 5 -- iris may rebloom in late Fall (temperature down to -30 degrees)
Zone 5 -- iris rebloom in September - October (minimum temperature -20 degrees)
Zone 6 -- iris rebloom in October (minimum temperature -10 degrees)
Zone 7 -- iris rebloom in July (lowest temperature of 1 degree F)
Zone 8 -- iris rebloom in August or September (10 degree F lows)
Zone 9 -- iris rebloom in September, and other months (20 to 30 degrees for a minimum temperature)

It is best to check the Zone of bloom instead of the State where you grow iris. Be cautious of your own Zone. Just because a map may show the whole state, or your section of that state is in a particular Zone, you may actually be in an area that is a different Zone. Three (3) examples:

Coastal parts of Virginia are zone 8; then, moving northwestward it becomes colder changing to Zone 7; then, to Zone 6b and 6a (minimum temperatures -10 degrees); and even colder in the highest Virginia mountains to Zone 5.

Much of California is Zone 9 (20 to 30 degrees for a minimum temperature) but does have Zone 5 through 8 and 10.

Arizona has Zones 9 through 5 (some higher altitudes can even be colder to Zone 4 and Zone 3).

This does not necessarily mean that if your zone is warmer, that a particular iris variety will rebloom for you.

Rebloom could depend on your cultural conditions; if you water when dry or apply extra fertilizer, etc. Sometimes a microclimate in a colder zone could still have rebloom. Also, hot summer nighttime temperatures can trigger dormancy that cutails rebloom, as does frigid winter temperatures. The best thing to do is just to give them a try.

Don't give up after one or two years since some rebloomers need to be well established before they rebloom. Also, some rebloomers are sporadic with only a tendency to rebloom yet may rebloom now and then even in cold zones.

Why do Rebloomers rebloom?

It is certain that the rebloom trait is not controlled by just one gene, rather it is a group of genes, some with dosage effects, that determines the quality and quantity of rebloom. Probably, having a homozygous recessive condition that makes inactive the dominant gene that controls the requirement for vernalization is found in all rebloomers, making vernalization (a prolonged chilling period) unneeded to induce bloom stalk production.
Possibly, genetically controlled levels of plant hormones, such as abscisic acid*, and their receptor proteins influence by environmental conditions and supported by hybrid vigorous growth and increase may allow certain irises to rebloom. A substance produced in the leaves may be the stimulus that causes development of the rebloom stalk.

What Special Effort?

Rebloomers take a little more care than the once bloomers but are definitely worth the extra effort. They like a little more fertilizer and water since they need an extra boost to flower again. You should fertilize them in early spring and again after spring bloom. Be careful not to put the fertilizer directly on the plant, but sprinkle it on the ground around the root zone area.

Use a fertilizer with a fairly low Nitrogen content, such as 5-10-5, mixed 50/50 with superphosphate (0-45-0). For the rebloomers we apply a liquid foliar fertilizer once or twice in September. Basically, you don't want to let the rebloomers go dormant. Water them adequately at least once every other week if the rainfall hasn't been sufficient, so they don't dry out completely. You may want to grow the rebloomers together or mark them in some way since the once bloomers may rot with the extra water and fertilizing.

Before the first hard frost, below 28 F, cut all flower stalks with well developed buds and bring them in to bloom indoors. For aesthetic reasons you may want to cut off all the other developing stalks close to the ground. Uncut stalks usually freeze and fall off over the winter without causing rot.

Since most rebloomers increase faster than once bloomers, you may need to separate and replant them every second or third year. Since some cultivars need to be well established before they rebloom, you should divide and reset half of the clump and leave the rest to rebloom.

Irises are among the easiest perennials to grow. But irises, and especially rebloomers, are heavy feeders; and when they are divided and replanted preparation of the soil should be top priority. Dig the clump deeply to save as much roots as possible and break up the whole clump. We don't advise cutting back the fan, just remove loose leaves.

Irises grow best in beds raised 4 - 8 inches and thrive best with a foot or more cultivated depth for root growth. Dig the top four inches of soil and place in a pan or wheelbarrow. Dig down eight inches more and turn over the soil. Prepare the sand, nutrients, and organic amendments as follows:

Nearly fill a wheelbarrow with one part concrete sand to two parts organic material, such as well aged garden compost, composted manure, and/or leaf mold, and mix thoroughly. To this we add a cup of each of the following: rock phosphate, green sand, granular dolomite (for Calcium and Magnesium), wood ashes (excellent source of some trace elements), and a good balanced nutrient mixture of slow release Nitrogen, Phosphorus, and Potassium plus boron, zinc, iron, copper, molybdenum, and manganese (such as Nursery Special 12-6-6 by Sta-Green).

These nutrients are mixed in a pan and then mixed in the wheelbarrow with the organics and sand. This sand/organics/nutrients mixture is then added about 1:2 to the eight inches of soil that has been dug and thoroughly turned over. Add back on top of the relatively nutrient poor topsoil and dig in the iris plants so their roots are straight down into the rich, mixed soil below.

Point the toes of the plant inward in the new clump with fan ends facing outward. Don't cover the top one fourth of the rhizome with soil. We use and position all the old pieces of bare rhizomes with roots and fans. The bare rhizomes are covered with 1/4" soil as they are excellent sources of the trace elements the rebloomers need, and many will sprout new increases with fans and roots. This method of replanting allows us to return the soil to a state similar in performance to soil 'that has never seen an iris."

Others strongly recommend the use of alfalfa pellet or meal that has the growth stimulant Triacontanol. We control soft rot with the treatment of Dial or a generic antibacterial liquid hand soap with Triclosan diluted 1 part soap to 5 parts water. Use the undiluted soap solution if required. A second follow up treatment usually cures soft rot.

At the very first sign of leafspot, treat with a fungicide, such as low toxicity Daconil. Removing dead leaves and surrounding debris in fall and late winter to earliest spring will help remove borer eggs before they hatch hiding places for slugs, and leafspot fungal spores. You may need to change fungicides periodically since the fungi can become resistant to it. If you have an outbreak of iris borers, aphids or Japanese beetles, you should choose a systemic with Imidacloprid, such as Merit (has a caution and not a warning label) that only kills the insects that eat the leaves and does not harm beneficial insect predators. Apply the granular Merit in early spring and for even better borer control use powdered Merit (1/16 teaspoon per 2 gallons) as a follow-up spray one-month later. Merit remains effective in the soil for about six months.

You can also apply beneficial nematodes (one source is Gardens Alive, 5100 Schenley Place, Lawrenceburg, IN 47025, phone: 812-537-8650) to help control borers, but you need to apply the live nematodes when the soil is moist and its temperature is about 50 F.

Interplanting with other perennials helps to curb outbreaks of iris pests. Many members of the daisy and mint families provide food and shelter for beneficial insects. We control slugs with Epsom Salt or wood ashes sprinkled lightly around the plants. They need to be reapplied after a rain. Slugs eat at night and are easy to see with a flashlight. They can be removed with a gloved hand and then dissolved in a container with table salt. We can have an environmentally friendly garden by using alternatives to harmful chemicals.