A quick guide to increasing your irises
Irises produce several offsets from the sides or growing end of rhizomes during a season. These new plants can be broken off and planted to increase the variety. Irises are best divided in the fall when the offsets have reached sufficient size and the new growing cycle begins.
Often, growers will have more plants than they need after digging, and excess irises can be shared with friends or swapped for new varieties. Those replanted will benefit from thinning, and it is a good opportunity to enrich the soil by adding fresh organic material and renewing the mulch.
Offsets from a rhizome always produce identical flowers. They do not change color over time despite what is sometimes said. When an unexpected flower appears in a clump, it most likely is due to other irises “traveling” into it by rhizome growth, the regrown remnants of a rhizome of another variety previously in that spot, or a seed dropping and forming a new hybrid.
A rhizome will bloom only once. It is the offsets that will flower in subsequent years. However, you can use the spent rhizome to increase a variety. Cut a 2-3 inch section of the rhizome and place it in a medium of sand and vermiculite, sand and peat, commercial potting soil, or a similar material. Cover the rhizome section with a quarter inch of the medium and keep moist.
It is helpful to treat the rhizome cuttings by immersing them for about ten minutes in water and bleach (10 percent) to deter any fungus that might cause them to rot. In most instances, new offsets will form at the leaf scars. When these grow to six inches or more and have formed good roots, remove them carefully and plant them into the garden or a container. The best practice is to plant the rhizome cutting with intact offsets since the new plants will presumably extract some nutrition from the old rhizome.
Some varieties more readily produce offsets by this method than others. When you examine a spent rhizome, you can sometimes see a small green “eye” emerging along the side of the rhizome. Such a rhizome will quickly produce a new plant when treated as described here. In other instances, it may be months before new offsets emerge. Sometimes, they never do, and the rhizome section will rot.
Growing from seeds produces plants that will vary to a greater or lesser extent from the parents. Because hybrid Louisiana irises have descended from species that vary widely in color and form, it is difficult to predict what seedlings will look like. This characteristic makes Louisiana irises a fascinating subject for systematic hybridization. Hybridizers make controlled crosses, applying pollen from one cultivar to the flower of another cultivar. Bees randomly do the same work.
Unlike inter-species hybrids in some plants, Louisiana iris species are closely enough related that their offspring generally are fertile. Consequently, they may be used in further hybridization. The cumulative consequences have meant that Louisiana irises have become an increasingly varied group of plants. Their evolution continues with each generation.
Seed pods will become apparent within days or weeks following the bloom season, whether from deliberate cross-pollination or busy bees. In Louisiana, these seeds will mature in midsummer. The Fourth of July is an easy target date for taking the seed along the Gulf Coast, both because they generally mature about that time and because of a long weekend. This represents about a 90-day wait from pollination to mature seeds, so growers elsewhere can adjust the dates accordingly.
If a seed pod begins to brown and crack open earlier, it should be harvested. Seeds that dry out in the pod may take another year to germinate.
Carefully cut open the pods and plant seeds about 3/4 inch deep in pots of garden soil or potting mix. If the pots are kept moist, the seeds should begin germinating with the first cool nights in October or November along the Gulf Coast. The seedlings can be planted in the garden in early spring. Most can will bloom the following spring.
Pots of seed can remain viable for several years. If good germination does not occur, hold the pots, and you may get a flush of germination a year later. Some crosses will produce seed that germinates readily, and others exhibit delayed or poor germination. After the second year, significant additional germination is unlikely.
Store seed pots in a shady spot and keep them damp but not soggy. It is essential to keep the seeds covered with soil or another medium not only to preserve moisture but to prevent the seed from being eaten by insects. The corky coating on seeds may cause them to rise to the surface over time, so additional soil may have to be added to pots. A fine wire screen covering the pots will prevent most potential insect damage.
Rarely, offsets occur at a leaf node toward the bottom of a bloom stalk, as seen in daylilies. This is much likelier on varieties that produce thick, husky bloom stalks. When the proliferation has produced visible roots and the stalk has begun to yellow in summer, cut the section out and plant.
It is also possible to cut a bloom stalk and induce the development of offsets. Two methods were described in an LSU Cooperative Extension pamphlet printed in 1966, “Louisiana Native Iris,” by Joe G. Richard, but no record of experience with the technique is known.
Method 1: Cut the stalk after blooming when it is still green but before the seed pod forms. Place the flower stalk in a container of water so that the water line is slightly above the base of the leaf node. Keep in the shade.
Method 2. Place the stalk almost horizontal in a sand propagating box. When an off-shoot and its roots have developed, remove the off-shoot and transplant.
The likelihood of success is low, and the technique is so seldom used that no description of experience with it is known.
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