Bearded iris (Iris germanica) grows best in the ground and is a popular perennial grown for its showy flowers in late spring. It can be dug bare root and sold, but bare root digging is restricted to midsummer. Irises do not transplant well while in bloom and lose their flowers if transplanted. Therefore, the best way to offer a blooming iris for sale is to get it to grow in a pot. This article will detail how to get irises to successfully grow in pots, and is based on personal experience growing iris in a greenhouse while employed by a garden center.
Dig iris rhizomes in July and procure the pots in which to plant them. I used the square, five-quart pots that were about five inches wide and deep, but an eight inch mum pot will also work perfectly. Mix potting soil with about ten percent sand for drainage. The potting soil will already contain sufficient organic matter. Put some soil in the pots. Trim back the leaves of the irises to about five inches tall. The rhizomes should be about two to three inches in length and have a strong fan so it will set flower the following year. Without a flower, the plant is unlikely to sell.
Then plant the iris rhizome, filling soil in around the roots. Plant with the top of the rhizome on a level with the soil and gently pack the soil. There should be about an inch of space from the top of the pot, so unless the soil is hard packed, leave on a level with the top of the pot to allow it to compact. Plant the rhizome on the edge of the pot with the fan end facing inwards. If the plant is off-center, it will grow into the center. Do not put the fan against or near the edge of the pot, lest it push into the pot or break it.
Feed the irises with a time-release fertilizer, and with a liquid feed weekly if desired. One type of feed is sufficient. Make sure not to overfeed or feed too frequently; iris will be fine with poor nutrition and the feeding is to promote more growth and blooms. Overwinter the potted iris in an unheated greenhouse or similar setup to protect the plants from the cold. This is to increase chances of survival of the plants and of bud stalks. Monitor to prevent rot and diseases from breaking out. Good ventilation is helpful for keeping irises free of disease. Never allow irises to sit in water or remain saturated. It is essential to provide good drainage for the irises lest they rot. Water whenever soil begins to dry. This can be daily in summer and sporadically in winter.
In the spring, monitor and remove any leaf material that exhibits holes or water-soaked spots that resemble the damage of iris borers. By keeping the borers at bay through early mechanical control, your iris plants will remain in sellable condition when in flower. Monitor bud stalks for borer activity and eradicate swiftly, before they can get to the buds. Iris borers in the bud stalk lead to bud stalks that rot and exhibit poor or no bloom.
The irises I grew exploded, with each fan making about seven to nine shoots and one to three bud stalks in the following spring. During the winter, however, an aphid infestation occurred and my manager put the irises outside from the unheated greenhouse in February rather than treat the aphids, and only about a quarter of the bud stalks actually bloomed. The rest never developed due to damage from the sudden exposure to frigid temperatures.
Irises that do not sell will need to be dismissed in July. Harvest any good shoots and re-pot again for sale the following spring. The rest of the iris should be planted in the ground and used as stock material for harvesting fans in following years for selling or potting.