Monday, September 16, 2013

Are Your Irises Going to Pot?!

As boxes of rhizomes arrive at homes around the country, and gardeners talk about their new treasures, the discussion sometimes turns to the practice of potting. Some gardeners view potting rhizomes as an unnecessary extra step. Other gardeners view it as an integral part of their iris gardening. Though the practice was once considered unusual, it appears that many gardeners have added potting to their gardening routine as they learn more about the potential benefits offered. But, just how many? A recent survey of iris enthusiasts showed that 12% of respondents potted all of their newly dug rhizomes each year. Another 10% responded that they potted any rhizomes that required special care. The remaining 78% indicated that they planted all of their rhizomes directly in the garden. With 22% of respondents potting to some extent each year, let’s take a closer look at the “why” and “how” of this practice. 

Of that 22%, the majority indicated that they employed potting to help protect newly dug rhizomes from extreme environmental conditions while establishing new roots. In the South and Southwest, gardeners wanted to minimize exposure to excess heat and humidity. In many cases, these gardeners also wanted to delay the chores of reworking the soil and planting until the cooler weather of fall. In the North, gardeners potted rhizomes that arrived late in the season, when planting directly in the garden increased the risk of freeze damage. Gardeners who indicated they employed potting with rhizomes that were small, with rhizomes that appeared to be struggling in some way, and with rhizomes that warranted special care came from all geographic regions.  

Perhaps the most widely read information about potting rhizomes was written almost twenty years ago by Walter Moores, who grows and hybridizes irises in Yalobusha County, Mississippi. Mr. Moores has introduced almost 90 cultivars since 1977, and 18 of those have received AIS awards. His earlier work focused on reblooming irises, some of which can still be found in commercial catalogs today -- almost 40 years later! His recent work focuses more on tall bearded irises and species crosses. Having written about the subject of potting in an issue of the AIS Bulletin, Mr. Moores followed up with an article in the Tall Bearded Iris Society's Tall Talk (September 1998). Demonstrating that this information can prove useful in diverse climates, the article has been reprinted by many local clubs throughout the United States and now also appears on the current webpage of the Canadian Iris Society. Rather than trying to rewrite a classic, Mr. Moores’ original article is presented here, with just a few additional notes at the end

Starting Bearded Iris In Pots
By Walter Moores

Over the years the reasons for potting irises to give them a head start in growth have remained constant, but the techniques have changed somewhat. What follows is my experience with potting irises for over twenty years.


Why Do I Pot?

I agree with Bob Strohman [of Louisville, KY] that when the new irises arrive, "It´s over 90 degrees, the ground is too dry and hard to dig, and we´re leaving for a vacation trip tomorrow anyway." So, if you live in the hot, humid South or Southwest and you bought a $45 introduction, you need to think seriously about setting that plant or any other for that matter out in the broiling heat. The alternative is to pot incoming rhizomes and to place the pots in a shady location.

Potting irises has unexpected rewards. While it is so hot at planting time, winter weather and rhizome heaving seem remote. But that extra step of potting back in July and August to protect rhizomes from heat will eliminate rhizome heaving when the ground goes through the freeze/thaw cycles of winter. The root systems developed while the irises were potted will enable the plants to remain where they were planted. Potting does reduce losses during these two susceptible seasons. If bloom is expected on first year plants, potting them and getting them established early makes bloom a sure thing. There are more reasons for potting irises than not, and growth and bloom seem to be the priorities for doing so.

The most desirable month for planting or transplanting irises in hot climates is September, when cooler temperatures make it more bearable to be outside, but it is often difficult to find a commercial source that still has blooming-size rhizomes for sale. Also, it has been my experience that when I have ordered early and requested late delivery, I have received inferior rhizomes. I have also had cancellations or gotten substitutes when I ordered in early spring and requested late August or September delivery. Typically, September is the driest month in the South, and the thirst of newly planted rhizomes can cause an added expense by inflating the water bill. So, to ensure you get quality plants of desired varieties, order early and pot the rhizomes. The boost the rhizomes get while in pots will almost guarantee first year bloom in your garden.

How I Pot
When a box of iris rhizomes arrives, open it immediately and check the rhizomes for mold or rot. If you find any, trim it off. Also, cut or shave off all roots. Then prepare a mixture of one part liquid bleach to nine or ten parts water. Soak the rhizomes for at least thirty minutes. Allow the plants to air dry before planting. The clorox bath is necessary to kill any rot potential that might have developed in transit and to prevent it from forming while the irises are potted. Plants may stay out of the ground indefinitely, but if a good head start is desired, the rhizomes should be planted within a day or two of receiving them.

For years at iris sales or auctions I had always seen a few irises potted in black plastic pots. I used gallon pots when I first started potting irises, for I thought I needed at least a gallon of soil for the plant to survive. I did not trim the roots, nor did I provide a bleach bath. Most of the time I just used garden soil that became as hard as a brick when the pots dried out. At planting time it became a chore to move the heavy pots to their blooming spot and to dig a hole large enough to accommodate the contents of a gallon pot. After experimenting with a few four inch plastic pots, I have decided that they are perfect for the potting procedure. I have not found a rhizome too large to fit in one. If a rhizome has a snout (an extension of growth on the toe), cut it off and consider it to be a second rhizome and plant it in the same pot with the mother rhizome.

The potting mixture should neither be friable or compact. A happy medium consists of one-third Magic Earth (a potting soil with fertilizer), one-third garden soil, and one-third sand mixed well. No other fertilizer is necessary.

Place soil mixture in four inch pots up to the rim and soak with water. The soil may settle and more may be needed. Set the rootless rhizome half exposed in the soil and firm it with your fingers. Make sure the soil level is at the top of the pot so no water can stand in the pot. It would be almost impossible to place a rhizome in such a small pot if it still had the roots intact. New roots will form quickly and wrap around and around the soil in the pot. Tag or label the plant as usual.

Watering the plants may become necessary depending on the weather. It is best to water the pots from the bottom up. Add water to a level of three inches in a galvanized tub, and place the pots in the tub. Allow the water to be absorbed through the drainage holes. Remove the pots when the soil is damp.

It is important to move the pots occasionally so roots don´t find anchor through the drainage holes. A few weed seeds may sprout, but these are easily removed.

Plants may be left in the pots until October to be planted where they are to bloom. In the meantime, preparations should be made for the iris beds or rows that will accommodate the new plants when it is time to unpot, one tap with a trowel will loosen the soil and root ball. That same trowel should have been used to dig a hole about four inches deep. For an extra boost, a balanced fertilizer might be added to the planting hole. Firm the soil around the plant and water.

In summary, many people would consider potting irises to be double trouble. It really isn´t when one considers the growth and bloom potential of the potted irises over the traditionally planted irises. Losses are almost nonexistent.

The author has given his permission for this article
to be reprinted for educational purposes.

Mr. Moores recently stated that the one change he would make to the above article is to add this advice: "When potting rhizomes, remove the leaves as they die to prevent moisture accumulation."

Additional Notes: Magic Earth is recommended in the article above, but it is no longer available. Magic Earth had a guaranteed analysis of 9-6-5. As a substitute, look for an organic potting soil with a similar analysis, such as Miracle Gro Organic Potting Soil. Do not use Miracle Gro's standard potting soil, as it typically has too much nitrogen for use with bearded irises. Once you have selected an appropriate potting soil, it can be amended to best suit your climate. Common amendments used when potting bearded irises include Canadian sphagnum moss, coarse sand, horticultural vermiculite, bone meal, dolomitic lime, Super Phosphate, and granular fungicide. If adding fertilizer, choose one that is slow-releasing and low in nitrogen. Select free-draining pots from 4" to 6" in diameter.  

Do you have a technique or cultural practice that has proven beneficial in your iris gardening? Share it here!

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