Using Species in MDB Breeding, Part 1: Iris lutescens

by Tom Waters

I’m a hobby hybridizer, working on a very small scale (I can only raise about 200 seedlings each year). So from the outset, I planned to focus on niche projects, rather than trying to compete with the large-scale hybridizers and their well-established breeding lines. I identified some projects that I thought might have value, but that few others were working on. One of these is to develop a line of true-breeding miniature dwarf bearded (MDB) irises.

As I’ve written previously, most MDBs produced today are small selections from standard dwarf bearded (SDB) breeding. Because the lines of the top SDB hybridizers are so advanced, this approach indeed produces many fine MDBs that display all the variety of color and refinement of form found in the modern SDBs. With a few exceptions, however, hybridizers do not have dedicated lines aimed specifically at producing MDBs. Rather, they select them from among their SDB seedlings that happen to fall below the height limit separating the two classes. Since these plants are genetically no different from SDBs, they are likely to produce SDB-sized seedlings, and some are prone to growing out of class, showing foliage and stalks out of proportion to their size, or having flowers that are too large and coarse.

In earlier times, most MDBs were produced by crossing SDBs with the species Iris pumila, which indeed produced smaller, daintier plants than those from pure SDB breeding. Unfortunately, these plants are unbalanced tetraploids with limited fertility, making them dead-ends.

If one is seeking a true-breeding line of MDBs, there are several options. One is to start with the SDBs and keep selecting for smaller size. Another option is that promoted by Ben Hager: cross tetraploid MTBs with I. pumila, then cross those seedlings with small SDBs or MDBs from SDB breeding. The idea here is that the MTB X pumila crosses will produce genetically smaller plants, and the genes for small size can be stabilized in the breeding line.

A third approach, which I have been exploring, is to expand the gene pool by using species (or combinations of species) that are small in size but belong to the same fertile family as the SDBs. This not only has the potential of creating a line of true-breeding MDBs, but also increases genetic diversity, which may give more variety of forms and colors. This is the first of a series of posts on this project, focused specifically on my hybridizing with I. lutescens and its close relations.

I. lutescens is a dwarf species found in the Mediterranean lands of southwestern Europe, particularly Spain, France, and Italy. There are many related but different populations in this range, which are sometimes given species status, but more often treated as synonyms of I. lutescens. I. subbiflora in Portugal and I. bicapitata in Italy, are almost always treated as separate species, but are part of the same continuum of types found in I. lutescens. Before the 1950s, most dwarf irises found in gardens in western Europe and North American were forms or hybrids of I. lutescens. After the first SDBs were produced, hybridizers quickly abandoned the older I. lutescens dwarfs in favor of the SDBs, which showed wider color possibilities and improved form.

Given this history, I did not at first think of using I. lutescens in my own hybridizing projects. On reflection, however, it seemed that those hybridizers of the 1950s, ‘60s, and ‘70s were excited about exploring the potential of the new SDBs, and not really concerned with creating true-breeding MDB lines. I. lutescens might indeed still have something to offer toward that goal. In nature, its forms span the MDB and SDB height ranges, and flowers are smaller than those of typical SDBs.

S004-01,
I. subbiflora ex Spain
(SIGNA seed)

 

S019-01,
I. lutescens (SRGC seed)

Early on, I grew a plant of I. subbiflora obtained from the SIGNA seed exchange, collected in Spain. Apparently this raises some doubts about its identification, as there is some question whether there are populations of I. subbiflora in Spain, or only I. lutescens. My plant is not as tall as most descriptions and photographs of I. subbiflora, normally being about nine inches in my garden.

Crossing the subbiflora with SDB ‘Kaching’ (Black, 2009) produced a lot of deep red seedlings, mostly of small SDB size, although height varied from year to year and with location in the garden. Although I enjoyed these seedlings, they were larger than what I was aiming for; so I made an effort to use MDBs, rather than SDBs, in future crosses, and to seek out small plants of I. lutescens to use instead of this I. subbiflora. A cross of I. subbiflora with MDB ‘Circa’ (Johnson, 2015) produced attractive plants near the MDB height limit, but still not as small and delicate as I would like.

S027-07,
'Kaching' X S004-01

 

S044-04,
'Circa' X S004-01

I raised a small purple I. lutescens from seed from the Scottish Rock Garden Club (SRGC) seed exchange. It’s about half the size of the I. subbiflora plant. Crossing this with MDB ‘Miniseries’ (Keppel, 2011) gave me a couple promising seedlings: one with small reddish flowers have rather tall stems; the other has mid-sized purplish flowers on shorter stems. I’d be happier if I’d gotten the small reddish flowers on the small stems! But these are interesting, and I will continue working with them.

S039-02,
S019-01 X 'Miniseries'

 

S039-03,
S019-01 X 'Miniseries'

I also crossed the SRGC I. lutescens with MDB ‘Pearly Whites’ (Black, 2014) and MDB ‘Beetlejuice’ (Black, 2013); and a number of these seedlings bloomed this spring. They were all interesting to look at, mostly nicer in form than expected. It’s too early to be sure what height they will settle out at, but this year most of them looked more like small SDBs than like MDBs.

S055-05,
'Beetlejuice' X S019-01

  

S055-11,
'Beetlejuice' X S019-01

S056-01,
'Pearly Whites' X S019-01

I acquired some other small lutescens plants to work with: one from the Berkeley Botanical Garden, and one from Sean Zera, raised from SIGNA seed. I have not yet bloomed seedlings from these. I keep acquiring more I. lutescens seeds, hoping to expand my collection, particularly in colors other than purple. I. lutescens grows here, but doesn’t seem really happy. My high desert zone 5/6 garden in New Mexico is a long way from the south of France.

I am enjoying working with species in MDB breeding, but it must be emphasized that this is a long-term project. Any use of species in breeding must be followed up with several generations of crossing back to modern hybrids if one wishes to meet current expectations of width, ruffling, and substance. It appears that some persistence will be needed to combine the genes for short stems, small flowers, and narrow foliage in the same line. Still, I’m finding these explorations to be a very satisfying use of my small space and limited resources.