Tetraploid Hostas

Very simply, a tetraploid plant has four sets of chromosomes rather than two.  This can occur naturally, rarely, or can be induced chemically.  Diploids, with two sets of chromosomes, are the norm. In hostas, as far as I know, only H. ventricosa is a naturally occurring tetraploid species.  Induced tetraploids are very common in the daylily world, but are a fairly recent development in hostas.  'Patriot', and 'Grand Tiara', two of the earliest and best known, were both registered in 1991, but only fairly recently has there been a concerted effort to convert a large number of popular diploid hostas.

Certain herbicides, such as Surflan, are known to occasionally induce tetraploidy, and presumably the earliest conversions were accidental byproducts of herbicide use.  Recently, breeders, hobbyists, and especially tissue culture labs have been using chemicals to try to produce them intentionally, and the availability of converted plants has been increasing rapidly.

Tetraploid hostas generally have thicker leaves, more pronounced variegation, darker color, larger flowers on thicker scapes, and because the leaves and leaf petioles are thicker and stiffer, the form of the plant may be more upright. Because the leaves are thicker, it also seems logical that they may be more resistant to slugs and other causes of leaf damage.  

I think that one of the most important benefits shows up in the white-centered hostas, where the tetraploid forms are often less likely to burn and melt out in the centers.  An example is 'Island Charm' which I stopped growing some time ago because it didn't hold up well for me.  It's tetraploid form, 'Fantasy Island' is an excellent grower here, and may be the best small, white-centered hosta I've grown.  Another example is 'American Sweetheart', which has a much thicker, more durable leaf than 'Sea Thunder'. And just to show that there are always exceptions, there are 'Fire and Ice', 'Loyalist', 'Flash of Light' and probably others, all white centered sports of the tetraploid forms of 'Francee'. We have never had any luck getting these hostas to grow and no longer carry any of them. They just don't seem to have enough green tissue in their leaves to grow well.

Hosta 'Patriot', left, is a tetraploid form of 'Francee', on the right. In this case, I think that most would consider the tetraploid a distinct improvement, with wider margins and darker center.

As in the daylily world, though, not all hosta fanciers consider every tetraploid an improvement over the original form. While I can't think of many of these new introductions that I don't like, there are some that I don't necessarily consider to be better than the original, just different. 

In some cases, it's difficult to know for certain whether a plant is actually a tetraploid without testing.  Listed on the left below are some fairly common hostas, and on the right are their probable tetraploid forms.  'St. Paul' and 'Paradise Glory' both seem to me to be likely tetraploids, but the nurseries that introduced them do not indicate that they are, so I'm just guessing.  There are others that are not listed below, and new ones are introduced each year.  I suspect that someday many more of our popular hostas will have tetraploid forms available.

 

Diploid Form	Tetraploid Form
'Blue Cadet'	'Blue Mouse Ears' (maybe)
'Francee'	'Patriot, Minuteman'
'Golden Tiara'	'Grand Tiara'
'Invincible'	'Brave Heart'
'Island Charm'	'Fantasy Island'
'June'	'Touch of Class'
'Paul's Glory'	'St. Paul', 'Paradise Glory'  (maybe)
'Sagae'	'Liberty'
'Sea Thunder'	'American Sweetheart'
'White Christmas'	'Night Before Christmas'