Sterile Plants: All-season Bloomers

A lot of the plants in my garden this year have something in common: they cannot bear young. 

Lobularia 'Blushing Princess': no seeds but lots of syrphid flies!

One might say it’s a triploid revolution!

But let’s start at the beginning.

The flower exists for one purpose—to attract pollinators. Once its powdery pollen is transported by bee, moth, beetle, or some other vehicle to the ovule of another flower, its function is finished. Seeds form. Eventually, baby plants germinate. So as the summer wears on and flowers turn to seeds, the plant has less and less incentive to make more flowers. Unless, of course, some manipulative gardener beheads the dying blooms. 

But what tedium, what fussiness! 

Cleome 'Seniorita Blanca' in late August

How fortunate for the lover of all-season color that plant breeders have stepped in. What! you say? Breeders “play god” and manipulate plants just so we can enjoy cleome that doesn’t become a spidery tower of swaying seedpods, and sweet alyssum that doesn’t get ugly in August? What’s next—tomatoes that don’t rot?  

Well, maybe … but that’s another story.

Back to the point – how, and why, does a scientist make a flower that will not fulfill its function? 

The answer to how is varied and complicated. Why is a little more straightforward. Take lantana, a beautiful flower that is benign in cold-winter regions like Pennsylvania, but a menace in the south. In fact, it’s one of the top 10 weedy species of the world. And its chromosomes are to blame.

Lantana 'Luscious Berry Blend' makes no seeds.

Many organisms (us, for instance) are diploids, meaning we have two complete sets of chromosomes, one from mom and one from dad. Plants often have multiple sets. Lantana can have two, three, four, five, or even six sets of chromosomes. Those with even numbers of sets are fertile (sometimes very fertile), and those with odd numbers are largely sterile. Combine a plant with two sets (a diploid) with one with four (a tetraploid) and the result is: ta da, a sterile triploid! It will start to form seeds but they will never become blackberry-like and fertile. 

Selling sterile plants will not undo the damage caused by the rude tetraploids let loose in warm climates, but it will at least allow gardeners to plant lantana with a clear conscience. And non-seedmaking plants bloom and bloom. Yes, I am aware that lantana berries are ornamental. This, I suspect, is a rationalization that will be handily tossed when the juicy fruits are absent. 

Hibiscus 'Blue Chiffon': blooming itself silly.

Other sterile or mostly sterile plants in my garden are ‘Lilac Chip’ buddleia, ‘Seniorita Blanca’ cleome, ‘Blushing Princess’ sweet alyssum, ‘Diana’ rose-of Sharon (my ‘Blue Chiffon’ rose-of-Sharon is not a triploid but is blooming itself silly nonetheless). Some of my seed-free plants are triploids, others are the sterile hybrids of two different species. 

Burning bush triploids will be available in four to five years, says breeder Yi Li of the University of Connecticut. And other good versions of bad troublemakers are on the way. Scientists are working on making impotent Norway maples, barberries, privets, and callery pears so that they won’t wreak havoc on the few natural areas that have not yet been compromised. 

From the BIG perspective, creating a cleome or sweet alyssum that blooms all summer seems a monumentally trivial achievement. And the self-serving aspect—no self-sown seedlings means more sales—is not lost on me. Scientists working on safe-to-plant versions of invasive plants are providing gardeners with guilt-free alternatives. A sterile burning bush, however, will do little to save the earth. 

But in the evenings, when I revel in the exponential multiplication of the blooms (that still, by the way, attract scores of pollinators), and enjoy the sweet scents, I am happy that breeders tinker with odd-numbered ploidy.  

Now if they would just start working on squash bugs ….