IT’S A GREAT YEAR FOR FRUIT-SET HERE in most of my bird-friendly ornamental plants (and also my beloved old apple trees), but for some readers the opposite has been true. Why no fruit on a viburnum (that’s a well-fruited doublefile type, top photo), or a spicebush, or a holly, people have written in lately to ask?

That was the topic, or at least part of the topic, of last week’s radio podcast (and you can stream it here or get it free from iTunes here).

The answer is all about sex, of course–or at least about pollination, and whether a particular plant can provide its own pollen or needs another plants (and if so, which kind) to provide the necessary ingredient. In the case of spicebush (Lindera benzoin, a native shrub that I grow here for my enjoyment and the birds’), there are separate male and female plants–meaning Lindera is dioecious. I didn’t know this when I planted mine decades ago, but since I put in a few, I apparently got lucky and have at least one male.

With my hollies, specifically the winterberry types or Ilex verticillata and its hybrids, I knew from the start that I needed a male for each grouping of females (various estimates say from 4:1 to 7:1 female-male ratio). Each species and series of hollies has its preferred pollinator(s), so a little homework is needed–though usually the nursery will know or the label will say so. I found this fairly exhaustive list of holly matchups for best pollination, for reference.

Viburnums are another story, and again, having a lot of different species and varieties, rather than only one or several of one kind, has spared me worrying whether I have just the right companion. Individual plants are neither male nor female–all of them can get fruit–but there is an obstacle to them self-pollinating because they tend to be what’s called self-sterile. They’ve got all the parts, it seems, but often to no good effect other than beauty, since they cannot pollinate themselves, or at least not thoroughly in most cases.

That’s why multiple plants that are not the same variety but closely enough related to mate are required–so not just one doublefile, or Viburnum plicatum (such as ‘Shasta’) but also another (a ‘Shasta,’ a ‘Mariesii’ and a ‘Summer Snowflake, for example).

I have some viburnums, such as Viburnum sieboldii, that fruit heavily–even though I have just one plant, and no other individuals in that species in the garden. Hey, I’ll take it, even if I’m not sure why this one has always been so wonderfully cooperative. That is the miracle, and mystery, of genetics at work, right?

A P.S.–A fact sheet on pollination of fruit trees might be what you’re looking for if you didn’t get good fruit set on an apple, pear, peach or cherry. Of course, then there’s the issue of weather–which if it’s lousy enough at blossom time can mean all bets are off, even if you have the right match of trees.