the sex life of tomatoes
Do tomatoes cross-pollinate? That’s today’s burning question. Can you save seed from different varieties grown in the same garden? And how far apart do different varieties need to be? You’ll find as many answers as there are tomatoes, all contradictory, with the majority tending to the self-pollinating end of the spectrum, which is where I started when I first saved tomato seed. I did it casually, with no thought of isolation distances, and the first few times it seemed to work – the next year’s tomatoes were recognizably similar to the ones from which I’d saved seed.
Then I grew Big Rainbow, a beefsteak heirloom with swirls of red/yellow/orange inside and out, and so delicious I saved seed and eagerly waited for the next year’s crop. Which turned out to have the coloring of Big Rainbow, but a size closer to a cherry, and a taste so bland only the chickens would eat it.
There are two factors, it turns out, that contribute to a widespread belief that tomatoes do not cross-pollinate. The first is that sometimes it’s true. Modern open-pollinated varieties have flowers that are not capable of cross-pollination (as in the photo above). The pollen-carrying stamens are fused into a tube that encloses the stigma, which is the girl part that takes in the pollen and transports it to the flower’s ovary. You can grow these varieties right next to any other tomatoes and save the seeds with confidence. Mountain Gold is the only tomato seed Laughing Frog offers that has this kind of flower. It was developed twenty years ago by Dr. Randy Gardner at the North Carolina State Mountain Horticultural Crops Research Station.
Older tomato varieties, the ones we call heirlooms – as well as all potato-leafed varieties – have flowers in which the stigma protrudes beyond the tunnel of stamens, making cross-pollination possible. Tomato pollen is a bit sticky, so even with this open structure the flowers self-pollinate unless they get some help. Which brings us to the second reason tomatoes still tend to be regarded as completely self-pollinating. In the 1920s UC Davis conducted what was considered the definitive study on tomato pollination, which conclusively demonstrated that all tomatoes self-pollinate. Word went out – an isolation distance of ten feet between varieties was sufficient. The UC Davis researchers used the most up-to-date agricultural practices, including regular applications of insecticides on their tomato test plots, thus eliminating insect pollination entirely. It took the resurgence of organic farming many decades later for anyone to think twice about how the study’s conclusions were reached.
Any pollination of tomatoes requires a bit of shaking to loosen the pollen, and in the absence of insects wind does the job, but only enough for the plants to self-pollinate, and even then not as well. Studies show that native bees increase fruit set in tomatoes by over 40%. All kinds of native bees pollinate tomato flowers, but bumblebees have an especially effective technique. The bee lands on the steepled center of the flower, which droops with the bumblebee’s weight if it wasn’t already facing down. Hanging upside down and grasping the flower’s stigma with her feet, the bee then vibrates the flower by whirring her wings. Pollen shakes loose and falls onto the bee, who goes from flower to flower gathering this nutritious food and in the process transferring pollen. Gardeners without bees, say in greenhouse cultivation, have to get by with shaking the plants themselves.
So how far apart do heirloom tomato varieties need to be grown to safely save seed? The greatest distance I’ve heard is 320 feet – this would be in a flat open area with no hedgerows or other barriers, between very different varieties – like Big Rainbow and Black Cherry, where a cross would be noticeable and very unwelcome. Factors that can shrink that distance are physical barriers like buildings, hills, or tall plants. The shortest distance we use, with plenty of barriers, is 100 feet. If you don’t have space to separate varieties that much, consider growing one heirloom for seed with other modern tomatoes nearby. Plan to save seed from a minimum of ten plants. And when the flowers appear, get a bee’s eye view of that small but noticeable difference that makes the heirlooms so attractive to native pollinators.
If you have experience with saving tomato seed, please comment here.