When it comes to pollination, domestic tomatoes (Solanum lycopersicum) are curious plants. The flower is essentially closed, as the pistil (female part) is shorter than the stamens (male parts) that form a protective cone or tube around it.
That means the flower does not readily accept pollen from outside sources. That’s why it’s so easy to maintain heritage tomato lines: unlike almost any other vegetable, you don’t have to be too concerned about cross-pollination: pollen from another variety is unlikely to arrive and create an unwanted hybrid with mixed genes.
This wasn’t always the case. Wild tomatoes do cross readily, with a stigma that extends beyond the staminal cone, a structure designed to permit cross-pollination. But about 1500 years ago, undoubtedly somewhere in the Andes, a mutation with a short pistil appeared, so the stigma (at the tip of the pistil) remained inside the staminal cone, out of reach of pollinators. Clearly gardeners of the time appreciated the innovation, as once they developed bigger, juicier, tastier tomatoes, they could maintain them without fear their plant’s good traits would be overwhelmed by wild tomato genes. Soon, they grew them exclusively and self-pollination became the norm for domestic tomatoes.
Even so, tomatoes still need bees*, preferably bumblebees, whose vibrations release the pollen grains from the stamens so they can fall on the stigma. (This is called buzz pollination or sonication.) However, the pollen inevitably falls on the stigma of the same plant, resulting in self-pollination.
*Sometimes wind will pollinate tomatoes in the absence of bees.
Breaking the Rules
To hybridize tomatoes, in other words, cross one tomato variety with another, you have to break the rules. You’ll have cut open the flower to move the pollen from one flower to the stigma of another.
Let’s say you have a favorite tomato with outstanding flavor and great disease resistance, but it’s a weak grower and none too productive. Yet another tomato you grow is ultra-vigorous with a massive production of … not-so-tasty fruits and unfortunately, it’s terribly subject to disease. You could try crossing the two and attempt to create the vigorous, tasty, productive, disease-resistant tomato of your dreams.
Gather Your Tools
• small labels you can attach to the flowers (jewelry tags are perfect!);
• a pen or pencil;
• forceps or fine-tipped tweezers;
• a fine artist brush;
• two different tomato varieties
• reading glasses (for most of us).
Step by Step
This is very delicate work and at first you may end up damaging the flowers you’re working with. Take it slowly and be patient. You’ll soon get it!
You have to be growing the two tomatoes you intend to cross and both must be in bloom. Also, choose a period when there will be a few sunny days in a row. Tomato flowers won’t always mature in rainy weather.
1. Start by writing the names of the parents (and the date if you want) on the label. You’ll need one label per intended cross.
2. On the plant you decide will bear the hybrid fruit (the female parent), choose a flower bud at the right stage. It should be well developed and turning yellow, but not yet open. If it’s it already open, it’s too late and will likely have been fertilized by its own pollen.
3. Remove all other flowers from the cluster to prevent accidental pollen transfer from another bloom on the same plant.
4. Remove the petals with the forceps or tweezers. They’re easy to remove.
5. You now have to emasculate the flower (remove its male parts). This is extremely delicate and I found it took me about 5 tries before I could do it without damaging the pistil (the female part). I found after practice that if I carefully grasped the staminal cone with forceps and pulled straight forward, it would generally come off, although at first I was only able to remove the stamens by pulling them free one by one (very tedious!).
6. Repeat with 4 or 5 other flowers to be sure of success.
7. Attach the label to the pedicel (stem) of the flower so you can better find it the next day, as the pistil will not yet be receptive. You’ll have to wait 24 to 36 hours before pollinating it.
There is no need to bag the flower: without petals to attract them, insects will not visit the pistil.
8. The next day, harvest a fully mature (open) flower from the male parent.
9. Open its staminal cone lengthwise, inserting the forceps into one of the slits in its side and forcing it apart.
10. Run the brush against the inside of a stamen, starting at the base and dragging it upwards to pick up pollen. You’ll see white powder (the pollen) form on the tip of the brush.
11. Apply the pollen to the emasculated flower, at the very tip of its pistil, the part called the stigma, which should be a bit sticky at this point. Apply enough to cover the stigma in white pollen.
12. Repeat about 12 hours later in case the stigma was not receptive the first time.
You’ll likely want to make several crosses, as not every one “takes.” You’ll know it has a few days later when the ovary at the flower’s base begins to expand and a fruit starts to form.
Testing Your Results
When the fruit you pollinated is fully mature, extract the seeds, then clean and dry them, storing them in a paper envelope with the label attached.
In the spring, sow the seeds about 6–8 weeks before you plan to plant the tomatoes out. As the plants grow and produce fruit, choose a ripe one from the plant that best meets your goals (productive, tasty, vigorous, disease-resistant, etc.) and keep its seeds. The following year, sow them, then again collect seeds from the best plant. And repeat the following year, and the following year. And so on.
By annually choosing the plant closest to your expectations, you’ll eventually obtain a stable line (although that can take 6 years, sometimes even more!): one where the plants meet your needs and are essentially identical. Congratulations! You’ve just created a brand new tomato variety! You can now give it the name of your choice and share it with the world.
Not What You Hoped For?
You didn’t get exactly the results you hoped for? That’s not unusual. Some tomatoes just don’t give good results when you cross them. Try again with other parents, remembering you’re doing this for the pleasure of it.
After all, even professional tomato hybridizers, who have an excellent grasp of tomato genetics, can spend a decade or more developing tomatoes that exactly match their goals.
Even so, almost all those heritage tomatoes that abound on the market today were developed by home gardeners: ‘Brandywine’, ‘Aunt Gertie’s Gold’ and ‘San Marzano’ are just a few examples among literally hundreds. And there’s the famous ‘Mortgage Lifter’ developed by the American M.C. Byles, that brought in enough money to pay off his $6000 mortgage back in the 1940s. That proves that some home gardeners have hit the jackpot in the past and there’s no reason you couldn’t as well.
Best of luck!