I said last week that I would describe tomato grafting in more detail, and now's the time because I spent much of the day finishing our stock in the nursery. Before I dive into the nuts and bolts, I should note that our success rate so far over the past two years has been closer to 65 than 100 percent. That's not bad in my book, but it's something to keep in mind when you're buying rootstock seeds. I imagine it can be even more successful with practice and better handling.
The hundred plants that Ted grafted last week are looking generally good. As expected, several of them aren't going to make it, either because the stems never matched up and they wilted within a day or two, or because the graft wasn't strong enough by the time I started decreasing their humidity. I'm truly in awe of the fact that any of them live at all! To be able to continue growing and producing (better than before!) after having half their bodies chopped off and recombined with a foreign replacement is miraculous to me. Plants are just astounding.
I'd recommend grafting for folks with limited garden space and potential build-up of soil borne diseases, those who want an extended abundant harvest, and people with kids (or inner children) who want to geek out on plant science. This is definitely next-level gardening, so if you've never grown tomatoes from seed, try that the old-fashioned way first. If you're raring to go, here's what I've learned:
1. Plant rootstock seeds a few days to a week earlier than your scion (the tomatoes you want to eat) seeds. Our rootstock tended to be smaller than the vigorous Big Beefs we grow for our greenhouses, which makes it harder to match up stems. The Youth Farm has received a donation of Estamino and Supernatural rootstock seeds from Territorial Seed Company for the past two years. On our scale, their cost would be prohibitive, but for an enthusiastic gardener a small pack could be a worthy investment. If I were planting these seeds at home, I would plant the rootstock seeds in separate 2-3 inch pots so they don't need to be potted up later, and the scion seeds 6-10 seeds per pot. We just use 200-cell flats for both, and this last round experimented with up-potting the rootstock plants in 2-inch pots about a week before grafting.
2. Gather some tools. You'll need a razor blade (or a fancy grafting blade made specifically for this purpose), silicone grafting clips (you might need a couple different sizes), small stakes and ties to help hold the plants up, and a "healing chamber". We made ours with clear plastic propagation domes to keep humidity high around the plants, a space heater to keep the temperature consistent and heating mat for mild bottom heat, and a make-shift black box from PVC pipe and layers of landscape fabric to control the light. Get creative and do what you can!
Once the rootstock and scion plants are 2-3 millimeters in diameter, it's time. Have your healing chamber ready along with a spray bottle, and gather your tools and plants. I found it easiest to do eight grafts at a time, to finish a whole tray of 32 little pots before anything started to show signs of stress (if that's possible after being beheaded...). I'd definitely suggest starting with one plant and building up to a rhythm that works best for you.
3. Select rootstock plants that have relatively consistent stem sizes for each round. Place them next to each other in a tray and get in a position so that you can lean in and see the hairs on their stems really clearly. (You'll need to see this much detail to get a good match.)
4. Select scion plants that are relatively consistent with the root stock stem size you're working with. Cut them just above the surface of the soil, trim off the cotyledons and most of the first true leaves to reduce their need for transpiration, and lay them on a clean surface.
5. With a sharp blade or fancy grafting knife, cut the root stock stems just below the cotyledons at as close to a 60 degree angle as you can get. This exposes a larger surface area for fusion than a shallow angle would. If the angle is too steep however, the thin points tend to curve away and slough off, making it more likely for the graft to fail. So go for that sweet spot.
6. Cut the scion stem at a similar angle below the cotyledons, or where the diameter matches best with the rootstock. I had to cut pretty high up the stems (where the plant is narrower) for some of the grafts since our scions had gotten much bigger than our rootstocks. I'm hoping that adds an extra advantage of reducing legginess and keeping the plants strong and straight as they heal, since long plants tend to flop over.
7. Insert the angled tip of the scion stem into a grafting clip. I agree with Ted's pro tips here: have the opening of the clip aligned with the shorter side of the angled tip (so you can see the exposed cut when you're looking through the clip's opening). This makes it easier to know how to orient the stem on the root stock and see the two angled cuts nest together. Also, slide the scion stem in to fill most of the clip.
8. Align the angled cuts of your scion and rootstock, and slide the clip over the rootstock until the exposed cuts meet. Since the scion stem should be filling most of the clip, you can slide the clip down a bit to make sure the stem edges fit together well and that the meeting point sits half way up the clip. I sometimes smoosh them together gently to make sure they have full contact.
9. Stake and carefully tie the newly grafted plants. Label them with the rootstock and scion you used (especially if you're experimenting with lots of varieties, which I encourage!). If you're like me, hum to them and tell them it's all going to be just fine.
10. Once your tray is full or you've grafted all your plants, mist the inside of a propagation dome and carefully fit it over the tray. Place in your healing chamber, seal them off from light, and say some prayers.
11. Now's the tricky part! The next week is critical to ensuring successful grafts. Basically, the plants need to completely stop working so they can focus on healing and fusing together. After they've fused, they need to very gradually be reintroduced to normal conditions.
- They need to remain in complete darkness for just 1-2 days, after which you gradually reintroduce light, little by little. With our set up, I did this by removing all but one layer of landscape fabric after the first day (so faint light can enter), then lifting a couple sides up just a bit for a day to let in more indirect light, then lifting all the sides for a day, then finally exposing them to direct sunlight.
- The temperature should remain around 80-85 degrees for this whole process, so be careful with direct light since it could heat the area up more than you want. We use a space heater on a solid surface within the chamber, a safe distance from any plastic.
- The humidity should stay at 100% for around four days so the plants can fully heal before they're pushing fluid up and down their stems (ie, transpiring). After the third or fourth day, I open the vents on the domes for a day, then prop up one side for another day, then prop them up higher for another day or two until they're basically in normal conditions. Along the way, spritz the inside of the dome whenever the condensation dwindles.
- The plants might need water, but putting any weight (even little water droplets) on their leaves could cause them to topple. So, if dribbling water around each stem isn't feasible, put your pots in some standing water for a few minutes to let the soil soak it up from below.
- If at any point during the healing process a majority of your plants start to crash, go back a step (less light, more humidity) and try to acclimate them again later or the next day.
Not exactly a piece of cake, but it's fascinating, fun to try, and really does make for healthier, more productive plants. We're using these grafted tomatoes in our greenhouses, trying out a few varieties of cherry tomatoes as well as our tried-and-true Big Beef for slicers. If we have extra we might experiment with some in the fields as well. The first batch that Ted grafted last week is almost back to normal greenhouse conditions (domes hopefully coming off tomorrow!). I grafted another 130 or so plants today and am hoping we get at least 100 good plants... Oh the suspense! I know you're all sitting on the edge of your seats-- updates surely to follow.