One of the experiments we did this year in the field was a comparison of grafted and non-grafted tomato plants. After a season of patiently growing and observing these plants, I am excited to have some results to share with you.
First, a little background information for those of you new to this grafting idea. The basic idea of grafting is to attach the top of a tomato variety you would like to eat to the bottom rootstock of a tomato variety that has strong disease resistance and more vigorous growth. After the grafting process, you end up with a tomato plant that looks like the ones in the photo below. At the time of this photo, the stems were held together with little clips, but those fall off once the graft heals and the stems expand.
For our experiment, we grew one 50 square foot bed of regular Moskvich tomatoes (an heirloom semi-determinate tomato) and one bed of Moskvich tomatoes grafted to Fortamino tomato rootstock. We raised both sets of tomatoes in the same way but we did give the grafted plants a head start to account for the time they would need to recover from the grafting process. The seeds for our grafting tomatoes were planted on April 8th and the seeds for our non-grafted tomatoes were planted a week later on April 15th. The grafting process needs to take place when the plants are very young so I kept a close eye on our young seedlings and decided to graft them on April 27th when their stems were about 2mm in diameter. After grafting, the plants were kept in a covered tray out of direct light and it took them about 2 weeks to really recover. My grafting success rate was far from perfect, but I had grafted quite a few extra in anticipation of this so thankfully, we still had enough plants for two full trial beds.
We continued to raise these tomato seedlings as usual. After starting them in 3/4 inch blocks, we potted up to 2 inch blocks, and finally 4 inch blocks. The photo below shows how they looked at the time of transplanting into the field on May 26. This plant here was non-grafted, but the grafted plants were hardly distinguishable at this point. The only difference at this point was that the grafted plants were slightly smaller, so next time I will start our grafting plants earlier to give them even more time to recover from the grafting process.
We grew both of our Moskvich tomato beds interplanted with lettuce as shown below. This is a planting combination that we use frequently so I felt confident that it wouldn't disrupt this experiment. Plants were spaced 2 feet apart, pruned to 2 leaders, and supported with the same 5 foot high trellis system. Both beds had a north-south orientation and the second test bed was located just 3 beds to the left of the one shown. The close proximity made it easy to make sure that the soil, light exposure, temperature, and irrigation were the same for each bed.
It wasn’t obvious early on, but a few weeks into the season, the grafted plants began to show obvious signs of increased vigour. Looks can be deceiving though, so I suppressed any presumptive excitement and awaited the numbers. Since a hard frost ended things for these two tomato beds early in September, I have been able to finally tally up the totals. The non-grafted plants yielded an average of 5.5 lbs per plant and the grafted plants yielded an average of 9.3 lbs per plant. Those yield per plant numbers aren't that impressive but the difference between them sure is remarkable. The yield of the grafted tomatoes was 170% of the yield of the non-grafted tomatoes! The results of any experiment are valuable whether the predicted outcome happens or not, but it was especially satisfying to see this big of a difference.
I didn't stop the measurements there though. The tomato varieties selected for grafting rootstock are supposed to have increased vigour so I was also curious how things looked below ground. I dug up the plants and carefully extracted what I could of the root systems from both beds.
After snapping the photos below I cut off the stems at the top of each root mass and weighed them. The non-grafted roots had an average mass of 46.6 grams and the grafted roots had an average mass of 84.4 grams, 181% greater. The numbers alone are proof of the increased vigour, but you can see the noticeable difference in the root photos below as well.
After reading those numbers, you might be starting to wonder if grafted tomatoes are a cure for everything. To be truthful, I have to report that this is not the case. Grafted plants are still far from invincible. They still freeze when the temperature drops too low of course, and I still observed quite a bit of cracking on the fruit of our grafted Moskvich tomatoes. Therefore, it would appear that the tendency of a fruit to crack is much more dependent on the top of the grafted plant rather than its rootstock. The cracking would make me question our growing techniques, but we grew 6 other tomato varieties in the same plot this season and no others showed any signs of cracking. That gives me confidence that our growing techniques are good enough and the cracking is more just a weakness of this specific heirloom variety.
Where do we go from here? Well, the results from this initial grafting trial were encouraging enough to leave me wanting more so you can bet I will be dabbling in some more tomato grafting next spring. While the grafting process does take a bit of time, the appeal of growing 170% of the tomatoes in the same space is pretty strong. The little bit of time I spend grafting tomatoes upfront saves me the time I would have spent growing 70% more tomato plants, not to mention the additional space and soil preparation that those additional plants would require as well. It would seem silly not to take that deal.