In our last tomato grafting post we discussed why we’ve chosen to try grafting some of our favorite heirloom tomato varieties this season.
Tomato grafting isn’t new, and many commercial producers rely on this method of heirloom tomato production to improve disease resistance, and increase fruit yield. However, we questioned whether or not grafting would be worthwhile on a much smaller scale, even for the home gardener. By the end of this season, we hope to have our answer.
At the end of the last post, our newly grafted heirlooms were sitting in a darkened humidity chamber while the graft unions were starting to heal.
For the first three days the chamber was kept completely dark. Gradually, over the period of 3-4 days, the light blocking plastic was removed, but shade cloth was kept in place to enable the grafted plants to slowly acclimate to the increase in light.
The next step was to slowly decrease the humidity level in the healing chamber, by gradually increasing the ventilation. We simply opened up the sides of the polyethylene ‘tent’ a little more each day. As a result of some warmer than expected weather, this process took somewhat longer than anticipated, and it was at least a week before the plants were eventually acclimated to greenhouse temperature and humidity conditions.
On a number of occasions the humidity levels needed to be increased, by closing the ‘tent’ back up, to prevent the plants from wilting excessively, but eventually the plants adapted and were moved out onto the greenhouse bench.
Next season, we’d do a few things differently. For one, we’d be sowing seed on schedule, rather than being a few weeks behind as we’ve been this season due to too many projects going on at once. Although the grafts need warmth and humidity, excessive heat during healing makes it more challenging to acclimate the plants after healing.
The biggest change will be that next year we’ll try placing the plants, immediately after grafting, under a humidity dome inside the healing chamber.
Although the chamber worked, we did struggle to get the humidity up high enough, and maintain it high enough, during the first few days.
This is when most of our losses occurred, as some grafts wilted to the point of no return.
We’ll also be more aggressive when trimming our scions, ensuring we only leave the growing meristem intact, and all other lateral growth will be removed. The less the plant has to support during the first few days, the better off the plants will be overall.
Overall, we lost six plants of twenty total that were grafted, which translates into a 70% success rate. Not bad for our first attempt at grafting, but we’d like to try to improve that percentage next time, if grafting ultimately proves worthwhile this season.
Once the plants were acclimated to the greenhouse, it was just like raising any seedling in the greenhouse. We waited until the grafts were not only healed, but were showing clear signs of strong growth before transplanting them out into the garden.
For a fair, and direct comparison, of grafted versus non-grafted plants, this year we’ve planted the tomatoes out in pairs (one grafted, and one non-grafted plant) for each variety.
After the grafts were healed, comparing the grafted versus non-grafted plants, it was clear that the healing process had set the plants back significantly.
Here the grafts are half the height of the conventionally grown tomatoes.
However, just prior to transplanting we could see clear evidence that the grafted plants were showing increased vigor overall. Within 10-12 days, the grafts had almost caught up, and I won’t be surprised to see the grafted plants exceed the height of their non-grafted counterparts now the plants are in the garden. I wish I could skip ahead to see how all this all turns out!
The greatest consideration when transplanting out grafted tomatoes is that they can’t be planted too deeply. Normally we remove the lower leaves of a tomato plant, and bury the transplants deeply in the soil. The section of stem planted below the soil line produces additional roots to help support the plant during growth and fruiting.
However, with a graft, the whole purpose is that you want the disease resistant rootstock to support the plant. If you bury the transplant so that the scion stem (i.e. the heirloom part of the plant above the graft union) is below the soil line, you negate the entire purpose of using a disease resistant rootstock as soon as the scion produces its own roots. As such, it’s imperative to keep the graft union above the soil line when transplanting, so the only roots that grow, are from the disease resistant rootstock.
As the plants grow we will need to watch for, and remove, any suckering growth being produced from below the graft union, as this will be rootstock growth. Beyond that, these grafted tomatoes will be treated no differently than the other plants.
Thus far, even though the grafted tomatoes initially seemed to be at a disadvantage, the increased vigor of the Maxifort rootstock that we grafted to appears to be making up for the lack of growth during the healing period.
What we really want to know though, is how these plants perform in regards to fruit set, and disease resistance, throughout the summer.
For that we’ll have to wait and see, and hope that this summer the weather cooperates!
Helpful Tomato Grafting Information:
Grafting for Disease Resistance in Heirloom Tomatoes. NC State University and A&T State University Cooperative Extension
Grafting Tomatoes for Increased Vigor and Disease Resistance. Johnny’s Selected Seeds
University of Arizona Tube Grafting
University of Vermont Extension How to Graft Greenhouse Tomatoes
Ohio State University Grafting Tomato Plants