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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.

Once the grafts are prepared the plants require minimal light, and high humidity, while the graft union heals

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.

It's important to monitor the grafts for signs of wilting, and heat stress, during acclimation

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.

A vented humidity dome may make it easier to maintain the desired humidity levels during the first few days of healing

Although the chamber worked, we did struggle to get the humidity up high enough, and maintain it high enough, during the first few days.

The humidity chamber we built worked fairly well, but the volume of space made it challenging to keep the humidity high enough around the plants in the first few days of healing

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.

The single leaf on the growing meristem is all that is required to be left on the scion. More than that, and the plant may struggle to support itself

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.

We'll be interested to see the differences in growth, and fruit yield, between the grafted vs. non-grafted plants of the same 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.

The grafted plants on the left, although the same age, are much shorter than the conventionally grown plants on the right

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!

Although the grafted plants (left) were much shorter initially, by transplanting time, the grafted plants were almost as tall as the non-grafted plants (right)!

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.

Remember, the grafted plant is made up of two parts. The 'scion' is the heirloom variety you want to grow, and the 'rootstock', in our case Maxifort, is the disease resistant portion of the plant

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.

The graft union MUST be kept above the soil line to prevent the scion from developing roots of its own

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.

The tomatoes, grafted and non-grafted, are being grown in the same beds, under the same conditions

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.

The plants are now in bloom, and hopefully soon we'll see the first signs of fruit set

What we really want to know though, is how these plants perform in regards to fruit set, and disease resistance, throughout the summer.

Last summer was cold, damp, and dreary. This summer we're hoping for lots of blue skies!

For that we’ll have to wait and see, and hope that this summer the weather cooperates!

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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

How-To Videos:

University of Arizona Tube Grafting

University of Vermont Extension How to Graft Greenhouse Tomatoes

Ohio State University Grafting Tomato Plants