With unpredictable summer weather the last couple of years, finally culminating in a dismal tomato harvest last year, after many plants failed to thrive, we decided to try grafting some of our favorite heirloom varieties onto more disease resistant rootstocks this season.

Commercial growers often graft heirloom stock, to impart disease and pest resistance, as many heirloom varieties are not resistant to some of the more common tomato maladies, such as Fusarium Wilt, Verticulum Wilt, and Tomato-Tobacco Mosaic Virus.

We already graft apple trees to alternate rootstocks for the same reason, conferred disease resistance, so we thought we’d experiment with grafting for tomatoes.

Choosing Rootstocks

The first step for grafting was to determine which rootstock we should choose.  There are numerous rootstocks available with varying levels of resistance to a number of common diseases, including Tomato Mosaic Virus, Corky Root, Fusarium Wilt, Verticulum Wilt, and Bacterial Wilt. [1] Knowing which diseases are present in your garden can help with rootstock selection.

Despite employing rotation techniques, we encountered Fusarium Wilt in the garden last year for the first time, so the rootstock needed to be at least moderately resistant to Fusarium, which is caused by  Fusarium oxysporum.

We selected the rootstock Maxifort (F1) this year, as it is a vigorous rootstock, with moderate to high resistance to Fusarium Wilt, and it was readily available from our seed supplier.

For grafting, we began by sowing both our heirloom tomato seeds, and the rootstock seeds at the same time.

However, we staggered seeding the rootstock seed over the period of a few days.  This is because not all tomato varieties grow at the same rate. It’s important when grafting to ensure that the diameter of the rootstock stem, and the diameter of the scion (the heirloom variety being grafted) are the same.  By staggering sowing dates for the rootstocks, we ensure we have a variety of stem widths to choose from at the time of grafting.

As we had not grafted tomatoes before we also were sure to plant more seeds than normal for each variety.  Then we could afford a few ‘learning curve casualties’.

Although rootstocks sown from seed tend to have increased plant vigor, initial germination can be low, and may not be uniform.  That was certainly true for us.  The germination rate for the Maxifort seed we chose was stated by the supplier to be lower than their standard rate (they had included additonal seed to compensate for poor germination).  At grafting time, we only had 50% germination, compared to 95% germination for our heirloom seed. As such, overseeding of rootstock seed, expensive though it is, is important to ensure there are enough rootstocks for the available scions.

Preparing to Graft

While the seeds were busy sprouting, we needed to create an area to place the plants as soon as they were grafted. Ideally the grafts need to be kept at least at 80-95% humidity while the plants heal, initially with no direct sunlight [2].

To achieve both, I constructed a simple humidity chamber out of sheet polyethylene, and PVC pipe.

It’s important that water doesn’t condense at the top of the humidity chamber and drip onto the scions during healing, as this can disturb or dislodge the new graft.  To prevent this, a peak is constructed at the top, so any condensate will run down the wall of the chamber, rather than dripping on the plants.

I expect I’ll leave this frame up on the greenhouse bench year around now, as it actually could be used outside of grafting season to support shade cloth over tender seedlings.  As it turned out, the peak of the chamber ended up above the level of the misters that run across the propagation bench. This wasn’t intentional, but it will allow us to run the misters over the shaded seedlings too.

The PVC frame was covered first in sheet polyethylene, then with a layer of shade cloth.

The polyethylene was secured to the frame using PVC snap clamps.

It’s important to have good coverage, and to seal up the edges to prevent humidity from escaping.

Sometimes it’s the simplest inventions that prove the most useful.  We use these snap clamps for a lot of projects here, whether it’s for anchoring netting over strawberries, or securing row covers over tender seedlings, or building a humidity chamber for tomato grafting, we always keep some on hand.

The floor of the chamber was also fitted with polyethylene sheeting, and clamped to the frame using PVC snap clamps.

By lining the floor (bench top) with plastic sheeting, this allows the floor of the chamber to be flooded, both for bottom watering, and to help increase humidity within the chamber.

Over the top of the shade cloth layer, to block light for the first few day while the graft heals, we also installed a layer of light blocking plastic (in this case a couple of 33 gallon garbage bags taped together).

The idea is that when the black plastic is removed, the shade cloth layer will permit some light to enter, but not at full intensity while the plants slowly acclimate to the greenhouse environment.

Preparing the Grafting Area

On grafting day, I waited until late afternoon, as the temperatures were cooling, to graft the rootstock and scions together.  It’s imperative not to allow the grafts to dessicate while working.  Avoiding direct sunlight, and drafts, will prevent the cut ends of the grafts from drying out before the grafting process is complete.

Before grafting, the plants should have attained the second ‘true leaf’ stage.

With exception of the grafting clips, the tools required for grafting were quite simple and straightforward:

• A cutting board
• Rubbing Alcohol
• Paper Towels
• Double-edged Razor Blades
• 2.0 mm Japanese Tube-Grafting Clips
• Exam Gloves (optional)

To test whether or not the plants are large enough to graft, a grafting clip was first slipped around the stem of a few plants to ensure the clip was snug, and did not slide downward.  If the clip slips, try again in a day or two.

During grafting it’s important to keep the work area free of debris, as any dirt on the cut edges of either the rootstock or the scion can inhibit graft healing.  The cutting board, and razor blades, are kept clean throughout the grafting process with rubbing alcohol.

To avoid unnecessary bandaging of fingers, the double-edged razor blade was first snapped in half.

The single sided blades with the thick metal bands at the top are not recommended for grafting, as these single sided blades are too thick.

Grafting Process

First, the rootstock was cut below the cotyledons.

This is to prevent adventitious rootstock shoots from emerging along the stem.  I’ve read everything from a 30-45 degree angle for the cut.  It’s more important to be consistent with the angle of the cut from rootstock to scion, than it is the exact angle of the cut itself.  If it helps, a 45 degree angle line can be drawn on a paper towel, and positioned under the plants to guide the cut.

The scion stock is cut at the same angle as the rootstock, adjusting up or down the stem so that the thickness of the stem matches the rootstock.

With the rootstock, and scion cut, you should be left with this.

Before completing the graft, the side leaves are then trimmed from the scion stock so the plant has less to support during healing, ideally leaving only the central growing meristem.

A tube grafting clip was pinched open, and stretched over the rootstock stem, and the scion stem slipped down into the clip, matching the angle to the rootstock, and seated securely before releasing the clip.  The grafted plants were then potted into 4″ pots, and immediately set into the humidity chamber.

The grafts will remain in the humidity chamber through the first week.  A few of the grafts have wilted slightly since we grafted, and it looks like we may have lost 2 out of 20 grafts thus far, which is not unusual.  Losses are to be expected, and we may have a few more losses before transplant time.  We will slowly increase the amount of light entering the enclosure where the grafts are housed, and decrease the humidity, until the plants have acclimated to the greenhouse.  This part of the process can’t be rushed, and if the plants begin to wilt further, we’ll close the chamber back up, and try again the next day.

Once the graft unions are completely healed, and the plants have hardened off, we can transplant them to the garden.

Grafting tomatoes increases both the cost of production, as rootstock seed is expensive (approximately 50 cents per seed).  It is also slightly more labor intensive, although the actual grafting went quite quickly.  Whether or not grafting will prove to be worth the additional expense, and additional labor, we won’t know until the harvest at the end of this season.

We’ll follow up on the progress of these grafted plants in our next grafting post.  We’re crossing our fingers that we’ll have some plants to show you that have healed, and are starting to grow!

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[1] Grafting for Disease Resistance in Heirloom Tomatoes.  NC State University  and A&T State University Cooperative Extension

[2] Grafting Tomatoes for Increased Vigor and Disease Resistance.  Johnny’s Selected Seeds

Helpful Tomato Grafting How-To Videos:

University of Arizona Tube Grafting

University of Vermont Extension How to Graft Greenhouse Tomatoes

Ohio State University Grafting Tomato Plants