Introduction
Typically tomatoes for production have a tendency to be less vigorous. To compensate for this slow vigour, tomatoes can be grafted onto rootstock which is more vigorous than the variety desired for production. This increase in vigour can also contribute to higher stress tolerance in the plant. Currently in Alberta, grafted tomato plants are purchased from BC which incurs high transportation costs. Propagating and grafting tomatoes for commercial production in Alberta is a potential opportunity for the Alberta Greenhouse Industry.
Since 2012, the Greenhouse Research & Production Complex has been doing propagation trials for grafted tomato plants for our greenhouse vegetable research program. We have also been working with seed companies to assess rootstock variety combinations for commercial production. Over the course of the trials, we have had a success rate of 95-100% in our grafting program at CDCS and can produce a transplant in 40 days from the seeding date.
Materials
Grodan Kiemplug standard french trays with 240 plugs, rootstock seed, variety seed, clear plastic, Billy Grafting Clips (1.6 mm x 0.5"), grafting chamber, misting bed, flooding bed, Grodan Rockwool cubes (10 cm x 10 cm x 6.5 cm).
Methods
One of the key elements to a grafting program is to know your germination rates of both the rootstock and the variety you are grafting on. This is important to determine your seeding dates to have the best match of stem diameters of both the rootstock and the variety. Staggering your seeding dates will compensate for some of the germination variation and provide more variable material for union matching.
Once you select your rootstock and variety, seed them into trays (Grodan Kiemplug standard french trays with 240 plugs) and cover with plastic. From the seeding date it takes 10-20 days for the seedlings to be ready for grafting.
The rootstock seedlings and the variety seedlings are sorted by diameter into large and small groups for grafting. The large diameter seedlings are kept in a cooler greenhouse if required to slow their growth and smaller diameter seedlings are placed in a warm greenhouse to manipulate the diameter of the plant stems for grafting.
70% isopropyl alcohol is used to disinfect the work area where grafting will take place. Also, the grafters hands and all the tools required for grafting should also be disinfected with the alcohol prior to grafting. At time of grafting, select a cool space out of the direct sunlight. Keep a spray bottle of water nearby to mist the grafts as they are completed and also to keep your variety and scion material moist as it waits to be grafted.
Variety seedlings are grafted onto rootstock. Plants and tops are matched according to stem diameter size. Slice the scion below the cotyledons at a 45o angle and slide your clip on so the cut part is about half way. Then cut your variety at a 45o angle below the cotyledons and slide it into the clip so that the exposed vascular tissue of each is touching the complete length of the cut. The clip must not slide up and down the stem of either the rootstock or the scion for optimal grafting success.
After grafting, each graft must be placed in a dark mist chamber maintaining a high humidity. At GRPC we misted for 15 seconds every 20 minutes. The chamber must remain dark for a minimum of 24 hours before exposure to light.
Day 2: open the mist chamber sides 1/4, opening it a little more each day. This acts as a stressor to the plant and forces the grafting site to fuse.
Day 4: change the mist to 15 seconds every 30 minutes. This adds an additional stressor to force fusion.
On day 5, the dark cover should be completely removed. Watch this stage closely, stressing the grafts is important for fusion but if you notice severe wilting, cover them back up leaving the sides of the chamber up. Each day after day 5 you will need to expose them to the light until they no longer wilt. Most times this stage will take a day or two.
Do not leave them in the mist chamber after day 6 for more than short periods of time. Long periods in the mist chamber will cause them to develop aerial roots from the variety scion and then the cotyledons will also drop off.
After the grafts are ready to come out of the chamber, plant them directly in prepared rockwool cubes.
Wait another day or 2, and judge readiness for pinching by giving the grafts a gentle tug from the bottom to see if they are rooted into the cubes. If multiple heads are required, pinch the plant right above the cotyledons to give you two new strong heads.
When the plant is large enough transplant to the greenhouse.
Keep in mind that the steps to the finishing stage of the plant are variable depending on time of year and your greenhouse environment and may need to be altered slightly.