Grafting is an important step in the cultivation process of tomatoes and cucumbers. In the 1980s, rootstocks developed very fast because it was observed that grafted plants led to higher production. Since then, the industry has learned a lot about the grafting process. At the International Propagation Seminar, organised by Grodan, Johan Bruins presented some of these insights. His background at Plantise, where he was responsible for grafting and sowing at locations around the world, and supporting Globeplant with cucumber grafting, makes him the ideal teacher for this subject.
It starts with the rootstock
Johan starts with the first stage of the grafting process: the rootstock. "You need to sow the seeds two days before the variety, at 21–23 °C. Cover them with vermiculite and foil, or without foil – some companies are doing trials without it to save on material costs. It's possible, but make sure the humidity in the department is higher without plastic." The next step is selecting the plant after six days. "Hardening off is important – don't give too much water before spacing. Try not to water too much, as it stresses the plants. During this phase, the roots should search for water, strengthening them."
Johan Bruins
Cultivation schedule
The next step is to make a cultivation schedule, based on germination trials. "This helps determine the percentage of usable plants and whether the process is faster or slower than average. Based on this, you can adjust the schedule. When selecting after seven days, graft above the cotyledons. If the plants become too big, move them to a colder or harder department to slow down their growth, ensuring you stick to the planned grafting schedule."
Three selection categories
For selection, there are three categories, Johan explains, with 1 being the largest, and 3 is the smallest. "We count selections 1 and 2 as usable plants, while 3 is saved for incidents, though it isn't counted as usable. The advantage of this selection process is that the bad plants have already been removed by the time of grafting and the size is consistent. You can also start hardening off the rootstock for selection 1 earlier than for selection 2, so that on grafting day, both are at the same stage. After selecting and spacing the plants, put the trays above the floor for airflow around the roots, which promotes a more active climate and stronger plants. Avoid water pooling under the trays, as it's never beneficial."
Speaking of water, the plants should only be watered when it's needed, Johan warns. "Don't water when the plants don't need it – watering too frequently weakens the plants. The goal is to have the plants reach a particular size by selection day." Hardening of the rootstock usually starts around day 14. "Begin by lowering the temperature slightly and increasing the light. Decreasing nitrogen levels also helps to make the plants stronger, and the purple hue on the roots indicates healthy growth. This helps the plants during the healing phase."
Following the protocol
When grafting, following a strict hygiene protocol is also very important. "Decide whether the laborers will wear gloves or clean their hands between trays or varieties. Change the knives for each variety, and live by the protocol. Cut the rootstock at a 45-degree angle on the tray, which is more efficient because you don't need to remove any bad plants. Put the clips on the rootstock, cut the variety above the cotyledons, and connect them to the rootstock before placing them in the healing room."
Cutting above the cotyledons prevents shoot growth, which is another reason Johan prefers this method. "It also means the plants should be two or three days older before grafting. The healing process takes place in either a healing room or a tunnel. In a controlled climate room, the temperature is always stable at 23 °C, which is ideal for healing. The process may take one day longer in the healing room compared to the tunnel, but you'll get consistent results. Start acclimatization earlier in tunnels since the plants get more light there and grow stronger."
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Evaporation and humidity levels
Humidity is another aspect that impacts the grafting process, Johan elaborates. "In tunnels, wet the floor before placing the plants under plastic to increase humidity. Make sure the plastic is intact and check for any gaps or scratches. Condensation inside the plastic is a good indicator of high humidity. Acclimatization can begin after two or three days, depending on the plants' condition. Look for signs like a darkening of the youngest leaf to know when to start opening the plastic. Transplant after six or seven days, and gradually open the plastic to acclimatize the plants to more light and a colder environment. When using a climate room, watch out for edema, as the evaporation rate is lower than in a greenhouse. In the tunnel, make sure there is no direct sunlight, and use shading if necessary."
The last stage: hardening off
For cucumbers, there are four different types of rootstock, Johan goes on to explain. "Maxima Moschata and Moschata Moschata (both pumpkin varieties), which are resistant to soil diseases, and wild watermelon, which is nematode resistant and used in soil-based cultivation. The last type is Fusarium and Pythium resistant, suitable for rockwool cultivation."
Sowing for cucumbers before grafting depends on the conditions. "Hardening off is crucial to reduce root pressure, which can negatively affect the healing process. When grafting cucumbers, there's a debate about whether to graft with or without cotyledons on the rootstock. Grafting with one cotyledon can take longer, and I prefer doing it without cotyledons to increase efficiency, but it's important to manage root pressure to ensure success. The cutting method is similar to that used for tomatoes, though cucumbers are thicker and require careful handling."
After grafting, cucumbers take about five days to start acclimatization. "In a climate room, you may use a fogging system to gradually lower humidity day by day. When done correctly, you'll get strong plants ready for transplanting. Watering plays a crucial role in the process, especially for cucumbers, which are more sensitive. Giving water only when necessary helps ensure strong rootstock and successful grafting."
For tomatoes, the hardening-off process is similar, typically lowering the temperature to 16 °C. "Pay attention to water pressure and timing during grafting to avoid issues with healing. Keep the environment as consistent as possible to produce strong, healthy plants", Johan concludes.
For more information:
Dutch Horti Support
Johan Bruins
+31 (0)6 1014 2402
[email protected]