Rootstock selection is usually driven by growers aiming to overcome specific challenges including soilborne disease, low soil moisture or fertility stress, salt stress, and/or temperature stress. Like scion varieties, rootstock varieties have been bred to have specific characteristics. Individual RS varieties may be effective at countering one or more of these problems but it is best to review RS options carefully. The root systems of nearly all RS varieties are more vigorous than the root systems of standard scions; therefore, grafting may improve crop performance even when growing conditions are generally good.
Second, the stem diameters of the rootstock and scion seedlings must be similar and correct. Stem diameters being similar is critical for two reasons. Similar stem diameters are easier to align during grafting. And, more important, similar stem diameters allow the grafted plant’s vasculature (xylem, phloem) and cambium (growing ring) to stitch properly, assuring strong, unrestricted movement of all substances form roots to shoots and vice versa throughout the life of the grafted plant. Stem diameters being correct is important because seedlings can be too small or too large to graft. In our experience, tomato seedlings should have a stem diameter of approximately 3mm (a little more than 0.1 inch) at grafting.
Finally, newly grafted plants must be allowed to heal in carefully controlled conditions. The actual graft (surgery) may require less than one minute but the healing period may last up to three weeks. During that time, temperatures around the grafted plant should remain 77- 86°F (for tomato), the relative humidity should remain around 95% and the light level should not exceed more than 75% of normal sunlight (approximately 50% for the first week after grafting is best). Grafters often achieve these conditions with the use of simple open-framed structures covered with plastic and shade cloth. We have also found that a flooded capillary mat beneath grafted plants gives them both the little water they need and adds humidity to the air in the healing chamber. The VPSL attempts to prevent liquid water from ever coming in contact with grafted plants while they are in the healing chamber – therefore, we use no hoses or large droplet vaporizers.