The extremly low natural light level is the major limiting factor for winter glasshouse production in Iceland. Therefore, supplementary lighting is essential to maintain year-round production. Most of the growers are using ungrafted tomatoes. The objective of the study was to test if the yield can be increased by grafting of tomatoes.

Ungrafted and grafted tomatoes (cv. Encore, rootstock Maxifort) were grown in pumice under high-pressure vapour sodium lamps for a maximum of 18 hours light. In the year 2011/2012 the experiment was conducted with 3.33 tops/m² (grafted: 2 tops/plant, ungrafted: 1 top/plant) at 300 W/m² and in the year 2012/2013 with 3.13 tops/m² (grafted: 2 tops/plant + additional top from each other top, ungrafted: 1 top/plant + additional top from each other top) at 240 W/m². Fruits were regularly harvested and classified.

At the highest light intensity and with two weeks shorter growing period compared to ungrafted plants, decreased grafting the accumulated marketable yield and increased unmarketable yield. However, this was not confirmed in the following year at the lower light intensity and stem density. After one month harvest the positive effect of grafting was getting obvious and at the end of the harvest period yield of grafted plants was significantly higher than of ungrafted ones due to a higher amount of harvested fruits.