Tuesday, January 29, 2013

An Introduction to the Topic of Grafting in Greenhouse Tomatoes

An Introduction to the Topic of
Grafting in Greenhouse Tomatoes

By Dr. Natalie Bumgarner, Horticulturist
CropKing, Inc. Lodi OH

Grafting- What and Why?

Grafting has become very prevalent and important in greenhouse tomato production, and there are a couple of key reasons this is the case. Before we explore those reasons, let’s take one quick step back and discuss the basic premise of grafting. Many who are familiar with crop production are aware that most tree fruits are produced with a root system (rootstock) and above ground fruiting portion (scion) from different plants. This same principle is now carried out in many vegetative crops where two separate seedlings are produced and then joined together through the grafting process to produce one plant for transplanting. The use of grafting enables breeders to develop crop cultivars specifically for the demands of root and shoot environments and functions.

A tomato plant immediately after being grafted 
A young grafted plant for hydroponic greenhouse
production that has already been grafted and healed

Grafting can provide many advantages, but they mainly fall into the three main categories of disease resistance, stress resistance, and enhanced vigor and productivity. The use of grafting can reduce risks of disease (often affecting root systems) due to the incorporation of resistance traits in rootstocks not present in some popular scion cultivars. This is especially important in soil production. Grafting also is being used in many regions and systems to address heat, cold, salt and other forms of stress present in the growing medium or environment. The final key area of benefit is that rootstock cultivars can be more vigorous in their growth habit. This benefit is one of the most important for greenhouse producers because increased nutrient and water uptake can lead to enhanced fruit production if properly managed. However, it is important to remember that plant stress and disease resistance and vigor can sometimes be related. Increased rootstock vigor can enable the plant to better withstand stressful conditions and still maintain productivity.

A small batch of young grafted plants approximately one week after grafting. 

Who is Grafting and When may a Grower Choose Not to Graft?

Currently grafting of vegetable crops is carried out in many countries around the world for a variety of crops. The most commonly grafted plants are tomatoes and peppers (Solanaceous) and cucumbers and melons (Curcurbit). These are high value, longer season horticultural crops where the added investment in grafting tends to be the most economically advantageous. Getting a little closer to home, a large percentage of the large North American greenhouse growers are using grafted tomato plants to take advantage of the benefits discussed above. Currently, the use of grafted tomato plants tends to be less common in small to mid-size greenhouse operations for several reasons.

Plant availability is one of the main reasons that grafted plants are not used. Most large growers are acquiring their plants from specialized propagation houses, and the high shipping costs are overly burdensome for many smaller growers who need a few hundred plants rather than many thousand plants. Grafting can be done on site, and this is certainly an option for growers. In fact, we will discuss the main steps and practices of grafting in your own greenhouse in the next blog post. However, time and space needs for grafting on site are higher than for typical seedling production, so this option is not desirable for everybody. 
Whether ordered from a propagation facility or grafted on site, the cost of grafted plants can be higher than seedlings because we are seeding two plants. Exact cost differences depend of production and training practices such as whether one or two stems are trained from the grafted scion (more on this in the future). These potentially higher seed/plant costs linked with shipping costs can be prohibitive for smaller growers. Another reason for the use of seedlings rather than grafted plants is the ability for growers to produce all the plants they need in their own operation. By controlling the site and environment of production, growers can reduce some disease risks in their young crops.  The duration of production in a greenhouse tomato crop can also influence whether grafted plants are optimally beneficial. Some growers who have shorter seasons in their operation may not achieve the benefits required to offset the added investment in grafted plants.

So, it is clear that there are certainly benefits and some drawbacks in the use of grafted tomato plants in the greenhouse. We at CropKing are currently researching grafted tomatoes in our greenhouse and will continue to explore these questions and concepts in an effort to provide useful information and enable growers to make the best choice for their operation and crop.