Grafting

In many gardening catalogs, you can find trees that boast multiple types of fruit. This is done by grafting twigs from various trees onto a parent tree.

Apple tree grafting (Karelj) Public Domain

A man named Sam Van Aken created a tree with 40 different types of fruit growing on it. His Tree of 40 includes almonds, cherries, nectarines, peaches, and many other stone fruits, all grafted onto a single tree. Grafting is an excellent way of making the most out of a small gardening space and it can look pretty amazing. Grafting is an old technique used to join two plants together. A newer version, called budding, does the same thing, but in a different way.

How does grafting work?

The top half of a graft is called the scion and the lower portion is called the rootstock. Grafting works because plant hormones, called auxins, allow the vascular cambium tissues of both the scion and rootstock to merge. This allows water and sap to continue moving through the xylem and phloem. 

Before you jump on the grafting bandwagon, however, keep in mind that grafting is tricky. It takes practice. You also need to know that, after a graft is completed, even though a protective callus has formed, and the vascular tissues have fused, the wood does not. This means that graft unions should always be considered structurally weak. You should also know that the fruit and nuts produced on grafted trees contain seeds that hold the genetic information for the scion wood only, and not that of the rootstock. Also, if you have a plant that is putting out suckers, keep in mind that these are from the root stock, and not the productive aboveground portion of your plant.

Natural grafting

Grafting occurs in nature, but it is roots that graft more often than branches. Tree root grafting allows huge stands of trees to share water and nutrients. It also forces them to share soil borne pests and diseases, such as Dutch elm disease. Potatoes and strawberries frequently graft their roots onto neighboring plants of the same species.

Natural branch grafting occurs when two limbs repeatedly rub against each other, exposing the cambium layer. The most common aboveground natural grafting occurs in blackthorn. These are affectionately known as husband and wife trees.

Husband and wife blackthorn tree (Rosser1954) Public Domain

Advantages of grafting

It is pretty safe to assume that nearly all fruit and nut trees available today are grafted combinations of hardy, pest- and disease-resistant rootstock and highly productive scions. 

Grafting speeds up production because young scions can be grafted onto older rootstock. This is called precocity and it allows growers to skip the 5- to 12-year juvenile phase, when trees are focusing on root system development, rather than fruit production. Grafted trees can also selected for size. Simply graft a scion from a full-sized tree onto dwarf rootstock and you get a dwarfed tree that produces more fruit. In many cases, grafting is used to imbue a tree with pest or disease resistance. This method is also used on watermelon, tomato, eggplant, cucumber, and other vegetative plants for the same reason.

Grafting for the garden

Let me say this up front - grafting requires skill. It easy to do incorrectly. There are several factors that contribute to successful grafts, and all of them are important:

• Compatibility - Clones and members of the same species are the easiest to graft because they have identical structures [peach to peach]; two different species within the same genus can sometimes be grafted [peach to cherry]; plants in the same family, but different genera can only occasionally be grafted successfully [peach to apple]; monocots, because they lack vascular cambium, cannot be grafted.
• Alignment and surface contact - The vascular bundles of both the scion and rootstock must be in firm contact with each other, and oriented in the same direction.
• Timing - Successful grafting occurs when the scion is dormant, the rootstock is able to produce a protective callus, and temperatures are not too warm or too cold. In the Bay Area, most grafting is done January through March. 
• Site protection - Graft sites must be protected against dehydration, pests, such as vine mealybugs, and diseases until they have healed. This means using very sharp cutting tools that have been sanitized frequently throughout the process. Tapes and waxes are commonly used to seal the grafting site. Structural supports may also be needed.

Tools used in grafting

Having your tools ready ahead of time will increase your odds of successful grafting. The last thing you want is for plant tissue to dry out before you are done. [Many gardeners hold scions in their mouth as they work, to keep the plant tissue moist.] You will need sharp, sanitized pruning clippers, stretchable, biodegradable grafting tape, a sharp grafting knife, and sanitizer. You may also want to have some tree sealant or grafting wax handy.​

Grafting and budding methods

Budding has become the method of choice for vegetative propagation. Partly because it can be done before or during the growing season. There are two basic types of budding:

T-budding, or shield budding, removes a dormant side bud from one tree and inserts it onto a 1/4” to 1”, actively growing, one-year old rootstock plant. T-shaped cuts are made in the rootstock, 3 to 5” above the soil line, and buds are inserted into the stem and wrapped with grafting tape.

​Chip budding is similar to T-budding, except that a chip of wood is removed from the rootstock and the bud is inserted into the space.

T-budding (Chrizz) CC BY-SA 3.0

There are several different types of grafts:

• Approach grafting joins two rooted plants together to provide more support for the developing new growth. It is also used in pleaching and can be done any time of year.
• Awl grafts are hard to do well as it means poking an awl under the bark of rootstock, but not beyond the cambium layer, and inserting the scion.
• ​Bark grafting (also known as rind or inlay grafting) uses pencil-sized scions (1-1/8” diameter), cut into wedge shapes, but only on one side, which are inserted into 2” cuts in the rootstock bark, with the cut edge wedged firmly under the bark, against the exposed cambium layer. In commercial practice, the scion and its protective bark are nailed to the tree. You can use grafting tape, if you prefer.
• ​Bridge grafting is used in an attempt to save a tree that has been girdled by planting identical species around the injured tree and grafting them above the injury to create a nutrient bridge over the girdled portion of trunk. A similar method, called inarch grafting, uses an existing branch of the injured tree that emerges below to injury to reconnect the supply chain of nutrients.
• ​Cleft grafting is one of the simplest and most popular forms of grafting. It takes advantages of naturally occurring clefts by inserting 3/8” scions into larger (3/4” to 2-3/4”) notches cut into Y or V-shaped joints while both plants are still dormant.
• ​Four-flap grafts (or banana grafts) are a complex method of grafting, commonly used on pecans, in which bark from the rootstock is peeled back, like a banana, and applied to the scion.
• ​Saddle grafting, or stem grafting, uses young stems and inserts them into cuts made in rootstock stems with similar diameters. This is one of the easiest methods of grafting. Just be sure to wrap the graft with grafting tape when you are done.
• ​Stub grafting is similar to cleft grafting, but the incision is made in a branch, instead of a cleft, and the scion is placed at an angle no more than 35° to the parent tree. After the scion takes, the portion of the branch above the graft is removed.
• ​Whip grafting, also known as splice grafting, is best suited to herbaceous plants that knit together easily, and that grow stems no more than 1/2” in diameter. Identically angled cuts are made to both the scion and rootstock, which are then lined up and taped together. A variation on the whip graft, the whip and tongue graft, uses an extra notch in both cuts, providing extra contact and support. [Do not try this one your first time grafting - it takes a lot of skill to make sure all those angles line up.]

In rare cases, graft hybrids, called chimera, will occur. This happens when rootstock tissues grow into the scion wood. This can lead to trees that produce flowers of both plants, plus strange combination flowers. Chimera are almost impossible to reproduce.

Whichever method you decide to use, grafting or budding, be sure to seal the area completely, either with grafting tape, tree seal, or grafting wax. This will protect against desiccation, pests, and disease, while providing some structural support, as well.

Problems associated with grafting

Grafting can provide you with added control over plants, making them more suitable to your garden theme. It can also make your foodscape healthier and more productive. But grafting requires skill and is labor intensive. Also, it is important that the proper rootstock is selected for your scions. Incompatibility may not kill the tree until several years of watering, fertilizing, and pruning have passed. Check with your local County Extension Office or rare fruit growers club for more information on compatibility before you get started.

When you first try your hand at grafting, don’t be surprised or discouraged when the alignment and pressure are insufficient, or the graft union dries out before the scion “takes” to the rootstock. This happens to beginners all the time. 

If your graft works, make sure you plant your new tree at the proper planting depth. This is critical to its health and longevity. Placing graft unions below soil level invites fungal diseases, such as crown rot.

The interaction between rootstock and scion wood can be pretty amazing when it comes to plant hormones. Check out my post on photoperiodism.

Assuming you have already collected, labeled, and kept scions cool and moist, you are now ready to begin. [If not, read my post on scions first.]

Did you know that you can graft a tomato plant onto a potato plant and get food from both?

Now you know.