Why are fruit trees grafted?

16, 2014

Grafting fruit treesGrafting is a technique that has been practiced for thousands of years by many civilisations, including the Chinese. It is still one of the main tree propagation techniques used. Grafting occurs naturally in forests when two branches of two different trees touch each other, merge, and continue to grow. When people buy fruit trees, mostly apple and pear trees, they are almost always grafted. But, what is grafting?

Grafting is used by nurseries for many reasons. The first and most important reason is to produce a plant variety identical to the original source. For example, let’s say we discover an apple tree with exceptional quality. We will call it 'Spartan' for ease of explanation. The Spartan apple is so good that we want to create and orchard full of this apple. If we keep the seeds of the apple to plant them, the trees that will grow from these seeds will not be Spartan. The trees will have some characteristics similar to Spartan, but they will not be identical. Similarly, every human being has characteristics in common with their parents, but is not a carbon copy of them. Every seed will produce a different tree, just as a human being is different from each other. Even the seeds from a single apple will produce different trees. This is why grafting is so important; it allows us to reproduce an exact replica.

What is a graft?

Similar to a human body part transplant, we take an organ (in this case a bud from the original Spartan tree) and insert it into the body (tree trunk) of a receiver (another apple tree that we call rootstock). Nature does the rest. The body (tree trunk) and the organ (the bud) are unified. The sap then travels into the Spartan bud and revives it, and the bud will grow to become the new trunk of the tree. When we look attentively, we can see the scar of the graft (the area where the bud has been grafted). Even after a number of years we can still see the graft scar or an evident change in bark colour.

The grafted tree is in reality an association between two trees. Below the graft union there is an apple tree that we call rootstock. The rootstock provides the roots of the tree. Over the graft union we have the grafted bud, which in this case is Spartan. The Spartan does not grow by way of its own original roots, but on the roots of the rootstock. All rootstocks we use are seedlings. This method allows the tree to develop a strong tap root. However, rootstocks most widely produced in the market are not seedlings, but rooting, which do not have a strong tap root. Our use of seedlings is what makes our trees suitable for use in clay soils.

Once a tree is grafted, its fruit is identical to the original tree. We can then use the branches of our newly grafted tree to graft even more trees.

 

Grafting  a fruit tree

Knife grafting – we are preparing a rootstock in order to graft a bud. We then use an elastic band to hold the stem of the rootstock and the bud together.

What is the use of the rootstock?

The rootstock is the seedling on which we graft the bud from the desired variety. The rootstock will exert influence on the trees through many avenues. First it will determine the size of the tree. Depending on the rootstock, the final product will be dwarf, semi-dwarf or full-size. The rootstock will influence its size and lifespan. A tree on a full size rootstock will easily live for 100 years, but a tree on a dwarf tree has a lifespan of only 30 years, sometimes less. Whilst widely used in commercial orchards, dwarf trees are not appropriate if you wish for your future generations to taste the fruit of the tree you are planting. Dwarf trees are widely used in commercial orchards, as the trend for varieties changes every 30 years – the lifespan of the dwarf tree. For example, for the past 30 years the McIntosh apple has been popular, but Honey Crisp is now gaining in popularity.

When choosing a fruit tree it is pivotal to select one that is suitable to the area. If you live in Alberta in zone 3, you must choose a variety that is cold-hardy and appropriate to zone 1, 2 or 3. Most people take care with this; however what they forget is to verify the hardiness of their rootstock. For example, take a Norkent apple tree hardy to zone 2 that was grafted on rootstock with hardiness appropriate to zone 5. Good luck! Your tree will not survive the first winter, because a tree without roots is no longer a tree! Most of the rootstock on the market, even in Canada, which has been developed in England and produced in Holland, is not appropriate for all Canadian conditions.

Do I need to take care of the graft?

If your rootstock is hardy the graft does not require any particular care. However, it is good to locate the graft union. Any branch that grows under the graft union should be removed as its fruit will not be the variety you expect. If your tree is damaged, check whether the damage is below or over the graft union. If the tree is broken over the graft, you have not necessarily lost your grafted variety. If your tree is broken under the graft, you have lost the grafted variety, but the rootstock will continue to grow and bear fruit. However, the quality of the fruit may not be as pleasing as that from the grafted variety.

A graft union at different stages

In the photographs above, we can see a newly grafted tree with elastic in place to hold the rootstock and bud together. Notice the difference in colour between the barks. The second photograph shows how the grafted bud awakens after a few weeks. We then see the graft union of a 1 year old, 3 year old and 5 year old graft. The older the graft is, the less obvious it is, but we can still see the change in colour of the bark above and below the graft union.

Does grafting influence the tree in any other way?

Yes. Grafting influences the time it takes for a tree to produce fruit. An apple tree that is not grafted will not produce fruit for about 10 years, whereas a grafted apple tree will begin to bear fruit after 4 years. This phenomenon occurs because the bud used in grafting is already mature.

Grafting is also used to control some diseases. For example, greenhouse tomatoes are often grafted using a similar technique to that used on fruit trees. Some diseases that affect tomatoes are found in the soil, so using rootstock that has resistance to this disease will allow the grafted variety to have immunity from the disease. This is because only the resistant rootstock will touch the disease-containing soil.

Grafting is also commonly used in Europe to produce grape vines. This practice began in the 19th century due to a root disease called phylloxera. European grape vines were badly affected by phylloxera, and it was found that the only way to protect the vines was to graft them on to American vines. Many American grape vines are naturally resistant to this disease.

The Hardy Fruit Tree Nursery and grafting

At Hardy Fruit Tree Nursery, we graft all of our apple, pear and plum trees. All of our rootstocks are full-sized trees which, unlike dwarf trees growing commercially, give a longer life and more vigorous growth. Our rootstocks are hardy for zone 2, allowing them to survive cold harsh winters. Some apple trees on the market are propagated as rooting. However we prefer seedlings which allow a tap root to develop, thus providing trees with more stability.