Agricultural Literacy Curriculum Matrix
Walnuts: The Importance of Grafting
Students explore the science and economic importance of grafting walnut trees. Grades 3-5
For each student
For each partnership of 3-4 students:
- two colors of play-doh plastic knife
- 6 oz. plastic cup
- sand or soil
- white paint
- paint brush
- rubber band
fluctuate: to shift back and forth unpredictably
fuse: to become blended or joined
graft: to insert a twig or bud from one plant into another plant so that they are joined and grow together
indigenous: native, or original
propagate: to grow, generate, or produce
rootstock: a root or part of a root to which an aboveground plant part is grafted
Did You Know?
- Walnuts are the oldest known food from a tree dating back to 10,000 BC.1
- There are more than 30 varieties of walnuts.1
- Walnuts have Omega-3 fatty acids, magnesium, and vitamin E. They are a healthy treat.2
- A healthy walnut tree can produce crops for approximately 100 years.
Background Agricultural Connections
California walnut farmers have scientifically learned how to produce nuts with thinner skins, larger nutmeat, and disease and pest resistant trees through a process called grafting. Grafting is a common type of plant propagation where healthy and disease resistant rootstock is fused onto a scion. A scion is a shoot from another plant that contains the desired genes to be duplicated in future production. The scion is often selected due to its leaves, flowers, and in the case of the walnut, its fruit.
Black walnuts are indigenous to California, however their thick shell and small nut size are not ideal for human consumption. However, the English walnut that is native to Ancient Persia, now known as Iran, produces walnuts with thinner shells and larger nuts making them a better choice for consumers. English walnuts are more prone to diseases and pests because they are not native to California. This is why California walnut farmers graft black walnut rootstock onto English walnut scions.
There are a number of types of grafting. The most common type is called whip or tongue grafting. Grafting is done in the late spring after the rootstock has produced leaves and is less likely to fail. The most difficult step in grafting is making sure that sap doesn’t flow from the cut rootstock. To avoid this “bleeding,” farmers need to be mindful of weather conditions. In periods of heavy rain or strong temperature fluctuations, bleeding is more likely to occur.
Black walnut seeds are planted during October through December. The seedlings emerge in early spring. The seedlings continue growing for one full year until they are strong and large enough to be grafted. Scions, from English walnut trees, are selected for their well-developed buds are collected during the dormant winter months from December to February. They are placed in moist wood shavings or a plastic bag in a refrigerator until the rootstock is ready.
When grafting, it is important to choose a piece of scion wood that closely matches the diameter of the rootstock. The scion and rootstock are both diagonally cut during the whip grafting process. A small slit is cut into the center of the cut pieces. The two cut pieces are matched together and the small slits help lock the rootstock and scion together. The union is sealed with grafting or masking tape. A rubber compound called yellow cap is also used. The young rootstock is painted white to protect it from the sunlight. The top of the unexposed scion is sealed with a grafting wax to keep the top of the walnut tree from drying out.
- Introduce the walnut to the students. Show them either a picture or an actual walnut both inside and outside of the shell.
- Ask, "Where are these produced?" (they grow on a tree.)
- Show the Harvesting, Hulling and Processing California Walnuts video.
Explore and Explain
- Distribute activity sheet, Let’s Graft! to students and read together the informational paragraphs before beginning the activity.
- Divide students into groups of 2-3 students per group and distribute materials for the activity. Make sure each student will have enough play-doh for 5” long ropes.
- Explain to students that they will be simulating how grafting is done.
- Ask students to roll each piece of modeling clay or play-doh separately into ropes that are approximately one inch in diameter throughout.
- As students are working, ask them to compare their rolled piece of clay with their lab partner’s. Ask them to continue working until they feel the diameter of each rolled piece of clay is the same.
- Assign each colored piece of clay as the scion or rootstock to help students differentiate between the two pieces that are grafted together.
- Using the plastic knife, have students cut a diagonal cut that measures approximately one inch from one end of one of the ropes and leave the other end untouched. This piece of clay is the rootstock.
- Ask the students to set the piece of clay down once the appropriate cut has been made.
- Have students pick up the other piece of clay and make a similar one-inch diagonal cut on one of the ends. On the other side of the rope, ask students to cut the clay so it is a straight cut. This piece of clay is the scion.
- Using the plastic knife, ask students to cut a small notch on both pieces of clay in the middle of the diagonal cut.
- Place the ropes of clay end to end with the diagonal cuts and notches connecting together as shown in the diagram.
- Cut one end of a rubber band. Holding both ends of the rubber band, carefully place the middle of the rubber band in the middle of the graft. Tell students to think of the rubber band like a bandage.
- Carefully wrap the rubber band around the graft and tie the ends together in a knot.
- Tell students that farmers paint the rootstock white to protect it from the sun. Ask students to paint their rootstock white.
- Share with students that farmers dip the top of the scion in wax to keep it from getting dried out. Using the wax from a melted candle, have students coat the top part of the scion with wax.
- Place sand or soil into a plastic cup. Gently “plant” the grafted walnut seedling making sure to bury only the bottom of the rootstock.
Take the students on a field trip to a walnut orchard nursery. Ask a horticulturist to share with students how walnut seedlings are grafted. Have students make observations of the walnut tree’s truck. Can they see where the grafting took place? What do they notice? Compare the size of the young trees with the older trees.
Have a walnut farmer visit the classroom and share his work on the farm.
As a class, plant a walnut seed and see how long it takes to germinate. Offer students a prompt about the growth of the seed and ask them to write a creative story about what will happen to the seed when it sprouts.
Place a stock of freshly cut celery in colored water. Have students observe the changes in color of the celery the next day. Explain to students that plants need water to survive and they draw water up from their roots through their capillaries. The capillaries are hollow and act a lot like a straw. Share with students this is why when grafting it is important to match the diameter of the rootstock with the scion.
Obtain different species of walnuts (California Black walnut tree, Juglans californica and English walnut, Juglans regia). Have students compare and contrast the physical difference of each species.
Demonstrate the importance of matching diameters of the scion and rootstock when grafting using a celery stalk. Trim the bottom of a celery stalk keeping the leaves intact and place it is a clear cup filled half full of water adding 8 drops of red food coloring until the water is a deep red color. The next day the leaves of the celery will be red. The water and nutrients are carried up through the celery stalk by the xylem. The xylem is the woody tissue in plants that is responsible for the movement of water and nutrients throughout the plant. During the grafting process, it is important to match the woody part of the walnut cuttings so the xylem and phloem (food conducting tissue) have the best possible chance of growing together. This will increase the likelihood of the walnut tree’s success.
After conducting these activities, review and summarize the following key concepts:
- Technology and science is used to improve the production of farm products. Walnuts are an example.
- Scientists and farmers study inherited traits in walnut trees in order to produce the best walnuts.
- Walnuts are grown on trees.
- California produces the majority of all walnuts produced in the United States.
A partnership project of California Walnut Board and California Foundation for Agriculture in the Classroom
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