Bark grafts uncovered

    When I was out grafting trees last weekend, I also took the time to unwrap several bark grafts that I made last year. Its always interesting to see what's under the foil and how well the graft has calloused over (photo at right). A graft that has made strong top growth usually means you'll find a well knitted graft union.

      Here's a photo (at left) of a graft that didn't callous as well as the one pictured above. Even though the graft was successful,  the stock tree didn't provide enough energy to force the rapid expansion of the scion. Without fast scion growth,  the quick covering over of the wound made by grafting is impossible.  Grafts made late in the grafting season tend to to grow slower (like the one in the photo) than grafts made as soon as the bark slips in the spring.

  

     In contrast, a fast growing scion can nearly heal over a grafting wound in a single growing season (photo at right). Notice that callous tissue has formed all around the cut surface of the stock, leaving only a small circle of stump yet to cover. Here's a tree that will be producing nuts two years after grafting.

     When uncovering a graft, I often find a colony of ants tending their eggs in the warm, moist environment provided by that aluminum foil and plastic bag (photo at left). Ants usually don't interfere with callous formation, but they can enhance the decay of the stock stump. Once exposed to the sun, the ants will disappear.
   A technique for limiting ant nesting is to remove all wraps in August (in the year of grafting) and painting the graft union white. White latex house paint will protect the graft union from sun scald while eliminating the protected environment ants need for nesting.
     As always, make sure to support all bark grafts with a sturdy stake until the grafting wound is completely covered by new wood growth.

Using the arrowhead graft

    This Spring has been great for grafting pecan trees. Trees broke bud with a bang about 3 weeks earlier that normal and ever since we've had moderate temperatures ideal for grafting success. Last weekend, I was out in the field grafting more Kanza trees and came across a tree too large for a 3-flap graft but too small for a bark graft (photo at right). Sure, I could have placed a 3-flap graft up near the top of the tree. But, to make sure my graft will get a good push from the root system, I like to remove at least one-half of the top when choosing a spot to graft.

        In the photo at left, I'm holding my scion next to the location on the tree I intend to apply the graft.  The trunk of the tree in this photo is about 3/4 inch in diameter, while my scion is about 3/8 inch in thickness. In this situation, I use a grafting technique that is known as the arrowhead graft. This method is actually a modification of the bark graft developed specifically for small diameter trees (0.75 to 1.5 inches in dia.) with smooth, thin bark.

    To prepare the stock for grafting, I use the tip of my knife to slice through the bark in a straight line down the stem about 3 inches long.

    Next, I turn my attention to carving the scion. Using my grafting knife, I make a deep cut into the scion leaving a thin, tapered tongue with a curved shoulder at the top (photo at left).

    I turn the scion to look at the bark side of the tongue. Starting just below the shoulder, I remove some of the bark along the edge to expose cambium. I make sure I see white wood all along the edge to ensure I have cambium exposed the entire length of the scion.

    Next, I move to the other edge of the scion and make another cut exactly like the first. In the end, it should look like you a sharpening an arrowhead (photo at left). I make sure to leave a strip of bark down the center of the scion that comes to a point at the end.

    The last cut I make is simply to put a chisel point on the scion to make it easier to slip the scion under the bark if the stock (photo at right).

    Now that the scion is completely carved, I insert it under the bark of the stock right down the center of the incision made previously (photo at left). As I push the scion down, I make sure that it lifts the bark on both sides of the incision.

     I drive the scion all the way down until the scion's shoulder touches the top of the stock. Note that you can see the strip of bark I left on the back of the scion and that both edge cuts are now covered by bark flaps.

     For any grafting technique to work, the cambium of the scion must be tightly fit against the cambium of the stock. But, look how the scion has pulled the bark away from the stock and left gaping holes (photo at left).  I'll use my staple gun to snug things up and conform the bark of the stock tight against the scion.

   Starting at the bottom of the graft, I use a light duty staple gun (Arrow model JT-21) to shoot 5/16 inch staples into the bark of the stock just outside the edge of the scion. The staples bend the bark inwards, pressing it up against the cut surface (and cambium) of the scion. I alternate sides as I place the staples so the scion won't slide to one side or the other. Six staples usually does the trick.

   Now look at the graft from above (photo at left). After stapling, the bark of the stock sits firmly against the scion.

    With staples placed only through the stock's bark, this graft is weakly attached at this point. So, to secure the scion in place, I wrap the scion with grafting tape (photo at right). Note that I covered the entire graft union with tape, including the upper edge of the stock.

    This graft receives the same protective covers I used in the 3-flap and bark graft. I wrap aluminum foil over the graft union then place a sandwich bag over  the graft as well.
    I grafted this tree in an area that deer frequent. So, instead of my normally applied bird perch, I placed a 2 foot diameter deer cage over the graft (photo at left). The cage is constructed of 2" x 4" welded wire that is 5 feet tall and secured with a steel fence post. These cages not only keep curious deer away from my graft, the birds will perch on the cage and not the graft.

    Here's a photo of a successful arrowhead graft (at right). You can see that the scion has grown rapidly,  popping out from under the bark flaps of the stock. The staples are still there but the tree has grown over them.

Hickory shoot curculio

    Have you seen flagging leaves and shoot dieback on this year's new pecan shoots (photo at right). If you have, you are most likely seeing an attack of the hickory shoot curculio.
     After resting dormant all winter in ground cover thatch, adult curculios (Conotrachelus aratus) become active in the spring as pecan shoot growth commences. When the first leaf on new pecan shoots is about one half expanded, female curculios start searching of locations to lay eggs.

     Hickory shoot curculio females use their mouth parts to carve a moon shaped depression into the base of a leaf petiole (photo at left). In that depression she will lay an egg. The larvae that emerges from the egg will first tunnel into the leaf petiole causing the leaf to wither and die. It will continue to feed by tunneling into the center of the new shoot making its way towards the terminal, often killing the entire shoot.

     If you see signs of hickory shoot curculio activity, cut open an infested shoot and you should find a white, legless larvae with a red head (photo at right).
    Once the larvae hatches and begins to burrow into pecan shoot tissue, it is impossible to control this pest with an insecticide. In other words, once you see the damage it too late to take action this year. If your pecan grove has a history of curculio attack plan on controlling adults early next spring.

     You will probably never see an adult hickory shoot curculio in the field (photo at left) but watch the development of springtime pecan shoot growth to time an insecticide treatment. Spray when the first leaf on the new growth starts to unfurls all its leaflets.
     Hickory shoot curculio is one of those sporadic pecan pests that seem to appear out of nowhere and then a couple of years later disappear altogether. I've only seen a few curculio infestations that warrant chemical control. It seems that naturally occurring biological control agents usually keep this pest in check.  

Flowering type and catkin shape.

    In a previous post,  I talked about being able to identify the flowering type of a pecan cultivar at budbreak. Today, all the catkins are out and you can still easily distinguish protandrous from protogynous cultivars even before pollen shed.
    In the photo at right, you can see a branch cut from an Osage tree (bottom) in comparison with a Kanza branch (top). Osage is a protandrous cultivar and has short fat catkins while Kanza is protogynous as has long narrow catkins. Even though cultivars can vary in the number and size of catkins produced, the obvious difference in catkin shape between protandrous and protogynous cultivars seems to be universal.
  Remember, pecan trees have one of two flowering habits. Those trees that shed pollen before their pistillate flowers become receptive to pollen are said to have protandrous flowering (also known as type 1 flowering). Pecan trees that shed their pollen late in the pollination season and produce receptive pistillate flowers before they shed pollen are called protogynous ( also known as type 2 flowering). Having catkins and pistillate flowers mature at different times on a tree is nature's way to ensure cross pollination.

Yellow pecan aphid

    I was look looking at our crop of pistillate flowers when I spotted a few drops of honeydew on the leaves. Honeydew is the tell-tale sign that aphids are feeding on the foliage. In this case, I found yellow pecan aphids (photo above). Aphids feed on the underside of the leaves usually right along the midrib, just like the wingless nymph above. The winged adult didn't like the bright sunshine and was already moving off to find a feeding spot in the shade.

   Early season aphids are not normally a problem in our native pecan groves because we have plenty of beneficial insects to keep them in check. In looking through the foliage, I found a cluster of lady beetle eggs laid on the underside of a pecan leaflet (photo at right). Lady beetle adults respond to the presence of honeydew and lay their eggs in a location where they know that there will be plenty of aphids around to provide food for their larvae.

Twig girdler damage on pecan

    Between rain showers, I spent most of this weekend grafting pecan trees on my farm. While wading through the water and wet grass, I came across a tree that I grafted last year. At first, the tree looked like it had suffered terminal dieback possibly from last year's drought. But when I took a closer look, I noticed a profusion of new shoots all growing out below a nicely cut grove in the stem (photo at right).  This groove was cut late last summer by an insect called a twig girdler.

   The twig girdler is a long horn beetle that is 5/8 to 3/4 inch long (photo at left). The most distinguishing feature of this insect are the long antennae that are as long or longer than the entire insect. Female twig girdlers lay eggs in August through September in small pecan branches usually just below the point where a leaf is attached to the stem.
    After laying eggs, the female cuts a grove in the stem. The girdling of the stem causes all the carbohydrates manufactured by the leaves above the girdle to become concentrated in the stem rather that being set down to the roots. The eggs she lays in late summer don't hatch until the following spring. But when they do hatch, the larvae feast on the carbohydrate enriched twig.
     Twig girdlers are most often a problem in non-bearing pecan groves. Once pecan trees start to bear, insecticides applied in August and September to control pecan weevil also control twig girdler. In young orchards, picking up girdled branches every spring and burning them will help reduce the population of twig girdlers in the future.

3-flap grafting pecan trees

    The grafting season in SE Kansas is in full swing. These mild, cloudy days we've had lately have been perfect for field grafting. I had a pecan seedling pop up in the yard (photo at right) and its now growing fast enough to provide a good rootstock for a 3 flap graft.
    Rootstock growth rate, prior to grafting, is one of the most overlooked factors in determining grafting success. As a general rule,  trees should be grafted only after they have grown at least 2 feet of new stem the previous growing season. Weed control and nitrogen fertilizer are two key ingredients for promoting rapid seedling growth.

    The first step in making a 3 flap graft is to select a piece of scionwood that is roughly the same diameter at the seedling tree (photo at left). To ensure that there will be plenty of tree energy directed to pushing the graft, I like to place the graft union no higher than half way up the stock tree. This usually means that I use large diameter scionwood (1/2 to 3/4 inch) for 3 flap grafting.

   After choosing my scion and the location for graft placement, I cut the stock tree with a pair of pruning shears usually just below a bud. By cutting just below a bud, I ensure that I will have nice clear bark on the stem to create my flaps.

    Now I turn my attention to creating the bark flaps on the stock.  If I find a bud on the stock within the top 3 inches of the top of the stock, I rub off the bud and make my first slice through the bark right down the middle of the bud (photo at left). This will make peeling back the bark flaps much easier than trying the pull a flap down directly over a bud.
   I made this cut about 3 inches long using the tip of my knife to slice the bark straight down the center of the stem. It doesn't take a lot of pressure to make this cut since the bark of a young tree is relatively thin.

   The hardest part for some folks in making a 3 flap graft is making 3 equal sized flaps. After making the first slice through the bark, I mark the location of that cut by pressing my knife into the top of the stock. Then I make 2 more marks with the knife to divide the stem into thirds (photo at right). Following the marks on top of the stock, I make 2 more 3-inch-long slices down the center of the stem, just like the first slice.

    Once all three cuts have been made into the stock, I tie a 30-inch-long piece of plastic grafting tape onto the stem just below the bottom of the cut (photo at left). Getting the tape in place before peeling the flaps back serves two purposes. The tape will prevent you from pulling back the flap too far and you won't be fumbling around looking for tape when the scion is ready to be inserted into the stock.

   At this point I turn my attention to preparing the scion.  I use my clippers to cut about 1/2 inch off the bottom of the scion (photo at right). This cut ensures that I have fresh wood and live cambium in the area the graft is made.

    Next, I twirl the scion around to see if there are any buds present on the lower 3 inches of the scion. If I find a bud, my first cut is to remove that bud (photo at left). The cut I make is fairly shallow but deep enough to reveal the white wood of the scion. The cut is about 2 3/4 inches long.
    The most common mistake folks make in attempting the 3 flap graft is not cutting deep enough into the scion. You must see white wood all way down the cut surface to ensure maximum cambial exposure.

    Before making two more cuts on the scion, I hold the scion so I can see the bottom of the wood and mentally divide the scion in thirds. Breaking the boy scout rule, I cut towards myself  making each of two more shallow cuts just like the first.  After completing the 3 cuts on the scion, I now have triangular shaped bud stick (photo at right). In the photo, note that there is a solid strip of bark left between each shallow cut. With the bark still in place and white wood showing on each cut surface, I am certain to have lots of cambium exposed (For any graft to work, the cambium of the stock must touch the cambium of the scion).

  Now that I've got the scion fully prepared, I hold the wood in the pit of my arm to feel up my hands for preparing the flaps (this will keep the cut surfaces on the scion from drying out). Using my thumb nail, I pry the bark of the stock off its wood (photo above). In pulling back the flap, I only touch the upper most part of the flap. Note that I pull back the bark flap all the way down until it hits the pre-placed grafting tape.

   I repeat the bark pealing process for the other 2 flaps, then use a pruning shear to clip the wood out of the center of the stock (photo at right). Make sure not to handle the inside of the flaps. Your oily skin can damage the cambium on the inside of the bark flap.

      Once the wood is removed, the three bark flaps will spring back upwards. I then grab the tape attached to the stock and start wrapping it around the stem.

    As I make one or two wraps around the stem, just above the cut surface of the removed wood, the flaps fill form into a tube. I'm now ready to insert the scion.

     I insert the scion into the bark tube while still holding the grafting tape tight. In pressing the scion into place I make sure each cut surface of the scion will be covered completely by a bark flap. Sometimes the scion and bark flaps don't seem to line up just right. When this happens to me, I simply pull the scion out and twirl the scion 1/3 of a turn to see if I get a better fit. Once I'm satisfied with the alignment of scion and stock, I continue to wrap the graft with grafting tape.

   In wrapping the graft, I pull the grafting tape tight to compress the bark flaps up flat against the cut surfaces of the scion (photo at right). If one of the flaps gets pulled out of place, I slide the flap back into the right position before continuing to tape up the graft. I wrap all the way up the stem to completely cover the top of the flaps. I then wrap down the graft union to the bottom where I tie the tape in place. 

  When I'm done wrapping the graft, I can see the triangular shape of the scion (photo at left). I applied the tape so tightly that it has formed the bark to the shape of the scion ensuring maximum cambial contact.. 

    Since I'm grafting out in the field, I like to add some protective coverings to aid in graft callusing (photo at right).
First, I wrap the graft union with a piece of aluminum foil to prevent sun scald. Next, I cut the corner out of a sandwich bag and place over the graft. I use grafting tape to tie the bag above and below the graft union. The plastic bag holds in moisture and acts like a mini green house to warm the graft union.

   To complete the grafting process, I dab on a bit of white glue on top of the scion. This seals the cut surface of the bud stick to help prevent excessive moisture loss. 

    Just like the bark graft, I always attach a bird perch to the tree to protect the new graft. Even though the 3 flap graft is mechanically stronger than other grafting methods, I have seen plenty of these grafts broken over by perching birds. I attach a pecan branch or bamboo cane to the rootstock to provide a alternative perching site for common song birds (photo at right).
    Three weeks from now, I should be able to see the buds on the scion start to push.

Top working with a bark graft

    We have a tree in one of our research trials that was grafted to the wrong cultivar. What was supposed to be a Posey turned out to be a Pawnee. Back in 2007, this misplaced Pawnee tree was severely broken down by an ice storm leaving us with not much more than the tree's trunk. We trimmed off all broken limbs and let the tree sprout from the remaining trunk. What sprouted out was above the original graft so the tree remained a Pawnee. In the photo at right you can see the misplaced Pawnee full of leaves and catkins while the Posey tree right behind it is just starting to leaf out. This year, I decided to correct our grafting error and top-work this ice-storm-damaged tree over to Posey.

     The photo at left shows one of the main sprouts that grew from the ice-damaged tree trunk. Note that there is a good chunk of dead wood on top of the original tree trunk above the point of new sprout attachment.  The roll of callus tissue on the trunk marks the point of live tissue growth.

    The first step in top-working this tree was to trim out the dead wood and then cut the limb sprout off in preparation for placing a bark graft (photo at right). There are two things to note in this photo. First, I trimmed the trunk at about a 45 degree angle, removing as much of the decaying wood as possible. And second, I left other limbs on the tree un-pruned to provide photosynthetic support for the tree's root system.
    At this point, the tree is ready for bark grafting.

      Preparing the stock to receive a scion is as simple as drawing a straight line. Using the tip of my knife, I sliced the bark straight down the stem for about 3 inches (photo at left). I made sure to cut all the way through the bark so the bark can be lifted away from the wood when the scion is inserted.
     Bark grafting requires that the bark "slips" easily from the wood. The bark slipping period starts in early spring when new leaves are expanding. and continues through the pollination season. However, I've found that grafts applied early in the grafting season out-perform those made later in the spring.

    Now its was time to work on the dormant scion.  The first cut I made on the scion removed about 2/3 the diameter of the stick. In the photo at right, I present two views of the same scion: one from the side and one looking straight down on the cut surface. I made the cut parallel to scion except for the curved shoulder near the top of the cut. I call this first cut into the scion "the deep cut".

    Turning the scion over, I make a shallow cut pealing off enough bark to enter into the wood.  Note that I started the shallow cut below the point on the scion where I carved the shoulder of the deep cut. The photo at left is a composite of three views of the same scion. This shallow cut is made at an angle to the deep cut. In making the shallow cut, I start peeling of the bark in such a way as to leave as thin a strip of bark as possible along one edge of the scion (upper view). Turning the scion slightly, you can see a thick strip of bark on the other side of the scion (middle view).  At this point my scion wood has a wedge shape when looking at it from the bottom (bottom view).

    With the next cut, I expose cambium at a right angle from the deep cut (photo at right). I turned the scion so I was looking at the thick bark strip side of the stick. I then placed my knife at 90 degrees from the deep cut and shaved off the edge of the scion. This cut needs to be deep enough so you can see white wood all the way down the edge of the scion but not so deep as to remove all the bark from that side of the scion. After making this cut, my wood has a triangular shape when looking at the bottom of the scion (lower portion of photo above).

    For the final cut, I created a chisel point at the end of the scion to allow easy insertion under the bark of the stock. This cut (red arrow) is made on the opposite side from the deep cut. With a fully carved scion its time to join scion and stock.

     To insert the scion, I lifted the bark on the left side of the slice I had made into the stock.  The deep cut is facing the wood of the stock, while the shallow cut is covered by the stock's bark. Note that the 90 degree cut fits smoothly against the slice I had made in the stock's bark.

    I gently tap the scion down under the bark until the shoulder of the deep cut rests just above the wood of the stock. Note that the shallow cut is completely covered by bark.

    I use a light duty staple gun with 5/16 inch staples to press and hold the bark of the stock tightly against the scion.  In the photo at right, you can see that I used 4 staples to hold the bark flap against the scion. I put these staples in at a 45 degree angle so that both sides of the staple are fully driven into the tree.

     I added three more staples along the left side of the scion to compress the stock's bark tightly against the scion (photo at left). It is very important to eliminate all air spaces between the scion and stock when grafting pecan trees. So feel free to use as many staples as it takes to bend the bark to fit around the scion.

    I then covered the graft union with aluminum foil to block the sun from inhibiting callus formation (photo at right). The foil covers the entire top of the stock and fits tightly around the scion.

    To conserve moisture around the graft union, I placed a plastic bag over the aluminum foil (photo at left). I tore a corner out of the bag and placed the bag over the scion.  As I slipped the bag over the scion, I made sure that I didn't injure the buds on the bud stick.

     I then used green grafting tape to seal the plastic bag over the graft union (photo at right). I first tied the bag around the scion, then at the bottom of the bag around the stock.

    My next step in the grafting process was to seal the top of the scion from moisture loss with a dab of white glue.  At this point, the graft is complete. However, there is one more step crucial for protecting the graft from bird damage.

     When grafting in the field, I always attach a "bird perch" on every tree (photo at right). A single scion sticking out in the open becomes a very attractive location for male meadow larks or red-winged black birds to perch and sing their songs of romance. As small as these birds seem, they can land with enough force to break out the scion. To prevent bird damage, I attach a branch stick (cut from the removed top of the tree) to the tree using black electrical tape. The end of this "bird perch" is much higher than the scion and provides a more desirable location for birds to sing their mating calls.
     In three weeks, I'll be able to tell if my grafting effort on this tree was successful. In the mean time, see related posts on bark graft maintenance and the anatomy of a successful bark graft.