Topworking: rapid and inexpensive orchard varietal change

Topworking, or changing the variety of an established tree by leaving it’s rootstock in place and grafting a new variety on top, can be a very quick, cost-effective and practical solution to establishing new varieties.

In order to maximise profit in a modern orchard it is necessary to ensure that the varietal structure of the orchard provides efficient use of resources with production of desirable cultivars. Failure to produce fruit that is desired by the markets and consumers will ultimately lead to orchard financial failure. In today’s modern society, where consumer products are regularly updated, it can be expected that we will see an ever increasing turnaround in the desired apple cultivars. This means that in progressive orchards each year between 10 and 20% of the orchard will be replaced with the current desired profitable cultivar. The cost of removing the old orchard trees as well as the trellis and irrigation systems followed by soil cultivation, hilling up, fumigation and replanting along with installation of new trellis and irrigation systems is an expensive activity. With modern PBR apple cultivars costing close to $15 a tree from the nursery this represents a cost of around $45,000/ha to the replanting activity alone. If many of the costs associated with old orchard and infrastructure removal and new infrastructure installation can be avoided there are huge cost savings to be made when renovating an orchard. If this can be combined with extremely rapid production of fruit, profit margins can be dramatically increased and time demands reduced.

Consider topworking

A topworked 'Gala' tree in the third season after topworking.

A topworked ‘Gala’ tree in the third season after topworking.

So how can we achieve this? The simple answer is to consider topworking your current orchard to the new desired cultivar. This is particularly applicable to existing young high density orchard plantings in a trellised situation. While the rootstock is not replaced in this system, if the current rootstock is too vigorous and not desirable it is still possible to topwork this orchard incorporating a dwarfing interstem into the process. If the orchard to be replanted is an old orchard at wide spacings or an orchard on old rootstocks suffering from root diseases or severe virus infection then consideration should be given to total removal and replacement of these orchards rather than topworking them. One of the interesting side effects of topworking an orchard is that it almost totally overcomes the problem of poor tree performance of newly planted orchards due to specific apple replant disease. The cost of treating for this is often equivalent to the value of the land so this represents a major cost saving. The 1976 report ‘Studies on regrafting fruiting apple trees’ of a trial where 9 year old trees were topworked to ‘Golden Delicious’ showed that some of the orchard blocks produced higher average yields of fruit over the subsequent four seasons than the non topworked trees. This was also found to be the case in China where the report ‘Trial on topworking for apple trees on a large scale’ (2,000) shows that topworked trees produced fruit in their second season and they were out yielding non-topworked trees in the fourth season. These findings clearly demonstrate the rapid cultivar turnaround that can occur with topworking an existing orchard.

Preparation before topworking

The first step in topworking an orchard is to identify the orchard block where there is a desire to change the cultivar. Then assess this block for its suitability to topworking. Characteristics such as rootstock vigour, presence of rootstock diseases, planting density, tree spacing, tree virus status and infrastructure condition should be taken into consideration when deciding on the suitability of a block for topworking versus total orchard replacement. Having identified that an orchard block is suitable for topworking it is then necessary to determine what cultivar it is going to be converted to and what cultivars you want to incorporate as pollinator species to develop a planting design. Having decided to topwork a block the number of trees to be topworked can be counted to determine the quantity of graftwood required. It is then necessary to organise a supply of graft wood of the desired cultivar. The desired cultivar may be a patented variety with graft wood originating from a nursery or it may be that the graft wood can be collected from your own orchard or a nearby commercial orchard. Always consider plant patents before you collect budwood from your own orchard or that of a neighbours. Be careful if collecting wood from another orchard as wood collected from off type trees will produce off type trees in your new orchard. This risk can be managed by advanced planning and observing the trees from which you are going to gather wood when they are full of fruit such that fruit can be inspected and sound trees marked to ensure that the graft wood is coming from high quality trees. In this manner it is possible to actually select a superior line of the cultivar. This tree inspection at fruiting is part of the normal quality assurance process in commercial nurseries such that wood collected by commercial nurseries should be true to type. If planning to topwork to a patented cultivar it is necessary to contact the patented owner or their agent to organise appropriate royalty payments per tree and any other aspects of propagation that they require. If propagating wood from non-patented trees then it is also important to ensure that you are not violating any non-propagation agreements that you may have entered into at the time of purchasing the original trees. When organising graft wood it is wise to plan on a minimum of two grafts per rootstock although you may only need to pay royalties for each tree grafted, (final tree royalties do vary with some cultivars so check with the nursery agent during your planning stage). With topworking it is possible to graft multiple times into the rootstock stump and this reduces the risk of unsuccessful tree conversions. As close to the time of topworking as possible, or no later than August, collect the graft wood and store in a high humidity environment in a cold room until it is required. One of the common reasons for topworking graft failure is due to the use of graft wood that has aged and dried out prior to grafting, or the graft wood is exposed to ethylene when stored with fruit. The first step in the orchard to be topworked is to remove the top of the old trees. This is achieved by cleanly cutting the trees, usually at waist level, and leaving if possible a few limbs below the cut to act as nurse limbs providing energy for the early season rootstock and graft union and growth. Remember that if the existing cultivar is to be used as the pollinator, prior to cutting the trees clearly mark the pollinator trees which can be severely pruned rather than cutting down to waist level. After removing the tops of the trees and prior to topworking activities it is advisable to remove the prunings and any weeds from the tree line from the orchard and to slash the inter row space close to the ground. This helps to provide easy pedestrian access for grafters to the orchard block and assists in the reduction of the risk of somebody tripping over and injuring themselves on the very sharp grafting tools that they may be carrying. The next activity is to inspect and repair the trellis and irrigation system within the orchard block. It is advisable to carry out this operation prior to topworking to avoid the risk of these activities damaging any newly grafted trees. At this point in time access to the trellis poles, wires and irrigation system is uninhibited making this activity easier, rapid and less expensive. The orchard block is now ready to be topworked. This is the time to collect the graft wood to be used. As soon as possible after getting the graft wood the tree should be topworked.

Undertaking topworking

At the start of each day where topworking is to be carried out it is essential to ensure that there is sufficient healthy graft wood for the activity and that all equipment is in excellent condition. Tools should be sharp and provide extremely clean cuts and grafting knives should be surgeon’s scalpel sharp. This will ensure that when cutting the bark and wood of both the graft wood and the rootstock the cells are cleanly cut with no cell bruising below the cut surface. Bruised cells below the cut surface need to repair themselves before they can continue with normal activity and growth to facilitate rapid graft union and supply of water and nutrients to the graft shoot. Hence sharp grafting knives lead to rapid wound healing and a more successful graft take. As grafting knives are excessively sharp it is almost inevitable that someone is going to cut themselves such that a good, well stocked first aid kit should be added to the grafting toolbox. When grafting and budding it is wise to remember the function of the different structures within the tree stem. Looking across the top of a cut trunk there are four structural elements that can be observed. On the very outside of the trunk there is the epidermis which may have levels of corky tissue in it. Inside this layer there is a layer of fibrous vascular tissue which makes up the phloem of the stem and this tissue transports sugars from the leaves down and into the roots to provide the roots with energy for growth and soil nutrient uptake. Under the phloem there is a very thin layer called the cambium which surrounds the bulk of the central wood structure. This central wood structure is composed of the xylem vessels which transport water from the roots up to the shoots of the tree to allow for normal tree growth. The xylem structure normally has annual growth rings clearly visible. Of all these structures, for grafting and budding, the cambium is the most important. The cambium is the layer involved in new cell division and these new cells differentiate into new xylem cells on the inside edge of the cambium and new phloem cells on the outside surface of the cambium. The cambium, due to its high rate of cell division, is also responsible for the production of callus tissue for trunk recovery from damage. For grafting and budding it is therefore critical for the cambium layer of the graft stick to be in close contact for as large a surface area as possible with the cambium layer of the rootstock. This allows for cell division of the cambium in both the rootstock and the graft wood to intermingle, eventually leading to new xylem vessels developing from the rootstock into the graft as well as new phloem vessels to develop from the graft into the rootstock. This is essential to allow for normal canopy and rootstock growth and performance. If the cambium layer of the scion and the cambium layer of the rootstock are physically separated from each other then the callus material from the graft wood and the rootstock will have insufficient time to intermingle and develop common xylem and phloem vessels leading to graft failure. Hence it is absolutely essential that whatever method you use to topwork your trees the contact of the cambium layers between the scion and the rootstock needs to be maximised. Furthermore it is essential that the scion and the rootstock are held firmly together to prevent ripping and separation of the callus material due to physical movement.

Identifying the cambium layer

The structural elements of a rootstock that has been topworked.

The structural elements of a rootstock that has been topworked.

For the scion, which is typically 1cm in diameter, for practical purposes the cambium layer is located within 1 mm of the external surface. For the rootstock, however, the cambium is located deeper in depending on the thickness of the phloem vessels which may be impacted by the rootstock cultivar, tree age and environmental conditions. An easy way of visualising the location of the cambium layer on a large rootstock stem is to imagine what happens when a trunk of an apple tree is hit by a slasher or other implement. Commonly a large chunk of bark is peeled off exposing the white wood underneath. The bark that is removed in this manner is the phloem vessels while the white surface that remains on the wood of the trunk is actually the surface of the cambium and not that of the fibrous, woody xylem which is underneath the cambium layer. When topworking an apple tree it is important to place the cambium of the scion wood, just under the bark surface, in contact with the cambium of the rootstock which remains attached to the wood of the rootstock when the bark is peeled back. This means that with graft unions where there is a large diameter difference between the rootstock and the graft wood the graft will be placed stepped back from the rootstock outer bark to allow for the different bark / phloem thickness when lining up the cambium.

Inserting the scion

While many methods of physically inserting scion wood can be successfully deployed there are two simple methods which I will now describe. The first method is a cleft graft which can be used on thinner rootstock material up to 3 cm wide depending on how hard the wood is to cut. This method involves placing your very sharp grafting knife across the cut surface of the rootstock and then pushing down to neatly split the rootstock stem for about 5cm. After this split is made a smooth wedge is cut into the base of the graft wood. A budding or grafting knife is sharpened on one side only and flat on the other side in a similar manner to a chisel. To make a flat smooth wedge on the base of the graft wood the flat surface of the knife blade is placed against the graft wood surface and then slid to peel excess wood off the base the graft wood. This is where an extremely sharp knife not only makes a clean cut on the wood but it also makes the physical cutting of the wedge extremely easy. After the wedge is cut it is then inserted into one side of the split rootstock lining up the cambium on one side of the graft stick with the cambium on one side of the rootstock. A second graft stick can then be inserted into the other side of the split of the rootstock. When completed these two graft sticks should be firmly inserted and not prone to falling out. To ensure continued contact between the cambium of the rootstock and the scion the rootstock should be tightly bound to prevent further splitting or opening of the graft split. This can be achieved easily with the use of duct tape. The final process of the graft is to waterproof the top to prevent disease infection or dehydration of the newly forming callus tissue. The usual material to use for this activity is a specially formulated bituminous plant paint which should be thickly applied to ensure a waterproof membrane for the entire following season. There are also a range of other sealing paints/materials, some of which have ‘healing benefits’ such as promoting cambium development or added fungicides, all work well, provided you seal the grafted tree. The second method of grafting, which is more suited to rootstocks with a large diameter, involves placing a slit through the bark and down to the wood in the rootstock with this slit extending down for 3 to 4cm from the top of the rootstock. After this slit is made the bark on one side can be lifted off the cambium for a small distance. The graft wood can be prepared in a similar manner described above for the cleft graft although it is also acceptable for the cut on the base of the graft wood to be on one side only. This cut surface on the base of the graft wood will expose an oval of the cambium layer and this surface can be slid down against the wood and cambium layer of the rootstock and held in place by the flap of bark that had been created during the rootstock preparation. As experience grows additional slices can be removed from the scion base to allow for neat bark flap contact on the back of the scion and neat contact with the unlifted bark on the rootstock which will ooze callus tissue as it heals allowing for further graft scion bonding and a more secure graft union. This process can be repeated on the other side of the rootstock or indeed on larger root stocks in multiple locations. As for the cleft graft, the graft should be securely held in place by the use of duct tape and again it is important to waterproof the top surface of the rootstock to prevent disease infection and dehydration of the newly forming callus tissue. The number of buds to leave on the freshly grafted scion will depend on the situation. The research paper ‘Scion shoot length for frame-working apple trees’ from 1978 compared 2, 4, 6, 8 and 10 buds and showed that early yield was related to a higher number of buds. The downside of large scions was that wind damage was more severe so larger scions will need support in the following season. If the rootstock is of higher vigour than desired, it is possible to topwork the tree with a dwarfing rootstock such as M9. The resultant M9 shoot can then be topworked the following season to form an interstem although for those of you who are prepared to take the risk the scion can be grafted, by normal whip and tongue procedures, onto the M9 rootstock prior to its topworking into the existing rootstock in the one season. As outlined above this will potentially lead to increased wind damage and as such it is a high risk approach however it can provide a rapid orchard turnaround resulting in earlier financial returns.

After topworking

A one year old topworked 'Gala' apple tree and the fruit on occasional trees in the season after topworking.

A one year old topworked ‘Gala’ apple tree and the fruit on occasional trees in the season after topworking.

Topworked trees can grow extremely rapidly leading to a large canopy on a narrow main stem attached to a chunky rootstock. This tree structure, present for the first 2 to 3 years after topworking, is very prone to wind damage with whole limbs being easily blown out of the rootstock. Initially after topworking, it is essential to stabilise the graft which may be performed by the application of duct tape tightly around the top of the rootstock and over the graft union or alternatively nail the graft to the rootstock. Both these activities assist in a tight graft scion/rootstock contact assisting in graft success. As the trees grow during summer, to further avoid wind damage, it is important to periodically tie the graft growths to the trellis system or if no trellis system is installed then on multi-graft rootstocks the growing shoots may be tied together to provide mutual support for each other. The nurse limbs that were left on the rootstock should remain for the first season of growth to ensure healthy rootstock and graft union growth but should be removed during the late summer/winter after the first season of active growth.

Why plant certified nursery trees?

Planting certified plant material has been proven around the world to have benefits over non-certified material. Viruses in non-certified material have a direct impact on the tree’s performance and its ability to achieve maximum yields and pack outs. The Australian Pome Fruit Improvement Program (APFIP) has implemented a plant certification system in the Australian apple and pear industry. This system provides certified nursery trees (tested negative for known viruses) though the use of its APFIP Certified Nursery Tree Tag. Licenced nurseries propagate certified rootstocks and cultivars, and nursery trees carrying the certified tree tag are now produced and delivered to growers. Growers should plan early when thinking of redeveloping orchard blocks and order certified trees well in advance from nurseries. A list of APFIP licenced nurseries can be found on the APAL website, talk to them today about certified trees and availability. APFIP Certified-sideMore info: APFIP Mark Hankin 0408 503 528 mark@apfip.com.au

By |June 29th, 2014|Grafting|

About the Author: