There have been several new cherry orchards planted around Brentwood, Calif., in recent years. Numerous questions have been raised on keeping trees small and bringing into bearing earlier.
There are several approaches that can help you achieve these goals.
On excellent clay loam soils, the traditional training methods and the standard rootstocks (Mahaleb, Mazzard, Colt) tend to produce tall, vigorous trees that are non-productive until at least year six. The newer Giesla rootstocks offer varying degrees of dwarfing and all are precocious so that you may get fruit as early as year three or four after planting. Giesla 6, Giesla 12 and Giesla 5 are the most commonly available and they will produce trees that are respectively about 10, 25, and 60 percent smaller than a standard rootstock. Because these new rootstocks set fruit early and heavily, there is a concern that the heavy set may contribute to smaller fruit size in later years. This could potentially be offset by diligent pruning but it is too early to really know. The fruit size issue may be less of a concern for U-Pick than shipping operations.
Whatever rootstock selected, keep the trees smaller and encourage early bearing, if needed, by employing one or more of the following techniques;
Promalin: This is a plant growth regulator that can be used to encourage branching on unheaded, one-year-old shoots. It is typically used during the first and second dormant seasons. There is a very narrow window of opportunity to apply this material and get the desired effect. It should be applied at the end of the dormant season just as the vegetative buds on the one-year-old-shoots show about one-eighth inch of leaf tissue. It is typically mixed with latex paint (one part Promalin to three parts paint) and painted on those sections of the branch where you want branching to occur. Results are not always consistent due to variability with budbreak. If the Promalin doesn't work, follow up with summer pruning or limb bending to get some lateral branches to break.
Summer pruning: Waiting until the summer to make any heading cuts has the advantage of both keeping trees smaller and encouraging more branching (which can lead to more fruit set). The general idea is that after the trees have put on 24 to 30 inches of growth, head the scaffold branches back by about one-third to promote branching. Make sure to supply enough water after this heading that they will grow several more inches that season. Any vigorous branches that are competing with scaffolds or growing in an undesirable direction should be completely thinned out. This can be done during the dormant season (less devigorating) or the summer (more de-vigorating). Make sure to keep all the weaker shoots (pencil sized or smaller) as these tend to fruit earlier.
Limb positioning: Pulling vigorous, permanent limbs to a nearly horizontal position will devigorate the limb and encourage it to begin flowering and fruiting the following season. And bending limbs downward will encourage lateral branching at the bend. However, these techniques have some practical disadvantages.
First of all, there needs to be something to tie the limb to — like a trellis (expensive!) or strings attached to the trunk/ground (which get in the way) or the adjacent tree in the row (least expensive). Secondly, the tips of the branches tend to turn up as the branch continues to grow, so you need to keep moving the tie outward over the course of the season to keep the branch horizontal. This makes this approach more costly than other methods.
Cut back water
Deficit irrigation: If trees are growing too vigorously for their space, you can slow their growth by cutting back on the water in mid-to-late season. The general rule is to cut back to about 50 to 60 percent of their full water use requirement once you want to slow the growth.
You want the limbs to stop growing and set their terminal buds, but you do not want to see yellow or dropping leaves before autumn. Be careful with this approach as you can sunburn limbs and permanently stunt young trees. If the trees are old enough to fruit, do not impose water stress before harvest or from mid-August thru mid-September. Water stress during this late summer period can lead to spurs, doubles and sutures in the fruit next season.
These are the basic tools to use to keep your cherries small and to encourage early production. The trick is to incorporate them into a system that works well for your soil, rootstock, spacing, varieties and wallet.
Spanish Bush System
One training system that has received a lot of attention for producing small, highly productive trees is the Spanish Bush System. This system is commonly used throughout Spain where they plant Mahaleb rootstock on extremely poor, stony soils and are able to maintain high productivity on 8 feet tall trees. This involves very successful integration of all the concepts discussed earlier.
(A videotape about the system is available from Caprile's office. It was made in Spain by Steve Southwick, former UCCE cherry specialist.)
This Spanish system is not likely to work exactly the same domestically as it does in Spain, as American soils and climate are different. Mahaleb trees on our deep, clay loam soils are not likely to stay as small and be as precocious as trees on Spain's rocky soils, but these same techniques can help us achieve orchards that are smaller and more precocious than our traditional training methods. In fact, a Spanish Bush type system may work better on Giesla rootstock as the continued summer pruning may help to maintain large fruit size.
Irrigate trees well during the first season.
At planting: Head tree 8 to 12 inches above the graft union
April-May: After 24 to 30 inches of growth, select three primary scaffolds and head them to remove 8 to 12 inches of growth. Thin out any strong competing branches; leave small, pencil sized branches.
July-Aug: After 24 to 30 inches of growth, select secondary scaffolds and head them to remove 8 to 12 inches of growth. Thin out any strong competing branches; leave small, pencil sized branches. Irrigate well to encourage several more inches of new growth. Then reduce irrigation gradually to set the terminal bud in a position where you would like to have fruitful laterals developing (about waist high).
Sept-Oct: Stop irrigating.
Feb-Mar: Make thinning cuts to remove crossing branches, open center, remove excessively vigorous branches. Make few or no heading cuts.
Optional: Apply Promalin to induce lateral braching on 1 year old shoots when vegetative buds show 3 to 8 mm ofgreen tip (early March).
April-Aug: The goal is to get 12 to 24 inches of shoot growth this season, so limit irrigation. On deep soils little or no irrigation may be needed to get this growth. After reaching the growth goal set terminal bud by withholding water. Then give enough summer water to keep the leaves from turning yellow and dropping. Do not give enough water to stimulate shoot growth (maybe 50 percent ET). If growth is too strong, head in April. If you need to head in summer too, get several inches of new growth before setting terminals.
Sept-Oct: Stop irrigating.
Repeat practices of year two.
If there is fruit, don't water stress trees before harvest.
Dormant: Thin to open center if needed.
Tie limbs in the row to adjacent tree to reduce upright growth if needed
Spring: Provide normal irrigation through harvest.
May-June After harvest, set terminal buds by reducing water
Jul-Sept: Apply full irrigation to maintain healthy fruit bud development. Prune to reduce water loss, shape tree, check growth.
Sept: Reduce water to set terminal buds.
Oct: Stop irrigating.
Year 5 and beyond
Dormant prune sparingly, as needed.
Repeat practices of year four.