Most nut species grow more readily on loamy or even sandy soils than they do on heavy clays. They also prefer well-drained soils, but pecans can tolerate heavy soils that flood occasionally.
All species are affected by shallow soils that have hardpan or rock layers in the upper meter of soil. Trials by the UC Davis, California on almonds show that where irrigation water penetrates to a depth of 24 inches, 75% of all roots are found in the top 0-16 in. However, trees will root much more deeply and those roots at 1.5m depth contribute significantly to water uptake during harvest dry-down, when surface layers are dry. Nut trees prefer slightly acidic soils (pH 5.8-7.0), but walnuts also do well in neutral soils.
Extreme soil pH values result in nutrient tie-up, or toxicity, adversely affecting tree and fruit growth. It is important to amend the pH on acid soils before planting by incorporating lime.
While most nut trees are tolerant of winds, very exposed sites can both shorten and distort top growth and may result in premature nut drop. Most nut trees will have reduced crops on lower lying sites where frost usually comes later in the spring and earlier in the fall.
Spacing varies according to tree type, but increasingly closer plantings are being practiced. Almonds tend to come into production more quickly than crops such as hazelnuts and pistachios.
Root growth commences in early spring – in almonds when leaves are at 70% expansion – and diminishes through the summer. There is a second, but much smaller period of growth in late autumn/early winter.
As the days become shorter and cooler in the autumn, trees stop growing, relocate nutrients and energy to the tree wood, lose their leaves and enter a state of dormancy which protects them from lower winter temperatures.
Once dormant, they will not resume growth, including flowering and fruit set, until they experience a level of cold or chilling. This chill requirement is defined as the accumulation of hours below 45˚F and above 32˚F.
Different nut tree species and varieties vary according to their chilling requirements. Most almond varieties require a shorter chill period compared to walnuts or hazelnuts.
During an extremely mild winter the chilling requirement may not be satisfied, resulting in a lack of synchronization between the open opening of male and female flowers, leading to an uneven bloom and poor pollination; fruit set and nut quality is also adversely affected. Conversely, if a tree is grown where winter cold satisfies its chilling requirement too soon, the end of dormancy and loss of hardiness caused by a warm spell could lead to late-winter freeze damage to the tree and/or a too-early bloom. Chilling that exceeds a fruit tree’s minimum requirement can lead to a stronger bloom and a heavier crop that will require more thinning for best fruit quality and size.
Additional factors that affect fruit set include the age of tree, its nutrition, the availability of compatible pollen and weather conditions during bloom. Vegetative growth peaks in the summer before the tree puts energy into fruit production.
Crop nutrition needs to closely match these different stages and ensure that supplies of macro and micronutrients is not limiting.
Almond trees are typically 13 - 20 feet high and have a round and bushy shape with a trunk that is up to 12 inches in diameter. The fruit of the almond is a drupe, with an outer hull and hard shell around the seed or ‘nut’.
Flower buds surround a vegetative bud on the previous year’s shoots and also on spurs produced from old wood.
The table below gives the typical bud initiation and development cycle for Northern Hemisphere almonds. Compared to other nut species, bud burst is relatively early. Flowers open in January/February, just before the leaf buds.
Pink buds can tolerate 25 to 21˚F, but at full bloom, temperatures of 28˚F will cause damage to fruit set. Almonds will tolerate dry conditions, but in areas of less than 20" rain per year and under prolonged periods of drought, yields are reduced unless crops are irrigated. In California, for example, all almonds are produced using irrigation, regardless of rainfall.
Almonds are relatively tolerant of poor soil conditions, but prefer lighter textured, deeper, well drained soils. Almonds are a little more tolerant of waterlogging than walnuts, particularly between bud break and bloom. However, once fully leaved, waterlogging is detrimental. Flowers are mostly self-incompatible and insect pollination is critical. A few varieties are, however, self-pollinating.
The USA depends on managed pollination and in February one million bee hives are trucked-in to groves in California every year to ensure good production.
Almonds are particularly sensitive to navel orange worm (Amyelois transitella) and peach twig borer (Anarsia lineatella) damage. Both insects will allow Aspergillus fungi to infect nuts leading to aflatoxin contamination. Integrated Pest Management (IPM) using predatory mites is increasingly practiced to minimize these risks. Fungicidal sprays around bloom may also be necessary to control a range of diseases to ensure good fruit set.
Nut maturity occurs 7 - 8 months after flowering. Almond trees start fruit bearing in the third year after planting and full yields of between 0.9 - 1.6 t/ac (kernel basis) can be expected around 7 years after planting.
In California, all almonds sold to the public are pasteurized to reduce risks of salmonella.
The United States Department of Agriculture (USDA) maintains strict standards and grades for California almonds covering everything from defects to discoloration.The top grades, such as US Fancy, Extra No. 1 and US No. 1 (Supreme) require strong visual appeal with low levels of chips or scratches (5-15%), only 1% of kernels should be split or broken and the sample should contain less than 0.05% ‘foreign’ material.
Levels of dust also need to be kept to a minimum in all grades at <0.1%, and harvest management is critical in this respect. Grades such as US No. 1 Whole and Broken, or US No. 1 Pieces, accept higher levels of chips or scratches and broken nuts. Nut size and levels of other defects are less critical and these nuts are sold at a lower price for use in food ingredient manufacturing.
Pruning encourages consistent, high yields of good quality nuts. It is also used to shape the tree to ensure it has the right pattern of branches to carry the load and allow good light penetration and air circulation as well as to remove diseased or weak branches and those that grow into the working space between the tree rows.
Pruning young trees helps them develop a desirable shape and branch structure. As nut trees become larger, unless it is needed to allow better light penetration, pruning is usually limited to removing dead or damaged branches. Common practice is to prune the tree branches at planting. This results in several shoots competing for the position of the new leader or main trunk. As a consequence, when the new shoots are 8 - 12 inches long, growers select the strongest and straightest as the leader and pinch out the growing tips of the competing shoots.
In late winter or early spring each year, the lower branches are shortened. If any of the lower branches grow vigorously during the growing season, the practice is to pinch out the growing point. As the tree grows taller, these lower branches can be cut right back. Leaving the lower branches on the young tree, aids early growth by maintaining its photosynthetic area; it also provides shade for the trunk during the growing season. Eventually all branches arising from the trunk within 3 feet of the ground are removed. This facilitates mowing and other cultural practices.
Hazelnuts, which are commonly multi-stemmed, need more aggressive pruning to thin out the smaller and weaker suckers each spring, cutting them off at ground level.
Alternate bearing – where a high crop load in one year is often followed by a low yield in the next year – is an issue in most nut tree species.
The reasons for alternate bearing are not fully understood, but it is assumed that parameters such as hormonal imbalances, or the competition for carbohydrates or nutrients during flower bud differentiation/fruit development, are to blame. Long term studies of flower and fruit production in almonds show that individual spurs alternate between years in which flowers and fruits are produced, and years in which only leaves are produced. As a result, if the majority of spurs on a tree bear fruit in one year the tree will produce a greater than average crop and the following year most of the spurs will mainly produce leaves, resulting in a lower than average crop.
Researchers suggest that where high levels of a tree’s energy and nutrient resources are used in one year, there are fewer reserves available for the spring flush in the following crop. In an off-year, leaves have lower resource demands than the fruit and result in increased local storage for the following year when fruit are produced. In addition, the increased vegetative growth enables the tree to produce more carbohydrates for storage due to increased photosynthetic activity.
Because alternate bearing in almonds is largely due to competition on a spur level and there is a balance between fruiting and leaf-bearing spurs, annual fluctuations in yield are not as great as with some other nut species. Pistachio has perhaps the greatest tendency for alternate bearing. Research at UC Davis, California shows that after pollination, as pistachio fruit grows, new flower buds develop within the branch.
These growing fruits suppress developing flower buds that would otherwise flower and develop fruits in the next year. As a result, during years with greater than average fruit yields, the majority of developing flower buds are suppressed so that the following year only a few flowers and fruit are produced. This creates regular oscillations between high and low yields in alternating years, with more blank nuts produced in the off-year (Table 11).
Weather conditions that impact on flower and fruit development can exacerbate the alternate bearing tendency of individual trees or whole orchards. For example, a severe frost during bloom would kill developing flower tissue and result in a very low fruit set. Diseases that lead to early defoliation of the tree will also encourage alternate bearing. As a result of the low fruit set and reduced use of stored carbohydrates at each individual spur, the following year the majority of shoots will produce flowers and set fruit. If all spurs on a tree are synchronized to flower during the same year it will generate a whole tree, and even whole orchard, pattern of alternate bearing. This in part explains why groves across a complete region will have off- and on-years that correspond. Trials confirm this. In an off-year annual biomass growth declines, but shoot and leaf growth increases. As a result, in the off-year, while there is less nitrogen used by the tree, most of it is found in tree reserves. The research work also shows that in the on-year, more nutrients are found in the kernel compared to those in the lower yielding off-year.
Crop nutrition that ensures nutrient supplies are sufficient from the end of the dormancy, right through the season, can help to mitigate the effects of alternate bearing.
With 40 acres of fertigated and established almonds, and 40 acres of irrigated and established walnuts, the Yara Incubator Farm in Modesto, California is a center for research, solution trials and knowledge sharing.