Planning a truffle orchard
Site selection
The primary considerations for site selection include climatic conditions, soil texture and structure, slope, past use, aspect and availability of water for irrigation.
Climate
Truffles and their host trees are native to south-west Europe, namely France, Italy, Spain and parts of the former Yugoslavia. The climate in that region is continental to Mediterranean, with cold winters, warm to hot summers and rainfall throughout the year.
Where trees are not irrigated, sufficient summer rain is needed to maintain some degree of moisture in the root-zone for the developing truffle. It appears that declining rainfall, especially in the summer months, is a factor in the observed decline in European production of truffles in the last 80 years.
Truffles grow best in regions with warm summers and mild to cold winters. The warm summer temperatures are needed to initiate truffle formation while the cold winter temperatures appear to be necessary to trigger final maturation of the truffle and development of the aromas.
Manjimup has an average maximum summer temperature of 26.4°C and an average minimum winter temperature of 6.8°C. Rainfall averages 1000-1100mm annually, mainly between April and November. The Pemberton climate is very similar.
Truffle orchards in Western Australia (WA) require irrigation from late spring for optimum production.
Soils
Key soil considerations are good aeration, good drainage after rain or irrigation, 'fluffy' friable structure to allow the developing truffle to grow, and a high pH with abundant calcium. While pH is amenable to adjustment, drainage and structure can be more difficult to significantly alter and sites should be considered with these two features in mind. Drainage is of paramount importance. Rainfall in WA is predominantly in the autumn-spring period when truffles are growing and maturing. Waterlogging during this period will lead to increased risk of rot and reduction of saleable truffle.
Further, both hazelnuts and oaks are susceptible to root rots if their roots are in very moist or wet soils for any period of time, especially when actively growing in spring and summer. Some sites may be engineered to reduce waterlogging but this is an expensive option. Slopes are likely to provide good drainage and such sites are preferable, providing machinery access is safe.
The best soils appear to be the sandy loams to clay loams with 10-30% clay content. Very sandy soils will drain rapidly and provide little water-holding capacity, while heavy clay soils will remain wet for long periods and may inhibit the growth and proper maturation of the truffle. In their native areas and in WA, truffle production occurs in soils with some stones or gravels which assist with drainage, contribute to aeration and may provide protection against soil compaction. Many of the soils in the Manjimup area contain pea gravel which provides good drainage but this can affect the shape of the truffles. Truffles obtained from soils with less gravel may be rounder and have better shape.
In Europe, truffles grow on well-drained alkaline loams with a relatively high level of calcium. The acidic soils in the Manjimup/Pemberton area and most of southern Western Australia require considerable quantities of lime, usually in the form of agricultural lime, to increase pH to greater than 7.5 (measured in water). Dolomite lime supplying magnesium as well as calcium can also be used if magnesium is deficient. Lime may need to be added periodically for the life of the orchard.
A soil test should be carried out on each different soil type in the proposed orchard to determine the soil depth, the pH and the levels of macro and micro-nutrients. Truffle production is best in soils with low to moderate levels of all plant nutrients. Excess nutrients as a result of previous intensive farming activities may adversely affect maintenance of the mycorrhizae.
Planting source
Truffles are grown in association with a host tree, most commonly hazelnuts (Corylus avellana), or oaks such as English oaks (Quercus robur) or Holm oaks (Quercus ilex). In orchards with hazelnut trees, the hazelnuts are generally not collected commercially.
Inoculated hazelnut and oak seedlings for commercial growers are available from specialised nurseries. If planting stock is being brought from interstate, Quarantine WA biosecurity requirements must be met.
Inoculation techniques are theoretically quite straight forward, however problems occur due to contamination with unwanted competitive species, or poor colonisation of the roots. Growers are advised to obtain inoculated plants from a commercial nursery. Commercial production methods are still closely held secrets. Growers are strongly advised to have their plants independently assessed for tree quality, for percentage of roots successfully colonised and for purity of the colonising species. An independent tree evaluation/certification process is operated through the Australian Truffle Growers Association. Several truffle orchards in eastern Australia have been found to be contaminated with Tuber brumale, an inferior truffle species that may outcompete T. melanosporum. It is incumbent upon growers to ensure their plants have been correctly inoculated before they plant them as it is virtually impossible to remove contaminants such as T. brumale from the field. Further, it is not feasible to attempt to re-inoculate mature trees if they lack the truffle fungus at planting out.