Arrangement and spacing of the trees is a subject of some debate and different approaches are used. Generally speaking, Tuber melanosporum grows beneath relatively isolated trees or trees at the edge of forests in its natural habitat. Thus, many plantations in Europe are fairly sparse with as few as 100 or fewer large trees per acre. In other cases, the trees are packed close with as many as 1000 trees per acre to encourage the movement of the fungus from one tree to the next through vegetative growth of the fungal mycelium. The spacing of the trees also depends to a large extent on the size of the tree species used, soil fertility, and the willingness of the farmer to thin the trees when they begin to crowd, possibly removing some that may be producing truffles.
It is not necessarily true that more trees will produce more truffles. Trees stressed by overcrowding may have fewer resources to give to the truffles and closure of the canopy is generally considered harmful to survival of French black truffles. Further, while root contact between trees will facilitate spread of truffle mycelium, it also facilitates vegetative spread of competing fungus species. Plantations established with high densities are fine when the trees are young and small, and by having more trees the likelihood of early production increases, but they must be thinned later to prevent crowding.
One plantation arrangement promoted in New Zealand is to mix large, and small trees to take advantage of early production beneath the small trees and longer production beneath the large trees. In this approach, the trees are planted at high densities with, for example, two hazelnut trees per oak tree. The hazelnuts should begin production several years earlier than the oaks, but their production will decline after 20-30 years. The oaks take longer to begin, but they can maintain production for up to 50 years. The plantation density is calculated so that the oaks will be at a desirable spacing once they begin production and the hazelnuts are removed.
The site should be plowed prior to planting to remove existing vegetation and set the stage for planting and maintenance of the plantation. Any lime and/or fertilizer applications are easiest at this time before the trees are planted. This is also the easiest time to install buried irrigation lines. New World Truffieres ships it trees in the winter while they are dormant and they should be planted well before bud burst in the spring. Planting holes can be dug by hand or with a mechanical augur to a depth of approximately 10 inches and a width of at least 4 inches. When filling the hole to bury the roots of the truffle tree, the soil should be broken up to eliminate clods so that no voids are left around the tree’s roots. The roots should be buried to just above the root crown. Healing the trees in by drenching the soil in the planting hole with water will collapse any remaining void spaces making better contact between roots and soil and it will improve growth and survival through the first year. Where rabbits and deer are common, the seedlings should be protected from browsing until they grow large enough to endure it. Once the trees are planted it is very important to control weeds and grass growing near them. Grass will kill your trees by depriving them of water and nutrients. At this stage mowing is not sufficient within a few feet of the trees and light tilling or hoeing is more effective to prevent weed establishment. Complete weed and grass control in the vicinity of the trees is essential until they are well established. Finally, irrigation through the first year or two is helpful to ensure survival.