Inoculum and truffle reseeding: How to proceed?

Once the truffle cavage period is over, it is time to reseed the truffle parcels. This technique consists in (re)energizing the mycelian life around the truffle plants by adding inoculum. When, how and for what purpose? This is what we are going to see together in this focus dedicated to the reseeding of truffle soils by inoculum amendment.

When, how and why to reseed truffle soils?

In truffle growing, as in any other culture, nothing is ever taken for granted. As soon as a harvest is over, it is necessary to prepare the next one. To put all the chances on one's side, and to ensure a good yield, one might as well do everything possible to encourage the appearance and development of new truffles in the orchard. This is precisely the interest and relevance of truffle reseeding !

The reseeding consists in amending the soil of a truffle field with truffle spores. By germination, these spores will accelerate the development of mycelium and thus increase the level of mycorrhization. This operation aims at several concomitant objectives :

  1. to improve the performance of the producing trees,
  2. to maximize the production potential of each tree,
  3. to have a positive influence on sterile trees.

➽ Why sow your truffle orchard ?

The reseeding by adding spores contributes to the development of mycorrhizae of truffle plants and their root system. It is not a matter of fertilizing or even spreading fertilizer to make poor soil fertile. It is rather a question of recreating the specific conditions for the development of mycelium, an essential prerequisite for the formation of young truffles. Much more than a tilling of the soil, reseeding is therefore an operation that ensures the perpetuation of mycorrhizae and the fructification of a truffle plantation. It provokes the necessary symbiosis between the producing plants (oaks, hazel trees, pines, lime trees...) and the truffles.

➽ When to reseed truffle fields ?

The reseeding of truffle beds by adding inoculum takes place once the cavage period is over and the soil has been worked (in early spring, after the last frosts). Note however that reseeding is not only done on truffle fields that are already producing. It is quite possible (and recommended) to proceed with the first spore inputs near young plants planted only 3 or 4 years ago. This "pre-production" reseeding will facilitate and activate the entry into production of young truffle trees. In both cases, care should be taken to use only controlled inoculum

➽ How to reseed a truffle soil ?

There are several ways to reseed a truffle plot (all of them are detailed later in this page). They consist in bringing to the soil a surplus of truffle spores, in the immediate proximity of the roots of truffle trees, in the burnt areas. One can rely on ready-to-use products or make one's own inoculum recipe enriched with truffle fragments. In both cases, particular care and attention must be paid to the composition and quality of the substrates used.

Advice in the choice of a truffle inoculum

If the addition of substrates enriched with truffle spores seems to be an excellent idea, its implementation can, in some cases, be counterproductive. Indeed, it is important to keep in mind that the addition of inoculum (or any other organic matter) will disturb the soil of your truffle bed. It is therefore essential to respect a few simple rules before proceeding with any new inoculum addition around your truffle plants.

Here are our recommendations :

  • do not use the first substrate or potting soil that comes along (even if it is "truffle" labelled). Choose only controlled truffle inoculum (whose nature, origin, quality and maturity of spores have been controlled). This is essential to avoid contaminating your truffle field with undesirable truffle spores!
  • pay attention to the density of spores in the inoculum (if this is not mentioned, don't bother!)
  • make sure that your inoculum contains only one type of spore (Tuber melanosporum, Tuber uncinatum or Tuber magnatum) and that it corresponds to your truffle field.
  • make sure that the chosen inoculum is "sterilized" (to avoid disturbing fungi and other parasites)
  • check that the origin, quality and maturity of the truffles used in the preparation of the inoculum are guaranteed by a serious and recognized organization in the truffle industry (INRAE, CTIFL...)

➽ The Robin truffle inoculum and its variations

ROBIN Nurseries offer a range of controlled inoculums. Our truffle inoculums are composed of vermiculite enriched with truffle spores carefully selected and controlled twice: 1st control under microscope and 2nd control by biomolecular analysis carried out by INRAE. This double check guarantees an optimal development of your truffles. This specific substrate for truffle growing is ready-to-use. It favors mycorrhization and truffle fructification. Our inoculum is offered in 5 liter bags containing a high concentration of spores of Tuber melanosporum, Tuber magnatum or Tuber aestivum var. uncinatum.

How to reseed a truffle field ?

The best way to boost the mycelial life around your truffle plants (and to boost the development of new truffles) is to increase the density of truffle spores present in your soil. To do this, you have two options :

  • do not dig up all your truffles and leave a part of your harvest in the ground.
  • add truffle spores to your orchard (by using a specific controlled truffle inoculum).

It is this contribution of truffle spores that is commonly called "reseeding". It consists in improving your soil by adding diaspores that will in a way remycorrhize the soil. This enrichment can be done in different ways and each truffle grower fine-tunes his own year after year. Some make their own substrate (at the risk of contaminating their truffle fields with truffles of undesirable species, because too often the truffles used are not rigorously controlled) while the most numerous have recourse to controlled inoculums "ready to use".

Once the choice of inoculum has been made, the next step is to incorporate it into the soil. Here again, several techniques compete for the favor of truffle growers. Some opt for a diffuse contribution, while others prefer the practice of "truffle traps" (or "truffle nests").

➽ Diffuse reseeding

This technique consists of spreading the truffle inoculum over the entire surface of the burned area, at a rate of 2.5 liters per tree. It is then buried 15 to 20 cm deep using a tool or a rotovator. 

➽ Reseeding with truffle traps

This technique consists of targeting the spore supply on 3 to 4 holes previously dug (with a spade or auger) around the truffle plant (at a distance of about 40 to 50 centimeters from the trunk depending on the size of the tree). Each hole, 20 to 25 cm in diameter and 25 to 30 cm deep, then receives a large handful of inoculum before being filled in with a soil + inoculum mixture.

➽ Reseeding in furrows

A third technique consists of digging furrows 5 to 15 cm deep near the truffle plants (at a distance of 50 to 80 cm from the trunk). The inoculum is then poured into these furrows before filling them in. 

Whatever the reseeding technique used, it is strongly recommended to irrigate immediately after the inoculum is added by sprinkling or micro-sprinkling.

What is the effect of reseeding (or over-seeding) ?

The main interest of reseeding by truffle traps or by furrows lies in the inoculation it generates on the root system. Indeed, during the realization of the nests or furrows, roots and rootlets are cut. This naturally leads to the regrowth of numerous root apices which will progressively grow in contact with the spores contained in the inoculum. This phenomenon induces a kind of over-mycorrhization.

If the inoculum used is of good quality, and that the conditions are met (flexibility of the soil, humidity...) then this reseeding will give birth to new truffles, in general in the two years which follow the application. If this principle works for all varieties of truffles (black truffle of Périgord, white truffle of Italy, truffle of Burgundy...), its effect fades after a few years. It is therefore wise to dig a new series of truffle traps every 3 or 4 years, gradually moving them away from the trunk as the tree grows.

Questions and answers about truffle reseeding

The inoculum contribution (diffuse or by truffle traps) must be done in early spring after having finished the soil work. The operation can be repeated during the cavage period by incorporating a handful of inoculum at the site of the harvested truffle.

A 5-liter bag of inoculum can be used to create 15 to 20 truffle traps (i.e. 4 traps around 3 to 4 truffle trees).

In addition to the irrigation immediately after the inoculum is added, it is also necessary to water the soil regularly from May to September, depending on the weather conditions.

The results of truffle traps or diffuse reseeding are generally only effective 2 years after the inoculum is added.

To be effective, a truffle inoculum must contain at least 10 million truffle spores per liter. As a comparison, 1 gram of truffle contains about 1 to 2 million spores.

The inoculum can be kept for 10 days after reception at room temperature. After this period, it is recommended to store it in a cool room or in the refrigerator.

In order to find the truffle inoculum that will boost your truffle bed, turn to specialized stores (online or in nurseries). Avoid potting soils and other exotic substrates whose composition is more than uncertain and whose use could well result in the colonization of your truffle bed by invasive and inedible truffles such as the brumella truffle...

ROBIN Nurseries offers a complete range of truffle inoculum whose quality is controlled by the INRA. Count approximately 22 to 25 € the bag of 5 liters (decreasing price according to the quantity ordered)