Nurseries in Spain provide two main rootstock options: conventional almond rootstock (generated from seeds of sweet almonds) and peach-almond hybrids. It is said that conventional rootstock grows deeper and stands drought better, while hybrids are better adapted to calcareous soils. Hybrids have become the norm, perhaps because they are easier for nurseries to produce.
It is thought that hybrid rootstock is one of the reasons for a serious beetle plague to expand in Spain. Capnodis tenebrionis beetle larvae is deadly for almond trees. It parasites the roots and prefers sweet and superficial ones, either of sweet almond or hybrids staff.
An advisor explained me that the best way for a tree to stand the beetle is in situ generation form seeds. This simple system allows the tree to fully develop a main pivotal root that goes deep. On the contrary, planting from a pot or nude root plant makes a more diffuse root system. What is more, research has shown that roots of bitter almonds deter Capnodis tenebrionis.
There are plenty of bitter almond trees in La Mancha, so it is easy to find strong ones that grow naturally in extreme conditions. This is the the rootstock that I have used for the orchard I have sown in 2015. The selected parental trees are rustic and provide bitter small almonds.
Genetic variability is wide even for almonds of the same tree. An orchard generated from seeds would have variability. So the selection that I made was not based on future performance. Instead I selected almonds that sprout and grow quicker in spring.
I made a trial in 2014 for this purpose. I sown several sets of almonds in a field. Two extreme performances were observed. One was that of sweet almond collected in Autumn 2012. The other extreme set consisted of small bitter almonds collected in Autumn 2013. I treated the seeds with thiram to prevent fungal infections. I sown them in lumps of three or four seeds per site in February, 28. No irrigation was provided. Mild rains came in early April. Almonds sprouted in mid April. Two distinct performances were obtained. Young small seeds sprouted and grew much quicker, while old big ones performance was poor. The graph is very clear: 100% reliability for the young bitter group.
The full trial results appear in this excel file. Germinacion 2014