Fr. Z shares a link to the Vultus Christi blog, which tells us of this fascinating tradition on this day.
I like Catholic legends like this.
The aromatic herb, basil (Ocimum basilicum) has long been associated with the Holy Cross. Etymologically, it is related to basileios, the Greek word for king.
According to a pious legend, the Empress Saint Helena found the location of the True Cross by digging for it under a colony of basil. Basil plants were reputed to have sprung up at the foot of the Cross where fell the Precious Blood of Christ and the tears of the Mother of Sorrows. A sprig of basil was said to have been found growing from the wood of the True Cross.
On the Feast of the Exaltation of the Holy Cross it is customary in the East to rest the Holy Cross on a bed of basil before presenting it to the veneration of the faithful.
Also, from the practice in some areas of strewing branches of basil before church communion rails, it came to be known as Holy Communion Plant.
Blessed basil leaf can be arranged in a bouquet at the foot of the crucifix; the dried leaves can also be used by the faithful as a sacramental.