A mycorrhiza is a symbiotic association and can't be "added". It might make some sense to add fungi to sterile soil such as in household pots, but anything that's going to latch on to those palm tree roots is already there; soil is dirty already! Buying "mycorrhizae" in a bag is a lot more work than just lighting a twenty dollar bill on fire, but just exactly as effective. It is, however, excellent scam marketing.