MYCORRHIZAE is correct, “myco” = fungus and “rhiza” = root. Fungus roots. Mycophiles love mycorrhizae while mycophobes run for the hills at the first sign of them. You can start without mycorrhizae but 90% of plants have improved field performance mostly Green, Leafy Plants including Tobacco and most Commercially Produced Plants. Shrubs and foliage plants except for Rhododendron, Azalea, and Heath; Berries except for blueberries, cranberries and lingonberries; Nut trees except pecan, hazelnuts and filberts. Flowers, Vegetables except Brassica and beets, cultivated grasses except weedy grasses; Fruit trees including tropical fruits; many wetland/aquatic species except rushes and horsetails. Mostly taking about ENDO-mycorrhizal fungi with this group. ECTO-mycorrhizal fungi is the other 5%, mostly woody plants, conifers and oaks. All up that’s 95% of the world’s plant species form associations with mycorrhizae and require the association for maximum performance in the field.
For frost dates I work with last frost and work back for indoor starts. And then first frost and work backwards for fruiting/harvest to build my local “window” for the local growing season.