The ground around the young trees should be kept liberally mulched with weeds, straw, barnyard litter, seaweed, or any coarse material which is not injurious and will not pack and form a layer impervious to air and water. Through the winter a mulch is not necessary in California, but in Florida it has been found desirable, in some sections at least, to maintain one throughout the year. In Porto Rico, G. N. Collins observed that the avocado tree was seldom, if ever, found in perfectly open places, with the bare ground around the roots exposed to the sun. While this principle applies more particularly to Florida and other regions distinctly tropical in character, it may be proved to hold good in California as well. Definite knowledge on this point is still lacking. Up to the present it is the practice of many California orchardists to cultivate the soil regularly after each irrigation, as with citrus fruits. Deep cultivation seems to produce no harmful results in California, where the roots go far down into the soil, but in southern Florida it must be practiced with caution. In this region the feeding roots extend practically to the surface, and deep cultivation destroys many of them, thus cutting off a large part of the tree's food supply. On shallow soils the most healthy and vigorous trees are those which are mulched. The mulch should extend at least two feet in each direction from the trunk of the young tree, and as the latter increases in size and its roots reach out on all sides, the mulch must be enlarged to be always a little wider than the diameter of the crown.
Mulching serves two purposes: it prevents the soil from drying out rapidly, and it protects the delicate feeding roots from injury due to excessive heating of the soil. This protection is of particular importance in Florida, where in many places the land is sandy and becomes exceedingly hot if exposed to the sun.
When the trees are of mature size, the shade furnished by their own foliage, together with the fallen leaves which carpet the ground, aids materially in maintaining the soil in good condition; but additional loose material, especially during the summer, is highly desirable.
The use of green cover-crops between the rows is decidedly beneficial, but they must not be brought close enough to the trees to rob them of their food. In Florida, cowpeas and velvet beans have been used for this purpose, cowpeas being preferred. A clump of pigeon peas (Cajanus indicus) planted four feet to the south of each young tree will provide shade during the first summer or two, serve as a protection from wind, and aid in enriching the soil. In California, purple vetch (Vicia atropurpurea), common vetch, and the other cover-crops used in citrus culture will probably prove satisfactory. Up to the present time they have not been extensively tried in connection with avocado culture.