Question
I was told a orange tree should be weed free from the trunk to the drip line. One nursery said I could use round-up; the other said no. I thought roundup worked from the leaf down; not from the root up. My guess then; it would be safe for fruit; am I correct?????

Answer
According to the U of Florida extension Service Roundup is and can be used to control weeds and grasses around orange trees. Care should be taken not to spray any green part of the tree-leaves and even cut bark that exposes the cambium layer.

Here is the recommendation from Florida:
Glyphosate is marketed under the trade name Roundup. It first appeared in the University of Florida recommendations in 1979 as a nonselective, systemic herbicide that is actively translocated from the leaf and stem tissue to the roots or rhizomes of the weed. Glyphosate is registered for postemergence use for total or partial control of most weed species. Glyphosate does not have any soil activity for preemergence weed control. The 1979 rates were 1.5-3.7 lb ai/A with an annual maximum of 7.9 lb ai/A. Recommendations cautioned growers not to apply to trees which were less than two years of age and to avoid green bark, tree foliage and fruit. Glyphosate's application rate was 20-100 gallons of water per acre. By 1984 the application rates were reduced slightly to 0.75-3.0 lb ai/A with a lower rate of 0.75-1.5 lb ai/A in 5-22 gallons per acre for the control of annual broadleaf weeds and grasses. The rates were adjusted slightly in 1985 to 1.0-4.0 lb ai/A. Chemical mowing recommendations first appeared in University of Florida publication in 1992. Glyphosate currently appears in the University of Florida recommendations and is the most popular postemergence herbicide currently used in Florida citrus production.

Here is the web link that also list other herbicides that are used.
http://edis.ifas.ufl.edu/CH144