Greens should be offered as early as possible, in addition to crickets and other insects. Bearded dragons will need at least one serving of insects a day till they are 12-14 months old, then you can cut down on the amount of insects given, either 2-3 feedings a week, or a few insects daily in addition to a large portion of mixed greens and vegetables.
greens will never replace insects, but will become about 90% of their diet once they are adults. When young, it's teh opposite, 90% of the diet should be insects, gradually reduced every few months as follows:
Hatchlings - 4 months: 2-3 feedings insects, greens offered all day
4-8 months: 2 feedings insects, greens offered all day (about this time, dragons should be eating some greens)
8-14 months: One feeding insects, greens offered all day
14 months: Greens offered daily, some insects daily, or 2-3 feedings a week.
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PHLdyPayne