I have a very light crop on trees in greenhouse and outside that are normally loaded. Even my 20 year old Swingle has very little fruit on it. The Pineapple orange in the GH looks great just little fruit. The only tree that has the normal amount is a Ponderosa Lemon. The only tree that looks unusually bear of leaves is a Valensia 15' tall, all the trees around it look great. I realize we had an unusually cold winter but cold has never affected the Swingle before and most of the fruit that is on it is in the top not on the lower protected limbs. The trees in the GH were kept warm as we keep some flower pots with tender annuals in there to winter over and they had no damage. I realize some trees have alternate bearing tendencies but almost all the trees at the same time is unusual to me. All the trees bloomed normal with a tremendous amount of flowers and there was no cold after flowering unlike last year with a late frost. Any comments or thoughts?

If there is no other reason, your trees may have a lack of boron.
Give them boron when the flowers set and it will keep more fruits.

Has it been dry in your area? Citrus will adjust the load of fruit based on what it can support--especially between bloom and the end of June. There are two critical support factors in that period--water and nutrients. Here in our sandy soil, I normally do not water my mature trees unless there is a serious drought--except during the "bloom to June" period.

My sister in Alabama had a pretty good fruit set initially on her satsumas--she had surgery in early May while we had a dry period and was unable to water and fertilize--her trees dropped almost all of the fruitlets.