I always use green fertilizers in my gardens. One principle is that the soil should never be naked or uncovered. In nature you don't find bare soils unless the land became a desert. Insects and bacteria that are useful for the soil don't like bare soils. In late summer and autumn two green fertilizers that can still be applied are mustard and winter rye.
Phacelia can be used as a green manure. I sow it on fields that are empty for some time. Phacelia quickly makes big plants and fits well in a plan of rotation of crops as it isn't related to most vegetables. The field is buzzing with bees and bumblebees.
Mustard can be used as a green manure plant. It grows very fast.
Rye is a green manure plant.
Rye grows through winter frost. It is a useful green manure plant to sow on fields that are empty during the winter.