Both spring and fall planting are acceptable with good results. Aren't we lucky!! Fall planting does give one a slight jump on things--they'll flower a little sooner than a spring planted bulb of the same kind because they've settled in over winter but thats all. There are however, certain noncommercial groups that must be fall planted and Trumpet varieties that should be planted in the Spring if planted in far northern zones like 4a and 3.