As mentioned, the best time to transplant A. belladonna and other hysteranthous amaryllids is when the flwers just faded. This way you enjoy the bloom and let them plenty of time to reform their perennial roots. As for Oxalises (and Arums, etc.) it is convenient to let the plans develop fully and then weed, either removing with bulbs or just the upper parts. Consecutive weedings along the growing season will let the plants time to recover but later on the damage is too considerable for recovery.