Trim off the jonquil flower heads using a pair of garden clippers. Snip them off right under the flowers after they fade. Unclipped flowers use energy to make seeds that otherwise could have been stored in the bulbs to produce more prolific blooms the next spring.
Avoid trimming green foliage -- it is making plant food for next year. Ideally, wait until the foliage yellows before trimming. However, horticulturist Greg Stack of the University of Illinois recommends that antsy gardeners wait at least six to eight weeks after flowering before trimming.
Trim the jonquil's foliage after it yellows or six to eight weeks after flowering. Trim it close to the ground using a pair of sharp pruning shears. Remove the foliage from the garden to compost or discard.