Perhaps your friend would like to start vegetables and flowers indoors early. There are special seed starting kits and pots that a gardener may enjoy. Potting soil may also be useful. Your friend may enjoy some new flower pots or colorful jardinieres. If your friend likes house plants or herbs, these would be good gifts.
Some gardeners specialize in cacti, orchids, tropical houseplants, or any of many other types.
Local nurseries can also help you select such a gift. Gardeners are a varied lot, but there are many gifts that they would enjoy. Be sure to give your friend a chance to tell you about garden preferences. Just giving of your time and listening is a gift that almost any gardener would love. May you shopping be pleasant and your holidays be safe and joyous.