How big is the garden? Some are spread out over a person's entire property, some are in a side yard, and some are grown in containers on porches or countertops. All gardeners need to water their plants, but the size of the garden will mean that a watering can is better for some and long soaker hoses and sprinklers are better for others. Just about everyone with an outdoor garden could use sun protection. Many people will find gardening gloves and knee pads useful, but smaller or established gardens are less likely to need trowels and shovels. Gardens that take up only part of a yard are easier to maintain and look nicer with clear garden borders or even raised beds--talk with your recipient to see if those would make good gifts. People with container gardens have less space to work with, so new plants and seeds might not be as welcome, but perhaps some more containers are needed for new plants or ones that have outgrown their current containers. Terra cotta pots are nice for this: you can paint them to customize them for your recipient. Larger gardens can be time-consuming to care for--maybe the owners would like an automated watering system, or even just help and company while weeding. If there's room for more plants in any size garden, perhaps look for "fancy" ones like heirloom varieties or strains of flowers with especially striking patterns like galaxy petunias.
What is the garden for? For food, for relaxation, for landscaping, for beauty? Someone who grows food might appreciate complementary crops to grow, like herbs that can be mixed with the already-planted tomatoes to make sauces. If you're near a farmer's market, you might be able to find seeds or starts of local or heirloom plants--I like to find Walla Walla onions to grow over other varieties, for example. People who enjoy spending time in the garden might like garden décor like windcatchers or birdbaths, or some lawn furniture to sit in while appreciating the plants. You can make garden art, too: I helped my kids and my sister-in-law's kids make stepping stones for my mother-in-law to put in her garden. If someone has spent a lot of time and effort transforming an area into a garden, gifts that help keep it nice are great, like mulch and other fertilizer or weed control measures.
What sort of climate does the garden grow in? Not all plants grow in all environments. An aloe vera won't do well outside in cold climates, and aquatic plants like lilypads are going to be tricky to grow in arid climates. Someone living in a harsher environment might like a greenhouse though, and they come in a variety of sizes. Local plants will be easier to care for and grow better than exotic ones, plus any with flowers will provide good food for local pollinators: bees, butterflies, hummingbirds, and even some bats and other mammals will appreciate local plants along with your recipient. If you know someone whose garden needs extra attention, maybe you can offer to care for it when the person is on an out-of-town trip.
How much access do wild animals have to the garden? If your recipient is tired of rabbits, deer, birds, or other animals sampling the garden, fencing or other deterrents are a good idea. I personally use a dog to keep mammals away from my garden (she doesn't mind the birds and neither do I), but you really ought to talk to people before springing pets on them! For something as permanent and visible as a fence, definitely consult with the gardener first. For smaller and shorter areas, a frame covered in chicken wire and placed over the plants makes it difficult for animals to get in, although depending on the size of the holes, small animals like wrens and mice can still slip in, and of course insects and other invertebrates can as well. Beneficial insects like ladybugs and praying mantids are a good offense as are beneficial nematodes, eating small creatures like aphids that would otherwise feast on the plants, damaging them. There are also sprays, storebought and homemade, that can deter animals. Certain plants, for example crocuses and bluebells, work well to keep animals like deer away too, and marigolds deter slugs and snails; a gift of those plants to make a border might be nice. Used coffee grounds add nutrients to the soil (fresh ones would be too acidic), deter pest insects, and attract worms--a friend of mine works at a coffee shop and often brings me used ground for my compost pile. Motion-activated sprinklers, lights, and sound equipment can also help. On the other hand, if your recipient doesn't mind "sharing" with the local wildlife, maybe you can look for something to encourage the animals: bird feeders, plants that pollinators like hummingbirds and butterflies love, and places for them to roost like bird houses and bat boxes. A nice trail camera or traditional camera could work too.
How particular is the owner about the garden? For example, I grow both food and ornamental plants. There are some fruits and vegetables I don't care to grow, but I'm open to planting almost any kind of flower or succulent or other decorative plant. Some people might have preferences for specific varieties, too: maybe Roma tomatoes are better than beefsteak or cherry. Maybe you love the garden gloves you use, but your recipient likes another. If you're concerned you might not pick the "right" plants or garden tools, a gift card to a garden store is a good choice. You can pair it with the gift of time: take your recipient to lunch, then head to the garden store together.
