There is nothing more gracious than receiving a gift connecting to one's passion. If your loved one's passion is gardening, then show your thoughtfulness by gifting something that will be really treasured.
There are a lot of great gardening gifts that the only constraint is your own budget.
If your budget is small, fancy things like gloves, kneepads or even a shady hat. A pretty pot (or a watering-can) filled up with a small bag of potting mix, a packet of bulbs, some gloves and a small trowel or other tool will be welcomed with delight by most gardeners. There are a lot of hand tools at hardware stores that are moderately priced.
If you feel that is too ordinary, how about a subscription to a gardening magazine? A tiny bit more expensive maybe, but it will gift twelve full months of pleasure. A book on gardening is another thought, but make sure your recipient does not already have the one you select. Books are oftentimes heavily discounted at Christmas time, so you might get a deal.
On the other hand, a pot that carrying a flowering plant is commonly a welcomed gift. Make sure to select a plant that is suitable for your climate. Sometimes plants are sent from tropical to moderate zones and kept in artificial conditions in the store. These plants will not do well once taken from their surroundings. Shrub roses are hardy and attractive and grow in many climates. Tulips do best in the cooler climate.
If your budget is potent, a more expensive tool might be suitable. A pull-trolley is easier to use than a garden cart and, like some electric tools, is still not awfully expensive. Small electric tools like whipper-snippers can retail for as little as $20.00. Or if your friend has a hose but not a hose reel, then that would be a more valuable gift that he would really appreciate.
Automatic lawn mowers, electric cultivators, hedge trimmers and brush cutters are in the more expensive price range and you are the only one who can decide whether that is an appropriate gift. Nevertheless, when the recipient sees you have presented a gift that complements his love, expensive or not, it will certainly become the best gift your friend has ever received.