Even the newest gardener realizes that plants die without water what is not so well known is that plants die equally decisively, though not so quickly, if they are overwatered. Beginners usually decide to play it safe and keep their potted plants thoroughly wet. In consequence, death by drowning is one of the commonest disasters to befall the plants of a new horticulturist. Plants wither away if they don't get enough water, and this draws attention to their problem. A plant that has been slightly underwatered so that it droops strikes terror into the heart of its new owner. But it will, in fact, recover completely as long as rescue comes in time and the process is not repeated too often. Overwatered plants, unfortunately, do not give any such obvious signal slowly they cease to thrive and the first visible indication of serious trouble is a yellowing of the lower leaves. Unless the overwatered pot soil is given a considerable period without water, during which time the plant will continue to look wretched, it will suddenly collapse in exactly the same way as the underwatered plant but with no chance of being revived(复活)because the roots have rotted away. From the text we can infer a horticulturist is person engaged in ______.