1. a special personal quality or power of a garden making it capable of influencing or inspiring large numbers of people
2. a quality inherent in a garden which inspires great enthusiasm and devotion
Examples of its use in a sentence:
The Lurie Garden in Chicago had a special hortisma about it that caused people to visit there many times on a single visit to Chicago.
Her garden did not seem at first glance to be anything unusual, but she was under the spell of its hortisma and spent hours weeding, watering and watching it each day.
You don't see hortisma in a garden, you feel it.