- Hardiness Zone - All plants are tested and assigned a growing zone. The most commonly used gage is the USDA Hardiness Zone map. Even other countries and continents have been using this scale. It is based on the average lowest winter temperature and is useful for knowing which plants will survive your winter, but it has limitations. The USDA Hardiness ratings do not take into considerations the amount and length of heat during the summer, the humidity levels or altitude and proximity to large bodies of water; all conditions that can affect the health of plants.. Gardeners on the west coast often refer to the Sunset zones, which are more targeted in the areas they map.
- Sun Exposure - All plants need some sun, but the amount and intensity can vary widely. Plants that can handle full sun in a temperate USDA Zone 5 will require the relief of afternoon shade in a USDA Zone 8. Similarly, if a plant is planted in a dry, sandy soil, it will appreciate a bit of shade during the hottest part of the day.
- Soil - Soil is the most crucial factor for plants and often the most overlooked. Gardeners are tempted to think they can change the soil to suit the plant. While it is wise to amend soil before planting, you will but yourself in an endless battle if you try to grow something that wants a soil ph or a drainage requirement that you do not have. You would be far better off to devote your time to creating a soil rich in organic matter and letting the natural soil pH and structure have their way.
- Gardener’s Will - The final deciding factor and perhaps the ultimate challenge is knowing what you must have and what you are willing to do to make it grow. If you are determined to grow lavender or columbines, no matter what your growing conditions, you had better be up to the task. It can mean constantly watering, fertilizing, amending and primping. It’s fine to have one or 2 prim donas in your garden, but when your whole garden requires constant attention, you are setting yourself up for failure.
With sun exposure, your plants are the best gauge you have. If they are thriving and blooming, they are happy. If they are wilting, languishing or having serious disease or pest problems, they are probably in less than ideal exposure.