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More on irrigation
If trees will be irrigated infrequently (just several times) after planting, only seeds, seedlings or small saplings of drought tolerant trees should be planted. Small-sized nursery stock has a small top, so roots come into balance with the tree soon after planting.
Once roots come into balance with the top of the tree, they usually survive and grow well with rainfall alone in humid climates, or with just the turf and landscape irrigation system in dry desert climates. With infrequent irrigation, larger trees often grow poorly, die back or may be killed from lack of water because they take much longer to establish.
If trees will be irrigated regularly only until they are established, drought tolerant trees should be chosen, and nursery stock of any size can be planted. If trees receive irrigation during establishment and then regularly during the life of the tree, or if you are planting in the plant's native range and soil type, any tree regardless of drought tolerance can be planted.
Maximum tree size at planting can be determined by knowing the number of months trees will receive irrigation after planting (Table 1 below). For example, if irrigation can be provided for only three months after planting, trees no larger than one inch in caliper should be planted in USDA hardiness zone 9.
If trees can be watered for 6 months after planting, larger trees can be installed. Since root growth is slower in colder climates, irrigation during the growing season in the first twelve months would be required to establish a one-inch tree in hardiness zone 5. Consult establishment for specifics on amount and frequency of irrigation.
Table 1. Maximum tree size at planting based on irrigation capability at planting site.
| Hardiness zone | can irrigate for 12 months | can irrigate for 24 months | can irrigate for 36 months |
| 4 | 1 inch | 1.5 - 2 inch | 2.5 - 3 inch |
| 6 | 1.5 - 2 inch | 2.5 inch | 3.5 - 4 inch |
| 8 | 2.5 - 3 inch | 4 - 5 inch | 6 - 8 inch |
| 10 | 3.5 - 4 inch | 6 - 8 inch | 10 inch and larger |
