Many palms can suffer from being planted too deeply. This essentially suffocates the roots and invites root diseases. As a general rule, one should always plant a palm at the same depth as it was initially grown. In some palms planting depth of just a few inches is critical.
There are a few exceptions to this general rule. Several palms, notably the Date Palms (Phoenix sp.) have both a subterranean and aerial root initiation zone (air roots). Coconut palms are another palm that have naturally evolved to shifting sands and different planting depths. If drainage is perfect, it is not uncommon and it is not a poor horticultural practice to plant these palms with the rootball below grade. The aerial roots can be below grade, but not any deeper than the height of these roots. Another exception is when you plant boxed palms. Since these palms have a "pre-established" root system, planting depth is less critical. The following has to be in place when planting palms that have the potential to send out adventitious roots higher on their trunk:
1. Positive drainage under the rootball to a storm drain if possible, or a modified French drain, or very well drained site soil is an absolute necessity.
2. Washed concrete or plaster sand backfill. No fine textured soils. This must be thoroughly "jetted" in around the rootball. Note: This is the method used in the western United States for Phoenix species..
3. Stand pipes need to be installed - preferably wrapped with a "sock" of landscape fabric covering a "U" shaped perforated pipe installed under the rootball. The ends of the pipe need to have a grated cover that allows air to circulate under and around the rootball.
4. The rootball must never be allowed to set in standing water in the planting pit. If #1 above is carried out this will not happen.
5. The rootball should be drenched with a broad spectrum fungicide after planting. Follow this up at 6 weeks after installation.
6. The palm should be planted when the soil temperatures are warm, not during cool wet winter months. The growth of the roots of most palms begin to slow dramatically when the soil temperature drops to 65 degrees F., and new growth almost totally stops below 60 degrees F.
7. Watch for grade changes after planting and never allow site soil or other fine texture soil cover the sand backfill. This will effectively seal the rootball in a poorly oxygenated area which is conducive to many secondary root diseases. If this happens it will be necessary to scrape away the soil on top and replace with more sand. You may want to consider aerating the soil also.
If the above steps are carried out then it is acceptable to plant certain palms like Phoenix and coconuts with their rootballs below grade. For other field grown palm species follow the industry standard of planting at the original grade.
