Triangle Palms, or Dypsis decaryi have an unusual trunk, which gives it the common name. Long fronds with delicate leaves sway in the breeze, giving it an aire of granduer. We have two, flanking the entrance to the property. They look wonderful in that space; very stately.
Also from Madagascar, and in the same group as the spindle palm, the Triangle palm is also frost sensitive the first few years of being planted out, but as it ages it grows a tolerance to withstand light frost without damage. A great palm for zone 9B with protection the first 3 or so years, and grows without it in zones 10 and 11.
Triangle palms do their best in full sun placement, and need lots of water to get established. This palm also likes extra potassium in the fertilizer, which your standard palm fertilizer should supply. You can tell if the plant is lacking in potassium by looking at the fronds. If they begin to yellow and have small brown spots, and then begin to grossly curl, your soil is lacking in potassium.
Do not mistake this for the normal dying of the outer fronds. As the Triangle palm grows, the outer fronds will slowly begin to die, but turning solid light brown over a period of a season is more natural than the lack of potassium look I described above.
Plant out Triangle palms in fall or spring when conditions are favorable. If planted in fall, the root system will have more time to get established in the new bed, without the hinderance of trying to put out new top growth. This is the reason many plants are said to be better off planted out in fall rather than spring. Spring plantings allow for both root and top growth, so although the root system will end up less established, the tree will grow more in height.
Since stronger roots make for a healthier plant, I try to plant in fall, but the choice is yours. If you run into them at an unopportune time of year, and the price is right, go ahead and buy them. They are rarely seen in nurseries and Home Improvement landscape sites.
Plant Triangle palms out in the landscape as you would the Spindle palm.