Question
Hello,
I have a Jacaranda sapling that I just purchased and it's about 3 inches tall. I have never had a tree this small before and would like to know when I can plant it outside. I know I need to wait to the last frost, but when, and how tall should a jacaranda be before it can be subject to the elements?

Answer
Native to tropical and sub-tropical regions of South America, the two most common Jacaranda varieties, the J. mimosifolia & J. acutifolia can withstand temperatures down to 20 degrees.

There are 47 other species, not all commercially grown, and these may be more or less tolerant of the cold. It is condisered a noxious weed in Portugal, South Africa, and Australia.  Late frost depends on you area, and the USDA zones are based on that one factor.

The Jacaranda mimosifolia is rated to a low USDA zone of 7a, and the J. acutifolia only to rated to a 9.  Check your area:  

By zip code:  http://www.garden.org/zipzone/
By map: http://www.usna.usda.gov/Hardzone/ushzmap.html

It really depends on your location. San Francisco had unseasonably cold winter system push through this last weekend so I'm sure there are a lot of disappointed gardeners with dead plants, where as LA and San Diego Jacaranda owners probably did just fine.   Exposure is another consideration.  Facing south is going to be "warmer" than facing north.  My tulips will bloom 3 weeks sooner on the south-facing side of my yard.  

A deep hole is good for small seedlings - 2 times the pot size, but be sure you don't plant the tree too deep.  Soil level should be no deeper that the point where the trunk ends and roots begin.  

As to size, I have a number of seedlings (from 2" to 6" tall) in my yard right now - the danger is not so much the weather but my 60-lb dog and 10-year-old...

Hope it helps ~Marc