Question
what type  fertilizer should i use for Leyland cypress.  

when should i fertize the plants.

Answer
Leyland Cypress -- 'Cupressocyparis leylandii' to botanists -- needs just the right soil more than it needs the right fertilizer.

This is a plant that is on the sensitive side, as Trees go.  It withstands stresses much more easily when it is planted in a slightly Clay-ish soil with a high Cation Exchange Capacity and acid pH (below apx 6.2).  You must get your Soil tested so that you know what you have and what you're missing -- and then you can factor in fertilizer.

Growers generally slow-drip a dose of Ammonium Nitrate after their Cypress has been in the ground for 3 months.  To quote Jeff Lowenfels in 'Teaming With Microbes', 'Trees, shrubs and perennials prefer their Nitrogen in the form of Ammonium, not Nitrate.'  A Leyland Cypress growing as a specimen plant in the center of a large Lawn will bristle against the 'nitrate-fertilized Lawn', as Lowenfels puts it.  The BEST treatment you can provide a new evergreen, beyond excellent Soil, is a substantial dose of mycorrhizal Fungi (which you can purchase at your local Garden Center) at the transplant stage; they will form a friendly alliance with your evergreen and facilitate its speedy recovery.

Fertilize with Mycorrhizae any time; chemical fertilizer must not be done until Spring has Sprung in 2008.  Unless of course you are writing South of the Equator, in which case let me know and I will revise my answer.

It's late, and I've tried to get a lot of information into a short answer (for me).  Let me know if you need clarification on this or anything else.  Thanks for writing.