Citrus require 8-12 hours of sunlight each day to be healthy and productive. If your growing space receives at least 5-6 hours of full direct sun per day, supplementing with full spectrum bulbs or fluorescent plant lights can help trees perform well. IF, however, the space provides less than 5 hours per day of direct, full sun, more sophisticated grow light systems may be necessary. Are the lights you bought T-5s?

I agree with SabrinaB, one shop light won't cut it without supplemental sun light. That being said I have a 6 year old lemon tree that I've overwintered in MA without any supplemental light in the heated basement of a split level. It's not terribly bright but I leave the curtains open and it does get some sun. YMMV, but it shows that if you have a sunny window(pref south facing) that will be more help than one shop light. There is a separate forum for citrus here that may also be helpful to you. I've seen other cold weather citrus growers use CFLs with good results though. Fluorescent's need to be within inches to the plant to be useful and CFLs are much easier to position for anything other than seedlings.

I have 4 citrus trees and will be using a 2 bulb 4' T5 lighting.
Will I need to have the bulbs very close to the plant to be effective?

I've never used T5's but I hear that they throw off a good deal of heat(for a fluorescent light of course). Any light's power goes down in proportion to it's distance from the plant. So as a general rule you want lights as close as possible without causing any burn from the heat. With T8's I would had them within 1-3 inches of my plants and they loved it. Hopefully someone familiar with T5's can chime in, but I'd start around 6 inches and see how the plants react. Citrus love light and heat so they should be more forgiving than say seedlings.

I grew dwarf citrus under T5HO's a few years back for something to do, I can't remember the variety. T5's produce more heat roughly equivalent to the increase in wattage. I keep T5's 1-2" away from anything I grow, including seedlings. That being said I ALWAYS have a fan moving air. I have only had plant damage when the leaves actually touched the bulb. T5's are my preferred system for lighting even over HPS/MH.

SCG

I'm growing citrus seedlings under T8s. Currently 4 bulbs per shelf. However as they get larger I feel they will need more/better lighting, so I'm getting some narrow profile T8s so I can squeeze more light across the shelf - my current fixtures are about 7" wide so I can only get 2 of them on there; with the narrowest profile I could find, I can easily fit 5 in there for 10 bulbs total across the width of the shelf. I will hang lights vertically along the back as they get larger.

A single 2 light fixture will help, but 4 bulbs (or more) would be better. How big are the citrus plants at present? I'm guessing they shipped you something around 5" or shorter? If so you can most likely get by with the one fixture for awhile yet. More would be better.

Lowe's carries a 2.75" wide narrow profile double bulb fixture I will most likely be getting several of pretty soon here.

Metalux Narrow Profile double lamp T8 fixture

It's a tad more 'spensive than some of the Home Despot offerings - $25 compared to as low as $15 for a dual T8 fixture - but it's about half as wide, so I can put up twice as many in the same space. More lumens = more betta happy plants!

The bulb also makes a huge difference. Many of those fixtures that come with bulbs are fitted with low-lumen bulbs in the wrong color range. These are the type of bulbs you want.

EDIT: Currently the website is showing that light as "no longer available" - something that LITERALLY happened while I was typing this message - but after calling corporate headquarters I was assured that they are not, in fact, discontinuing this light. So if it shows up as "no longer available" when you go look at that link, apparently that's an internet glitch. My local store has 11 in stock and assures me they can order more at any time. So it SHOULD still be available.

Here is a link that might be useful: 4 ft. T8 32-Watt Daylight Deluxe (6500K)

This post was edited by zensojourner on Mon, Nov 17, 14 at 16:51