plants use nitrogen [as nitrate compounds] for healthy growth, making proteins for the plants new structures.
if you are adding nitrogen 'compounds' in the form of plant food then it has to be when the plant is growing. generally from spring onwards until the end of the growing season.

Seaburn - what are the likely plant food trade names for nitrogen compounds, just so I can get some ready for Spring.

Dawnsaunt, are you sure you wouldn't be better using a balanced fertilizer? Nitrogen is good for green growth, but you need potash for good flowers and phosphates to help with good rooting. Poultry manure is a good organic source of all three. You can buy it in big tubs. If you want nitrogen only a good organic one is dried blood if you can still get it. Otherwise you can use sulphate of ammonia. You don't need trade names for any of these.

On any bag of fertilizer, there are usually three numbers prominently displayed (by law, in the U.S.). the first number is the percentage, by weight, of the various forms of nitrogen in the fertilizer.

Dawnsaunt, most plant feeds are a balanced NPK Nitrogen, Phosophorus and potassium balance.

I use Growmore, Bonemeal/FB&B and homemade compost. :o)

Thanks everyone. I'm just interested in plant foods for shrubs and perennials .... Now I know :)

For Billiam:
Effects of nitrogen fertilizers on plants~ Faster growth; larger, darker green leaves; more branching, especially new stems from the base, sometimes less flowering--usually because the plant is deferring maturity to grow more while the nutrients are available--and less frost tolerance, due to all the succulent new growth.
Effects of nitrogen deficiency~ Slower growth; sparse, light green foliage; old leaves turn yellow and drop off; reduced bloom, especially on plants that bloom on new growth--such as Petunias--due to insufficient bud formation; and reduced heat and drought resistance.