The GARDENA gardening expert
Give the plants plenty of time to grow first. When they are not fed, they have to extend their roots to look for nutrients. This enables them to develop their root system and grow into a more robust plant.
Thujas have their strongest root growth at two times of the year: a small growth spurt from mid-April to mid-May followed by a stronger and more important one in July and August. For deciduous shrubs, the strongest root growth occurs in June instead, with a second somewhat smaller growth spurt in July, and an even smaller third one from mid-August to mid-September.
The plants should find enough nutrients to grow in "normal" garden soil. To encourage the plants, work about 1 to 1.5 litres of garden compost per square metre into the soil around the roots at the start of June.
The first proper feed can then be given in the spring of 2016:
For spiraeas: about 1 to 1.5 litres of compost per square metre at the start of May, followed by about 1 to 1.5 litres per square metre again in mid-June, or alternatively, 30 grams of compound fertiliser per square metre at the end of May.
For thujas: about 1 to 1.5 litres of compost per square metre in mid-April, followed by about 1 to 1.5 litres per square metre again at the end of June, or alternatively, 30 grams of compound fertiliser per square metre in early to mid-May.
I do not know exactly which type of spiraea shrub you have planted but I presume that, as you have planted a hedge, it must be the spiraea arguta or the "Anthony Waterer" spiraea. To prune these shrubs, you need to cut back the stems that have flowered to encourage growth in the straight young shoots. The easiest time to do this is from mid-March to mid-May, depending on the weather, using robust secateurs or telescopic loppers so you do not have to bend down too much.