Constructing a new rock garden

[dewberry]

We are planning a new rock garden as an extension to an existing bed.  We’d like advice on the construction of the bed, where to buy rocks and what soil mix to use (peat-free).  We want to grow flowers which attract insects.  We live in Peebles in the Borders.

Site
South-facing front garden sloping down slightly away from pavement.  Well-drained.  Unimproved top soil. 
Originally a field.  Difficult to dig to any depth due to stones and roots from a silver birch.

Description of existing bed
Half circle (slightly oval) with straight edge adjoining pavement.  Diameter 4m.  Pavement to back of bed 3m.  Raised up to a maximum of 30cm at back supported by wooden paling which is rotting away.  End of straight edge is 1m away from front path.
The bed is full of plants and bulbs which we don’t want to move.  Harebells (campanula rotundifolia), angel’s fishing rod (dierama pulcherrimum) and lavender are growing well.
A dwarf pinus mugo is planted behind the raised bed.
Surrounded by lawn.

New bed
Extend existing bed to form approximate quarter circle with sides adjoining pavement and front path.  Radius of quarter circle 5m alongside pavement and 4.5 m alongside front path.

[Tristan_He]

Hi Dewberry and welcome to the forum!

I think the first thing you need to decide on is a more detailed idea of what sort of plants you want for your rockery. Do you want a particular look / theme etc. Different plants have different requirements, so this will need to influence your design. For example, dwarf rhododendrons, Meconopsis and dwarf lilies / Nomocharis will generally prefer an acid humus-rich soil, whilst things like Dianthus and Campanula and other mediterranean species are more likely to appreciate some lime. Size of plants is also a factor - really small cushion / mat type alpines like Saxifrages or Androsace look brilliant in a crevice garden with loads of rocks but tend to get overwhelmed among larger plants. You need to watch out for the Dierama in this regard as it likes to self-seed into crevices.

I'm a great fan of height in a rockery. It creates a wider range of different niches from exposed drier bits at the top where you could grow Sempervivums for example, and also has the potential to create cooler but still well-drained crevices where you can try trickier species. The plants also look great like this. Obviously you need more rocks and soil for this though, so it adds to the initial cost. Larger rocks also add much more impact, so if you can afford it is it worth enlisting help to manoevre them into position.

So far as attracting insects goes, most flowers are attractive to insects. The main thing to avoid is double flowers (which often don't produce nectar or pollen). It's also good to think about getting things in flower over a long season - many alpines flower around March-May so keep an eye out for things to extend the season. Cyananthus is a big star in this regard. 

Hope this helps!

Best, Tristan