Posted: 04/10/2015 at 21:46
For me it depends on the plants....Monty recently planted 3 heleniums together. Why? Heleniums will make sizeable clumps in their first year and increasingly large year on year. 3 heleniums will make a huge clump. In this large space over winter what else can grow there?
i plant in groups and in singles. For example a salvia will make a large plant on its own. Dahlias too.....if I grew 3 dahlias together they would look odd. On average my dahlias right now are 5 to 6' and as much across. Grasses too.
Cosmos, my only concession to annuals in the garden, I grow as 3 or 5. I will grow some of the smaller echinaceas in 3's but not the larger ones....some grew 5' plus this year and 3 or 4' across. Rudbeckias I grow as groups of 5. Some of the shrubby verbenas too as a group. The shorter grasses like fescues or stipa tennuissimas are grown as groups. So, it depends on fhe plants and whether they can make an impact on their own.
i also vary plant heights......some taller ones will grow in the FRONT of the border, not very tall though. To have a change of height like this adds variety. A tall upright perennial can look superb in the middle of the border esp if seen from both sides.
Lavendar looks odd as single plants...a hedge or a group of 3 looks better. heathers too as a group. tiarellas better in 3's.
Most perennials grow enough in 2 years to be divided and then replanted as a group...hostas for example.