Periwinkle

Questions about periwinkle

Does periwinkle need sun or shade?

Periwinkle grows in part sun (4 to 6 hours of sun, including some afternoon sun), part shade (2 to 4 hours of sun, mostly in the morning) and shade (less than 2 hours of direct sun, or bright dappled light). It is one of the few that also put up with deep shade.

It wants ordinary well-drained soil, of average fertility, and little watering once settled. Once settled in, it stands drought.

What zones does periwinkle grow in?

USDA zones 4 to 8. It is hardy to zone 4, which means it survives winter nights down to about −20 °F (−29 °C).

Above zone 8 it struggles: the zones measure winter cold, but a plant's top zone usually means the summers are too hot or the winters too mild for it to rest.

Gardening in Canada? Use the USDA zone for your town: Canada's own zone map uses different numbers.

When does periwinkle flower in my zone?

In zone 5 it flowers mid April to late May. As a spring flower it moves about a week for every zone: earlier where it is warmer, later where it is colder.

Worked out from Hopkins' bioclimatic law: spring arrives about four days later for each degree of latitude north, and one zone is roughly two degrees. A warm spring or a cold one moves any garden two weeks either way.

How far apart should periwinkle be planted?

12 in (30 cm) apart, from the middle of one plant to the middle of the next. That is a little closer than its full width of 24 in (60 cm), so a group knits into one patch.

It grows 6 in (15 cm) tall. Give it the full distance from anything taller, or it will lean out towards the light.

How many periwinkle plants do I need?

At 12 in apart, set out in staggered rows:

• 12 to fill 10 square feet (about 1 m²)
• 11 for a single row 10 ft (3 m) long
• 38 to fill a 4 × 8 ft bed on its own

In a mixed border, plant it as threes and fives, along the front and just behind it.

Is periwinkle poisonous to dogs and cats?

Yes: mild toxicity to dogs, cats and horses. Contains alkaloids that can cause vomiting and diarrhoea in pets if eaten.

Rated from NC State Extension. Where references disagree, the more cautious rating is used.

What can I plant instead of periwinkle?

Periwinkle is listed as invasive in parts of North America, so the planner never uses it. These do the most similar job, and are native:

• Creeping woodland phlox: the same job in a bed, the same size, the same flowering time and the same colour; not evergreen.
• Robin's plantain 'Lynnhaven Carpet': the same job in a bed, the same size and the same flowering time; lavender, not blue, not evergreen.
• Dwarf crested iris: the same job in a bed and the same flowering time; 4 in narrower, lavender or white, not blue.

Matched the way the planner's Swap button matches plants: the same job in a bed, then size, then flowering time.

Do deer eat periwinkle?

Mostly not. NC State lists it as resistant to deer. No plant is proof against a hungry deer in a hard winter, but this one is low on the menu.

Will periwinkle spread?

It spreads sideways, by runners or roots; the risk of it taking over is high. Spreads by trailing stems that root as they go, and forms a dense evergreen mat that smothers other plants. Listed as invasive in many eastern and midwestern states. Check local guidance before planting.

It is listed as invasive in parts of North America, and the planner never chooses it.

What flowers after periwinkle?

Periwinkle is over by late May. These start as it finishes and carry the same spot on, so it never goes flat:

• Lady's mantle: from late May to early July, 18 in tall.
• Astilbe: from late June to mid July, 24 in tall.
• Carpathian bellflower 'Blue Clips': from late June to late August, 8 in tall.

What does periwinkle look like in winter?

Periwinkle keeps its dark green leaves all winter, so it is one of the plants that hold a bed together from November to March, when the perennials have gone.