Roof iris

Questions about roof iris

Does roof iris need sun or shade?

Roof iris grows in full sun (6 or more hours of direct sun a day) and part sun (4 to 6 hours of sun, including some afternoon sun).

It wants ordinary well-drained soil, of average fertility, and a good soak in dry spells. Once settled in, it stands drought.

What zones does roof iris grow in?

USDA zones 4 to 9. It is hardy to zone 4, which means it survives winter nights down to about −20 °F (−29 °C).

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

When does roof iris flower in my zone?

In zone 5 it flowers mid May to early June. 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 roof iris be planted?

16 in (40 cm) apart, from the middle of one plant to the middle of the next. That is a little closer than its full width of 18 in (45 cm), so a group knits into one patch.

It grows 14 in (35 cm) tall. Give it the full distance from anything taller, or it will lean out towards the light.

How many roof iris plants do I need?

At 16 in apart, set out in staggered rows:

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

In a mixed border, plant it as a group of three or five, with a second group further along in the other number.

In the bed above, the planner used three in one group.

Is roof iris poisonous to dogs and cats?

Yes: mild toxicity to people, dogs, cats and horses. The underground stems (rhizomes) and leaves can cause drooling and stomach upset if eaten.

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

Is there a pet-safe plant like roof iris?

These come closest to doing what roof iris does, and no toxicity is recorded for them:

• Bluestar 'Blue Ice': the same job in a bed, the same flowering time and the same colour; 4 in taller, medium leaves, not bold.
• Spike speedwell: the same job in a bed and the same colour; 4 in taller, flowers late June to early August.
• Wood sage 'East Friesland': the same job in a bed; 4 in taller, flowers late May to late July.

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

Do deer eat roof iris?

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.

What flowers after roof iris?

Roof iris is over by early June. 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 (a recorded partner).
• Creeping thyme: from early June to mid July, 3 in tall, to stand in front of it and cover its fading stems.
• Butterfly weed: from late June to early August, 24 in tall.

What does roof iris look like in winter?

Roof iris holds some of its leaves into winter, more in a mild year or a sheltered spot, and they look tired by March. Cut the old ones off as new growth starts.