Butterfly Pea Flower: Growing It and Using the Blue

Grown for one thing: a flower that brews an ink-blue tea and turns magenta the moment lemon hits it. The plant itself is an easy tender vine that asks for heat and a trellis.

What it is

A twining legume from tropical Asia, in the pea family and behaving like one: nitrogen-fixing roots, pea-shaped flowers, slender pods. The common form is a deep blue single; there are also double-flowered and white cultivars, and the doubles give noticeably more petal per flower for drying.

• Vine to 6–10 ft, occasionally more in a long hot season
• Flowers 2 in across, continuously from midsummer to frost
• Zones 10–11 perennial, annual elsewhere
• Frost kills it outright — there is no hardening it off into cold

Growing it

Sowing. The seed coat is hard and repels water, which is the usual reason a packet fails.

1. Scarify — nick the coat with a nail file or rub it on fine sandpaper, avoiding the pale scar where the seed attached
2. Or soak overnight in warm water. Seeds that have swollen to roughly double are ready; ones still hard get another nick
3. Sow ½ inch deep, somewhere warm — 70–85°F
4. Three to seven days to emerge
5. Start indoors 4 to 6 weeks before last frost if the season is short

Planting out. Only once nights are reliably above about 60°F. A cold snap in the first week sets the plant back further than waiting would have cost.

In a pot it does well: a 5-gallon container, gritty free-draining mix, an obelisk, and a sunny wall. Pots are also how you overwinter one — bring it in before frost, keep it bright and above 55°F, water sparingly, and cut it back hard in spring.

Pinching the growing tips when the vine is young gives a bushier plant and more flowering stems. Left alone it runs upward and flowers mostly at the top.

Harvesting and drying

• Pick in the morning, once the flower is fully open and the dew has dried
• Picking daily keeps it flowering — letting pods set slows the whole plant down
• Use fresh, or dry for storage
• Drying: a single layer somewhere warm, dry and out of direct sun for a few days, or a dehydrator at low heat. They are dry when they crumble rather than bend
• Store in an airtight jar away from light. The colour fades in sunlight

A productive vine gives a steady handful a day through late summer, which dries down to very little — expect to need a season’s picking to fill a decent jar.

The colour

The pigment is anthocyanin, and it reads pH like litmus.

Brewing: roughly 5 to 10 dried flowers per cup, water just off the boil, three to five minutes. Longer gives a deeper blue rather than a stronger flavour, because there is not much flavour to extract — it is mild, slightly earthy, caffeine-free, and nearly always blended with lemongrass, ginger, mint or citrus.

Beyond tea it is used as a natural food dye: blue rice, cocktails, syrups, ice cream, and anything where the colour shifts on serving.

Where it struggles

• Cool summers. Without real heat it grows slowly and flowers sparsely
• Shade. Fewer flowers, longer leggier growth
• Wet, heavy soil. Root rot follows quickly
• Rich ground. Foliage instead of flowers
• The first frost. No warning, no recovery — pick everything before it

It self-seeds readily in frost-free climates, and is listed as invasive in parts of northern Australia and some Pacific islands. Where winters kill it that is not a concern; where they do not, check before planting.