Argentinian vervain

How to recognise it

Habit

Airy, upright plant 1–1.8 m tall and 40–60 cm across, with few leaves and stems that branch at the top.

Leaf

Lance-shaped, 5–12 cm, toothed, rough and wrinkled, opposite and clasping the stem; mostly low on the plant.

Flower

Small, lilac-purple tubular flowers 3–5 mm across in short, dense spikes, together forming flat clusters of 3–5 cm at the tips.

Fruit/Seed

Four tiny nutlets per flower, which ripen in the dry clusters and fall out easily.

Stem

Square, stiff and rough-haired, thin but sturdy; stays standing in winter.

Look-alikes

The native common vervain (Verbena officinalis) is lower, 30–80 cm, with long, slender spikes of pale lilac flowers. Verbena rigida stays low, 30–50 cm, and spreads by rhizomes.

Flowering: July–October · Pruning: April

Why grow it?

• A butterfly magnet: the peacock, red admiral, painted lady, whites and the hummingbird hawk-moth take nectar from it for months.
• Bumblebees and bees visit the flowers too, well into autumn.
• Flowers from July until the first frost, longer than most perennials.
• A ‘see-through plant’: the airy stems take up little space and add height without hiding anything.
• Self-seeds, so it stays in the garden even though each plant lives only a few years.
• Copes well with drought and heat: ideal for gravel gardens and sunny edges.

Argentinian vervain: where does it grow best?

• In the wild: open grassland, roadsides, river banks and disturbed ground in South America, from Brazil to Argentina.
• Full sun; in shade it flops and flowers poorly.
• Well-drained, ordinary to poor soil: sand, loam or gravel. Wet soil in winter is fatal.
• Lovely in a gravel garden, among ornamental grasses, at the front or middle of a border and between paving stones, where it likes to self-seed.
• Allow 30–45 cm per plant, in small groups or scattered among other plants.

Goes well with

• Purple coneflower: Pink flower heads among the airy purple clusters: together a feast for butterflies.
• Mexican feather grass: A fine, swaying grass that complements the see-through plant beautifully in a gravel garden.
• Dahlia: Large flowers in warm colours that flower just as long.
• Globe thistle: Blue globes, also loved by bees and butterflies.
• Sedum: Flowers alongside in September–October and attracts the same butterflies.

Argentinian vervain: when and how to plant it?

1. Buy plants from a nursery or garden centre; never dig plants up from the wild.
2. Plant in May, after the last night frost, so it can root well before winter.
3. You can sow in February–March indoors at 18–20 °C; the seed germinates better if you first keep it moist in the fridge for 2–4 weeks.
4. Barely cover the seed and keep it moist; germination takes 2–4 weeks.
5. Plant 30–45 cm apart, at the same depth as in the pot, and water until established.
6. On heavy soil, mix coarse sand or gravel into the planting soil.

Care

Water

Rarely needed once established; it copes well with drought. Only water regularly in pots.

Feeding

Not needed: on rich soil the stems become floppy and it falls over.

Letting seed drop

Leave the faded clusters: it self-seeds and so replaces the older plants, which disappear after 2–3 years.

Seedlings

Too many seedlings? Weed them out in May or move them to a better spot; you can recognise them by the square stem and rough leaves.

Winter protection

Leave the stems standing and in hard winters put some leaves or straw over the base.

Tip: Scatter it among other plants rather than planting it in a block: the airy stems then float like a purple veil above the border.

Pruning and mowing

Pruning months: April.

When: Leave the stems standing all winter and cut them back in April, when the new shoots appear.

How:

• Leave the stems in autumn: they protect the crown of the plant and insects shelter in them.
• In April, cut the old stems back to just above the new shoots at the base.
• Want a shorter, sturdier plant? At the end of May, cut the tips back by a third.
• In summer you can cut off a few faded clusters for new flowers, but leave enough for seed.

Caution: Do not cut back in autumn: a plant cut to the ground survives a cold, wet winter less well.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Seed

Sow indoors in February–March, after 2–4 weeks of cold in the fridge.

Cuttings

In late summer, take 8–10 cm cuttings from shoots without flowers, root them in cutting compost and overwinter them frost-free.

Self-seeding

The easiest way: leave the dry clusters standing, and new seedlings will come up every spring; move them in May.

Argentinian vervain: how does it get through the winter?

Hardy to −10 °C: protect in harsh winters.

• A short-lived perennial that is only moderately hardy here: to about −10 °C in well-drained soil, but it often fails to survive cold, wet winters.
• The stiff, dry stems with their seed clusters stay standing and look lovely with frost on them.
• Even if the parent plant dies, seedlings come up in spring; that is how it stays in the garden.
• The old stems give shelter to insects.

Diseases and pests

• Winter losses: In wet, heavy soil the plant rots in winter. Plant in free-draining soil and only cut back in April.
• Too many seedlings: In a good spot it self-seeds freely. Weed out what you do not want in May; young plants pull out easily.
• Powdery mildew: White powder on the leaves in dry late summers. Harmless so late in the season.
• Slugs: They eat young seedlings and shoots in spring. Pick them off in the evening.
• Flopping: On rich soil or in shade it falls over. Choose a sunny, poor spot or cut the tips back at the end of May.

Through the year

• February: Keep the seed moist in the fridge for 2–4 weeks.
• March: Sow indoors at 18–20 °C.
• April: Cut old stems back to above the new shoots.
• May: Plant out after mid-May. Move or weed out seedlings.
• June: Optionally cut back the tips for a shorter plant.
• July: Flowering begins. Count the butterflies.
• August: Full flower. Take cuttings.
• September: Flowering. Seed ripens in the first clusters.
• October: Last flowers until the first frost.
• November: Leave the stems as winter protection.

Did you know?

• Bonariensis means ‘from Buenos Aires’, after the region where the plant was first collected.
• It is related to the native common vervain (Verbena officinalis), which grows in Belgium on verges and waste ground.
• In warm regions such as Australia and the southern United States it has run wild and is regarded as a weed.