Blue Giant Hyssop
Agastache foeniculum · Lavender Giant-hyssopUSDA | Grow Zone
3-9Outdoors all year in zones 3 to 9. Anywhere colder, keep it as a houseplant. Protect it below 40°F.
- Light
- Full sun, 6-8 hours
- Water
- Every 1-2 weeks
- Humidity
- 40-60%
- Temperature
- 65-75°F (18-24°C)
- Difficulty
- Beginner
- Growth
- Moderate
- Mature size
- 3-4 ft tall, 2-3 ft wide
Agastache foeniculum, commonly known as Blue Giant Hyssop, is a stunning perennial native to North America. With its vibrant blue flowers and aromatic foliage, it attracts pollinators and adds beauty to gardens.
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Ideal temperature
65-75°F is where it grows fastest. Below 40°F cold damage starts.
Spacing and harvest
- Mature size
- 3-4 ft tall, 2-3 ft wide
- Plant apart
- 24-36 in
- Frost
- Half hardy: light frost only
Everything it needs
9 essentials at a glance
Watering
Agastache foeniculum prefers well-drained soil and should be watered deeply but infrequently.
Water every 1-2 weeks during the growing season, allowing the top inch of soil to dry out between waterings. In cooler months, reduce watering frequency. Signs of over-watering include yellowing leaves and root rot, while under-watering can cause wilting and browning leaf tips.
Light
This plant thrives in full sun, requiring at least 6-8 hours of direct sunlight daily.
Ideal placement includes south or west-facing gardens. While it can tolerate partial shade, reduced light may lead to leggy growth and fewer blooms.
Soil
Agastache foeniculum prefers sandy or loamy soil with a pH range of 6.0 to 7.0.
Ensure good drainage to prevent waterlogging, and consider using a potting mix designed for herbs or a blend of compost and sand for optimal growth.
Humidity
This plant enjoys moderate humidity levels, ideally around 40-60%.
To increase humidity, mist the leaves or place a humidifier nearby. Signs of low humidity include browning leaf edges and stunted growth.
Temperature
Agastache foeniculum thrives in temperatures between 65-75°F (18-24°C) and is hardy in zones 3-9.
It can tolerate heat but may struggle in extreme cold, so provide protection during frost.
Fertilizing
Use a balanced fertilizer with an NPK ratio of 10-10-10, applying every 4-6 weeks during the growing season.
In fall, reduce fertilization to encourage dormancy. Signs of nutrient deficiency include yellowing leaves and poor growth.
Pruning
Prune Agastache foeniculum in early spring to remove dead or damaged stems and promote new growth.
Use sharp, clean pruning shears. Deadheading spent blooms throughout the summer encourages more flowering and keeps the plant tidy.
Repotting
Repot every 2-3 years or when the plant becomes root-bound, indicated by roots growing out of the drainage holes.
Choose a pot 1-2 inches larger in diameter. Gently remove the plant, loosen the roots, and place it in fresh potting mix, watering thoroughly afterward.
Propagation
Propagation can be achieved through seeds or division.
For seeds, sow in spring after the last frost, covering lightly with soil. For division, carefully separate established plants in early spring or fall, ensuring each division has roots and shoots. Water well after replanting.
When it's unhappy
Yellow leaves
Why it happens
Typically caused by over-watering or nutrient deficiency.
How to fix it
Check soil moisture and reduce watering if necessary. Fertilize with a balanced fertilizer if nutrient deficiency is suspected.
Root rot
Why it happens
Occurs due to poorly drained soil or excessive watering.
How to fix it
Ensure proper drainage and reduce watering frequency. If root rot is present, remove affected roots and repot in fresh soil.
Leggy growth
Why it happens
Insufficient light can lead to elongated stems and sparse foliage.
How to fix it
Move the plant to a sunnier location to encourage bushier growth and more blooms.
Browning leaf tips
Why it happens
Often a sign of low humidity or underwatering.
How to fix it
Increase humidity levels and ensure consistent watering, allowing the soil to dry out slightly between waterings.
Pests (aphids, spider mites)
Why it happens
Common pests can infest the plant, particularly in dry conditions.
How to fix it
Inspect regularly and treat infestations with insecticidal soap or neem oil, ensuring to cover the undersides of leaves.
Care through the year
Spring
In spring, prune back any dead stems and prepare the soil for planting. Fertilize with a balanced fertilizer to encourage new growth.
Summer
During summer, ensure consistent watering, especially during dry spells. Deadhead spent flowers to promote continuous blooming.
Fall
In fall, reduce watering as the plant prepares for dormancy. Cut back the foliage after the first frost to tidy up the garden.
Winter
In winter, protect the plant from extreme cold by mulching around the base. Avoid watering unless the soil is exceptionally dry.
Toxicity & Safety
Agastache foeniculum is non-toxic to pets and children. It is safe to handle and consume, with no known harmful effects.
Good to know
How often should I water my Agastache foeniculum?
Water your Blue Giant Hyssop every 1-2 weeks, allowing the top inch of soil to dry out between waterings. Adjust frequency based on rainfall and temperature.
What type of soil is best for Agastache foeniculum?
Agastache foeniculum prefers sandy or loamy soil with good drainage and a pH of 6.0 to 7.0. Avoid heavy clay soils that retain water.
Why are my Agastache foeniculum leaves turning yellow?
Yellow leaves can indicate over-watering or nutrient deficiency. Check soil moisture levels and consider fertilizing if the plant appears to lack nutrients.
Can I grow Agastache foeniculum in a pot?
Yes, Agastache foeniculum can be grown in pots. Ensure the pot has drainage holes and use a well-draining potting mix to prevent root rot.
When is the best time to prune Agastache foeniculum?
Prune your Blue Giant Hyssop in early spring before new growth begins. This helps to remove dead stems and encourages bushier growth.
Is Agastache foeniculum deer resistant?
Yes, Agastache foeniculum is known to be deer resistant due to its aromatic foliage, making it a great choice for gardens in deer-prone areas.
What companion plants grow well with Agastache foeniculum?
Companion plants that thrive alongside Agastache foeniculum include Echinacea purpurea, Rudbeckia hirta, and Salvia nemorosa, which share similar light and soil preferences.
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