But for a simple request to compare two plants, it was really helpful.
I wanted to compare Rocky Mountain penstemon (in my High Country Gardens order) and Black Adder agastache (I'll divide the pot of it that I have) both of which I'm going to plant in the new strip I'm adding between the birdbath and the left edge of the potting bench curve.
Both are purple spiky flowering plants, quite similar looking, but I needed to compare bloom times, sizes, preferred conditions, etc.
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| Rocky Mountain penstemon versus agastache Black Adder |
So I asked Google Gemini to compare the two and within seconds I had this really helpful and concise information, even a chart. I think it's accurate.
Response from Gemini --
Both
Rocky Mountain Penstemon (Penstemon strictus) and 'Black Adder' Agastache (Agastache hybrid) are popular purple-flowering perennials known for their drought tolerance and ability to attract pollinators. Their primary differences lie in their bloom timing, foliage scent, and growth habit. Key Comparison
| Feature | Rocky Mountain Penstemon | 'Black Adder' Agastache |
|---|---|---|
| Bloom Time | Late Spring to Early Summer (brief, intense) | Mid-Summer to Fall (extended blooming) |
| Flower Color | Deep lavender to royal purple | Red-violet flowers from dark, smoky buds |
| Foliage | Smooth, dark green; non-aromatic | Blue-green; strong anise/mint scent |
| Hardy | USDA Zones 3–9 (very cold hardy) | USDA Zones 5–9 (prefers well-drained soil) |
| Height | 24–36 inches | 24–36 inches |
Detailed Differences
- Flowering Window: Penstemon provides a spectacular but relatively short display in late spring, often acting as a bridge between spring bulbs and summer perennials. 'Black Adder' is a "season extender," blooming continuously for several months until the first frost.
- Aromatic Qualities: 'Black Adder' belongs to the mint family; its leaves release a spicy liquorice scent when crushed, which also makes it highly deer and rabbit resistant. Rocky Mountain Penstemon does not have fragrant foliage.
- Soil Requirements: While both are drought-tolerant, 'Black Adder' Agastache is particularly sensitive to "wet feet" in winter. It requires exceptional drainage to survive cold seasons, whereas Rocky Mountain Penstemon is more adaptable to various garden soils as long as they are not boggy.
- Growth Habit: Penstemon tends to form a basal rosette of leaves with tall, vertical flower spikes. 'Black Adder' forms a bushier, more branched upright clump with multiple "bottlebrush" flower heads.