Wednesday, February 3, 2021

Astilbe: A Horticultural Tragedy

Walk into any garden center and ask for perennial recommendations for shade. Likely the first thing to be recommended will be Hosta; the second thing will probably be Astilbe. If you're not familiar with Astilbe, they are perennials that provide essential early to mid-summer color for shaded gardens. And that summary is where our tragedy begins.

Shade tolerance is the start of our tragic tale. It's certainly true that Astilbe are shade tolerant. They're native mostly to Asia, with one species currently present here in North America, and are naturally found in forest edges, ravines, and other somewhat shady locales. Shade tolerance does not mean shade loving. They need bright light, preferably some direct sun, to perform their best. Given adequate moisture, they can even tolerate full sun in cooler climates. Placed in heavy shade they might grow fine, but are likely to bloom poorly, if at all, on less sturdy stems. Plants will likely be thin rather than dense. 

Adaptability is the next chapter of our tragedy. Most species of Astilbe are found in fairly moist habitats with rich organic soils. While they are somewhat adaptable to less wet environments and a range of soil types, they are not at all adaptable to dry gardens or clay soils. 

The best Astilbe I've seen were planted in an area that was receiving 2" of water per week from automated irrigation. They were lush and had hundreds of blooms on tall 40" tall stems; the best marketing images pale in comparison to these plants. The average homeowner, gardener, or landscaped business does not (and given future water availability concerns, probably should not) provide this amount of water. 

Most of us don't have ideal soil either; here in WI we mostly have heavy clay or sandy soils. Neither is ideal for Astilbe, which like organic rich soils. Fortunately, most shade tolerant plants also like organic rich soils, and if you plan ahead you can easily create raised areas to accommodate them. 

A special chapter of this tragedy goes to Astilbe chinensis, or if we use the currently correct name, Astilbe rubra. This chapter is titled: Drought Tolerance is a Lie. Astilbe rubra is found in equally moist habitats to other species. It tends to be more adaptable to average soils than other species, but this is NOT drought tolerance. Astilbe rubra and its cultivars and hybrids are probably the best choice for our gardens. They can go longer periods without irrigation than A. japonica or A. x arendsii cultivars and hybrids, but they will not survive actual drought conditions. In true drought conditions, plants can go weeks or even months without appreciable water. Many plants will survive drought conditions just fine. Astilbe, including A. rubra, will not. At the least they will go dormant and their health and vigor will likely be severely impacted  when moisture returns. 

Selection is the final chapter of our tragic tale. There are A LOT of Astilbe cultivars available. George Arends was instrumental in popularizing the genus, and introduced at least 74 cultivars over the span of 50 years. That quantity and pace has continued to this day. Just a quick browse of 4 wholesale suppliers turns up 70 cultivars. 17 of them are older cultivars but the bulk of the rest have been introduced in the last 25 years. 6 are new for introductions for 2021. A quick search of plant patents turns up 60 different patented cultivars. There is certainly room for a large pool of unique cultivars. But many of them are just incremental improvements over cultivars introduced prior to 1960. Slight differences in color, height, bloom count, etc. really bogs down consumers who are just looking for a great plant.  

The conclusion of this tragedy is that gardeners and homeowners are being set up to fail with a really fantastic plant. The horticulture industry needs to stop heavily recommending it to non-gardeners and beginning gardeners who are destined to fail because we don't educate them properly. The myth of drought tolerance needs to be dropped from our collective vocabulary in regards to this genus. At this point we need quality over quantity and inferior plants should be dropped from wholesale and retail offerings. To some extent this is happening; but probably not as much as needed. There also needs to be more extensive comparative trials in place, though the trend is for there to be fewer trials these days.

I love Astilbe, I intend to continue to expand the selection of cultivars we offer and trial varieties here in the garden to see which perform well for us and which can just fade into horticultural history. I'm even interested in breeding some more novel forms, but I don't know if I'll get around to that project. If you're struggling to grow Astilbe; water them more, maybe improve their soil if it's well-drained or heavy clay, move them into morning sun and afternoon shade or dappled light if they're growing in heavy shade. They will reward you with essential color in early to mid-summer. 

*a couple quick footnotes:

Astilbe crenatiloba is also a North American species, but it hasn't been found since the 1880s and is likely extinct or possibly just a variant of A. biternata

This post is absent of pictures because I've seen so few great looking Astilbe in the landscape that I've not managed to photograph them. When I *DO* see good ones it's not during bloom season! 

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