Entries are grouped by genus. Within the genus an attempt is made to provide information that is actually a learning experience, rather than the usual A-Z.
Acquiring plant knowledge takes time, and the acquisition of that knowledge can be helped by organising material in such a way as to make remembering it easier.
So if all species within a genus are relatively similar, the most frequently encountered one is chosen and described in detail, and others dealt with as variations on that.
In other cases, it is clear that there are distinct groups of species, so a genus may be divided up into groups, which may be botanically
(i.e. taxonomically) based, or on a pragmatic basis that makes sense to plant users.
In others again, species are quite disparate, so an A-Z ordering is used, but as much effort to illustrate similarities is made as possible.
Genera with large numbers of cultivars use tables to convey key information. All appropriate taxa with RHS-awarded AGMs are included.
Calendar
The timing here is an approximation of performance in north-west Europe.
Height
This is given as a range, with information culled from existing references sources, author's own information, and nursery catalogues.
Where the height of flowers is considerably greater than that of the non-flowering plant, this is noted.
Clearly, 'good' growing conditions: moisture, nutrients (notably nitrogen), and we should stress – the lack of competition typical of cultivation, will frequently result in heights at the top end of the scale.
At high latitudes, near non-stop sunshine from May to July may result in greater heights being achieved; as the late Kenneth Lorentzon observed “the best delphiniums you will ever see are north of the Arctic Circle”.
Spread
This is the least useful metric here. Clearly, for some rhizomatous species, they will carry on spreading, potentially for ever, whereas many non-clonal species will never spread much.
This is a very approximate measure of how much the plant's foliage growth will cover after three years.
The idea of spread is related to that of the planting distances typical of conventional horticulture. Since, in more ecologically-based styles, plants will be very much closer together, and the conventional notion of plant spread will, in many cases, go out of the window, I have taken quite a narrow view here of 'spread', suggesting that the information given under Spread Mode and Spread Rate will be far more useful.
Foliage
This is self-explanatory
Flowers
This is self-explanatory
Post-flowering
Refers to aesthetic value of seed heads or other post-flowering structures
Longevity
Generally defined within these categories:
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3 years or less
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3-5 years
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5 -10 years
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10 + years – generally long-lived, but not necessarily
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Long-lived – potentially decades
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Notably long-lived – as above but noted as being exceptionally long-lived and highly persistent – lifespan of decades
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Potentially long-lived – frequently fails to live out its promise because of growth habits that render it likely to fail, usually because of fragmentation of clonal growth.
Spread mode
The Łukasiewicz terminology will be used here (see - What goes on underground): Rhizophyte, Rhizocaulophyte, Caulophyte, and the terms: Phalanx, Guerrilla (See CSR theory), but in summary:
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Rhizophyte – tap root, minimal, very slow lateral spread, often non-clonal and often with definite lifespan
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Rhizocaulophyte – multiple tap roots developed by multiple shoots from crown structure which slowly expands, clonal but limited lateral spread
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Caulophyte – no tap roots, but rhizomes, allowing lateral spread
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Phalanx – lateral spread a steadily increasing circle
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Guerrilla – random appearance of new shoots at irregular intervals from parent.
Underground
Mention may be made of the presence of a crown (if rhizophyte, rhizocaulophyte) or rhizomes if caulophyte, storage structures, roots.
Any other structures pertinent to vegetative spread, nutrient storage or root foraging will be mentioned.
Spread rate
An approximate measurement of the rate of the outward spread of the base of the plant – i.e. where the shoots emerge from the soil, the vegetative propagation of the plant.
The figure is for one side of the plant only (i.e. not the spread across the diameter) and tends to represent the most rapidly growing ramets, so it is a measure that in many cases may tend to overestimate rate of spread.
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Minimal
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Slow – 1-2cms per year
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Moderate – 2-5cms per year
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Rapid – 5-10cms per year
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Very Rapid – >10cms per year
These measurements are absolute, rather than relative to the size of the plant.
So, the rapid spread of a low-growing plant will be perceived as much faster than the rapid spread of a tall, bulky one.
Persistence
See Understanding what goes on underground for full explanation.
Given for notably vegetatively spreading species only, so only for caulophtyes and some rhizocaulophytes.
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Low – rhizomes often annual or short-lived, leaving gaps, usually in centre of clump
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Weak – clumps solid with rhizomes that live 2 years or more, but often patchy or gappy, obvious signs of shoot die-back
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High – long-lived rhizomes; solid clumps formed
Clump Density
This metric is closely related to the above and given for notably vegetatively spreading species only, so only for some rhizocaulophytes and caulophtyes.
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Very Low – shoots very widely spaced, plenty of space for other plants
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Low – shoots widely spaced enough to allow some space for understorey plants
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High – very tight, no room for other plants
Standing
How well does the plant stand up in garden conditions.
Only discussed for where it is actually relevant, i.e. not for species that do not have significant taller or upright growth.
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Prostrate – naturally sprawling
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Lax – lax growth tending to flop or need support, often not naturally free standing, see Architecture B6. Flexible
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Weak – naturally upright but often flopping, especially in conditions of high fertility, or rain
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Firm – usually stands well
Seeding
Of all the metrics here, this is the most difficult to express.
The level of seeding depends on many factors, starting with the level of viable seed produced through to soil and prevailing weather conditions. There is also insufficient evidence in many cases.
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Almost never reported
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Occasionally reported
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Frequently reported
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Almost always – in most situations
Seeding may additionally be described as Extensive, so that even if it not frequently reported, there may be situations where it is potentially extensive enough to be problematic.
Soil
Most perennials are generalists, so 'average' suits them fine.
'High organic matter' generally correlates with woodland specialists which like cool soils that never completely dry out whilst being perfectly drained
Moisture
The normal range from dry to wet.
'Drought tolerance' implies that there is some hope it will cope with a Mediterranean summer, although possibly becoming dormant.
Light
The normal range of Sun, Light Shade, Shade.
Latitude makes a big difference here, most light shade plants do best in full sun at higher latitudes. The latitude taken as default is that of southern England, 51º of latitude.
Hardiness
The USDA hardiness zone for minimum temperatures is used.
Other indications towards climatic tolerances are given here.
Illustrations
Currently, these are all mine (N. Kingsbury), with drawings by Úna Scherer.
Links to images are also given, the idea here being to disseminate that small minority of website photographs that show up on image internet searches that show the habit of the whole plant, not just the pretty bits.
In some cases technical quality is poor, but this is worthwhile trade-off when truthful and comprehensive images are few and far between.