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

Foliage

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.

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.

This is self-explanatory

Architecture

For definition of these highly subjective terms see section §Perennial Architecture.

To summarise:
1.0	Dicotyledons
1.1	Basal leaves dominant
B1	Clump-forming
B2	Flowers held well above clumps of lower foliage
B3	Flowers above clumps with notably large foliage
B4	Narrow or rosette basal clump, with foliage decreasing in scale further up the flowering stem; flowers held above, sometimes dramatically so
B5	Flowers above very large foliage
B6	Flexible
1.2	Stem leaves dominant on upright stems
S1	Upright, unbranched
S2	Upright, but forming spreading clumps
S3	Upright, branching stems
S4	Upright, branched, not spreading
S5	Partially upright from narrow base
1.3	Ground-covering (including some monocots)
GC1	Creeping - rooting from near ground level rhizomes as it goes
GC2	Extensive underground spread through rhizomes or roots
GC3	Prostrate - spreading from a central crown, close to the ground
GC4	Sprawling - lax stems acting as stolons and rooting
2.0	Monocots
*2.1*	Grasses and grass-like plants
GR1	Cespitose clumps, flower/seed heads not more than twice the height of the plant
GR2	Spreading clumps, flower/seed heads not more than twice the height of the plant
GR3	Spreading clumps, upright flower/seed heads more than twice the height of the plant or more important visually
GR4	Turf and other rapidly-spreading grasses
2.2	Other monocots
M1	Rosette of basal linear leaves radiating out from base of the shoot, generally arching at length
M1b	As above but broad rather than linear
M2	Basal linear leaves emerging from base of ramet in a plane, often stiff
M3	Erect stems with leaves in whorls or otherwise radiating out
M4	A stem with leaves on each side, very much more like a dicot, sometimes bamboo like

Flowers

Post-flowering

Longevity

Spread mode

Underground

Spread rate

Persistence

Clump Density

Standing

Seeding

Soil

Moisture

Light

Hardiness

Illustrations

This is self-explanatory

Refers to aesthetic value of seed heads or other post-flowering structures

Generally defined within these categories:

3 years or less
3-5 years
5 -10 years
10 + years – generally long-lived, but not necessarily
Long-lived – potentially decades
Notably long-lived – as above but noted as being exceptionally long-lived and highly persistent – lifespan of decades
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.

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:

Rhizophyte – tap root, minimal, very slow lateral spread, often non-clonal and often with definite lifespan
Rhizocaulophyte – multiple tap roots developed by multiple shoots from crown structure which slowly expands, clonal but limited lateral spread
Caulophyte – no tap roots, but rhizomes, allowing lateral spread
Phalanx – lateral spread a steadily increasing circle
Guerrilla – random appearance of new shoots at irregular intervals from parent.

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.

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.

Minimal

Slow – 1-2cms per year
Moderate – 2-5cms per year
Rapid – 5-10cms per year
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.

See §Understanding what goes on underground for full explanation.

Given for notably vegetatively spreading species only, so only for caulophtyes and some rhizocaulophytes.

Low – rhizomes often annual or short-lived, leaving gaps, usually in centre of clump
Weak – clumps solid with rhizomes that live 2 years or more, but often patchy or gappy, obvious signs of shoot die-back
High – long-lived rhizomes; solid clumps formed

This metric is closely related to the above and given for notably vegetatively spreading species only, so only for some rhizocaulophytes and caulophtyes.

Very Low – shoots very widely spaced, plenty of space for other plants
Low – shoots widely spaced enough to allow some space for understorey plants
High – very tight, no room for other plants

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.

Prostrate – naturally sprawling
Lax – lax growth tending to flop or need support, often not naturally free standing, see Architecture B6. Flexible
Weak – naturally upright but often flopping, especially in conditions of high fertility, or rain
Firm – usually stands well

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.

Almost never reported

Occasionally reported
Frequently reported
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.

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.

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.

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.

The USDA hardiness zone for minimum temperatures is used.

Other indications towards climatic tolerances are given here.

Hardiness Zone		Average Annual Extreme Minimum Temperature
3	a	−40 °F (−40 °C) to −35 °F (−37.2 °C)
3	b	−35 °F (−37.2 °C) to −30 °F (−34.4 °C)
4	a	−30 °F (−34.4 °C) to −25 °F (−31.7 °C)
4	b	−25 °F (−31.7 °C) to −20 °F (−28.9 °C)
5	a	−20 °F (−28.9 °C) to −15 °F (−26.1 °C)
5	b	−15 °F (−26.1 °C) to −10 °F (−23.3 °C)
6	a	−10 °F (−23.3 °C)) to −5 °F (−20.6 °C)
6	b	−5 °F (−20.6 °C) to 0 °F (−17.8 °C)
7	a	0 °F (−17.8 °C) to 5 °F (−15 °C)
7	b	5 °F (−15 °C) to 10 °F (−12.2 °C)
8	a	10 °F (−12.2 °C) to 15 °F (−9.4 °C)
8	b	15 °F (−9.4 °C) to 20 °F (−6.7 °C)

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.

The Guide

Calendar

Height