Understanding the Coefficient of Wetness

Why This Matters

We preach "right plant, right place" because matching a plant to where you're putting it makes it more likely to thrive. It uses fewer resources than constantly babying a plant to keep it happy. And, holy cow, it can make gardening much less frustrating.

Plant tags list moisture requirements, but that doesn't give the complete picture. The Coefficient of Wetness tells you more about how a plant interacts with water and soil. No matter where you garden in SE Michigan, this information helps you make better choices.

The bottom line: This rating tells you how much a plant likes to have "wet feet." Negative numbers mean wet-loving. Positive numbers mean dry-preferring. Zero means the plant doesn't have a strong preference.

The Scale: -5 to +5

The Coefficient of Wetness (CW) is an 11-point scale ranging from -5 (wet) to +5 (dry), with 0 in the middle representing species with no strong preference.

Wet

-5 -3 0 +3 +5

Dry

-5: Obligate Wetland Loves wet feet

These plants love water. They almost always grow in standing water or soils that stay wet. They're typically intolerant of drought. In SE Michigan, think of plants like Caltha palustris (marsh marigold) or Asclepias incarnata (swamp milkweed).

Best for: Pond edges, drainage ditches, rain gardens that hold water, low spots where water pools

-3 to -4: Facultative Wetland Prefers wet conditions

These plants prefer wet conditions but can sometimes grow in drier spots. They're adaptable but still lean toward moisture. Lobelia cardinalis (cardinal flower) is a great example—stunning in a wet garden but can adapt to regular moisture.

Best for: Stream banks, bioswales, low rain gardens, areas with consistent moisture

0: Facultative Flexible about moisture

These are the flexible generalists. They occur in wetlands and non-wetlands about equally. They can handle a range of moisture conditions, which makes them versatile for many garden situations.

Best for: Most garden situations, areas with variable moisture, rain gardens that drain

+3 to +4: Facultative Upland Prefers dry conditions

These plants prefer well-drained soils but can occasionally tolerate wet conditions. If your site tends toward dry, these are good candidates.

Best for: Well-drained gardens, slopes, sandy soils, areas that dry out between rains

+5: Obligate Upland Needs dry ground

These plants need good drainage and will struggle or die in soggy soils. Don't plant these where water pools or soil stays wet.

Best for: Dry slopes, sandy or rocky areas, well-drained raised beds

Key Distinctions: Don't Get These Confused!

Heads up: There are several different moisture-related scales you might encounter. They measure different things and can cause confusion if you mix them up.

Coefficient of Wetness (CW): What We Use

Scale: -5 to +5 | Measures: Where the plant naturally grows

This is a biological classification based on a plant's natural occurrence in wetland environments. Negative numbers indicate wet-loving plants; positive numbers indicate dry-preferring plants. It's part of the Floristic Quality Assessment (FQA) system and is used by Michigan Flora Online. This is the scale used in this app.

Grower Scale: A Different Thing Entirely

Scale: 1 to 5 | Measures: How much to water

Some nurseries and growers use a 1-5 scale as a practical irrigation guide, where 1 means "keep it bone dry" and 5 means "keep it fully saturated." This refers to soil moisture management, not plant biology. Don't confuse this with the CW scale—they're measuring completely different things!

National Wetland Plant List (NWPL): Letter Codes

Scale: Letter codes | Measures: Same concept as CW

The U.S. Army Corps of Engineers uses letter codes (OBL, FACW, FAC, FACU, UPL) to classify plants by their wetland affinity. This measures the same biological concept as the CW scale, just with different notation.

NWPL Code	Meaning	Roughly Equivalent CW
OBL	Obligate Wetland	-5
FACW	Facultative Wetland	-3 to -4
FAC	Facultative	-1 to +1
FACU	Facultative Upland	+3 to +4
UPL	Obligate Upland	+5

The CW and NWPL systems both describe where a plant naturally grows—they're just different ways of expressing the same biological information. The Grower Scale is something else entirely: a practical watering guide that tells you how to care for a plant, not where it comes from.

Practical Tips for SE Michigan

Reading Your Site

Before choosing plants, spend some time observing your site:

Where does water pool after a rain? Those spots can handle plants with CW values of -3 to -5.
Where does the soil dry out quickly? Stick to plants with CW values of 0 to +5 there.
Is there a slope? The bottom will be wetter than the top—use different plants accordingly.

The Shade Factor

Here's a useful trick: a plant that typically needs lots of water can often get by with less if it's in shade. Less sun means less evaporation. A wet-loving plant in full sun needs consistently moist soil, but the same plant in part shade might tolerate drier conditions.

SE Michigan Considerations

Our region has clay soils in many areas, which can hold water longer than sandy soils. A spot that looks dry on the surface might stay wet underground. This can work for or against you—clay can sustain moisture-loving plants through dry spells, but it can also rot the roots of plants that need good drainage.

Learn More

The Coefficient of Wetness is part of the Floristic Quality Assessment system. You can learn more at Michigan Flora Online or look up any plant's NWPL status at the National Wetland Plant List.

Quick Reference

Coefficient of Wetness Scale

CW	Meaning
-5	Obligate wetland
-3, -4	Usually wetland
0	No preference
+3, +4	Usually upland
+5	Obligate upland

Remember:
Negative = Wet-loving
Positive = Dry-preferring
Zero = Flexible