What are Indicator Plants and what can we learn from them?

Indicator Plants are a bit like the canary in the coal mine, as they can warn us about issues with our soil.
Plant indicators (or indicator species) are plants that have very specific demands for it to thrive or have an elevated tolerance against something other plants do not. Armed with this knowledge we can make informed choices on the use of our soil.

Certain plants have some, or more, very specific environmental conditions that must be met for them to thrive optimally, these plants are called plant indicator or indicator species. Plant indicators can assist in determining the best use of the local soil.

There are plants that can alert one to the presence of a particular mineral or metal and can be valuable information for owners of land and geologists.
This is called geobotanical prospecting.

The Ocimum centraliafricanum, (the copper flower or copper plant) a perennial herb found in central Africa is a good example of geobotanical prospect plants. It is used by geologists that are looking for copper and nickel as it can tolerate a very high concentration of those metals in the soil.
Photo: H. Zell / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
The copper flower
The Ocimum centraliafricanum, (the copper flower or copper plant) is a perennial herb found in central Africa. It is a good example of a geobotanical prospect plant. It is used by geologists that are looking for copper and nickel as it can tolerate a very high concentration of those metals in the soil.
Photo: H. Zell / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

They help point out soil and water deficiencies and act as plant watering indicators


Soil structure plant indicators

Some plants, often only thought of as weeds, thrive under very specific conditions. Instead of thinking of such weeds as a problem, it might actually make more sense to change one’s mindset. What do they tell us about the conditions of the soil? Instead of focusing on simply removing them.

If you have Dandelions in your garden then that is a sure sign that your soil is very compact and improving the quality of your soil is what you should focus on. The Dandelions will not thrive in great soil.

Turn your problem into your solution

Sandy soil plant indicators

Sandy soil contains, as expected, a large proportion of sand and little clay or organic matter. It is:

  • light
  • warms up quickly
  • dry
  • often acidic

Furthermore, rain tends to wash out nutrients, so this soil type is low on these.

plantlatinsoil types
Australian PineCasuarina equisetifoliaprefer sandy soil, but adaptable
Bayhops / Beach Morning GloryIpomoea pes-capraesandy soil, salt tolerant
MustardBrassicaceaesandy soil
Chinese Wolfberry / Himalayan GojiLycium barbarumsandy soil, but adaptable
PanicgrassPanicumsandy soil


Silty Soil plant indicators

Silt is granular material of medium-sized particles, that is well-drained and holds moisture well. Silt soil has a high fertility rating, but since the particles are rather fine, they can easily be washed away with rain or become compacted.

plantlatinsoil types
New Zealand flaxPhormiumsilt, swamp, but adaptable
WillowSalixthrives best in silt, tolerates water around roots
BirchBetulathrives best in silt and sandy soil
SnowdropsGalanthussilt, sand, but adaptable
DogwoodCornussilt, but adaptable

Clay soil plant indicators

Clay Soil is a heavy soil type with high levels of nutrients. Only plants that can tolerate their roots being submerged in water can survive growing in clay soil, and those who tolerate wet and cold winters and also can resist dry periods in the summer.
Sandy soil does not transport water easily up from deeper layers through capillary action.

plantlatinsoil types
Cogongrass / Kunai GrassImperata cylindricaclay soils, but adaptable
Plantains / FleawortsPlantagoclay soil, compacted, sour soil
Vetiver / SunshineVetiveria zizanioidesclay soil, can survive deep water flow

Compacted soil indicator plants

Some plants thrive in areas that are compacted by, for instance, animals or people walking over an area or use of heavy machinery.

Quite a few plants can indicate more than one category.
Canada thistle - (Cirsium arvense) is one of them.
Compact soil and rich nitrogen soil
Quite a few plants can indicate more than one category.
Canada thistle – (Cirsium arvense) is one of them; Compact soil and rich nitrogen soil
plantlatinsoil types
Creeping Thistle / Canada ThistleCirsium arvensecompact
Large PlantainPlantago majorcompact , disturbed
Field HorsetailEquisetum arvensecompact, poor growing conditions
DandelionTaraxacum ifficinalecompact, potassium poor soil, calcium rich

Soil quality plant indicators

The main bulk and most important nutrients plants demand are; nitrogen (N), phosphorus (P) and potassium (K). These make up the trio often referred to as NPK. Other important nutrients are calcium, magnesium and sulfur.

Nitrogen plant indicators

Nitrogen is essential to photosynthesis because chlorophyll requires it. Without it, the plant can’t transform sunlight energy to make sugars from water and carbon dioxide.

If you observe a lot of nitrogen plant indicators, then that is an alarm. It is very important to be aware of this. It might also be useful to know if the soil is really nitrogen-rich.

Stinging nettle, not only useful to indicate that the soil most likely is a nitrogen-rich soil, but have many uses from soup to natural fertilizer
Stinging nettle, not only useful to indicate that the soil most likely is a nitrogen-rich soil, but have many uses from soup to natural fertilizer
plantlatinnitrogen
CloverTrifoliumpoor
Lemon Thyme / Broadleaved ThymeThymus pulegioidespoor
HawkweedHieraciumpoor
VetchesViciapoor
Creeping Thistle / Canada ThistleCirsium arvenserich
Stinging NettleUrtica dioicarich
White GoosefootChenopodium albumrich
ElderberrySambucus nigrarich
Winterweed / ChickweedStellaria mediarich
Wild Chervil / Mother-DieAnthriscus sylvestrisrich
Common HogweedHeracleum sphondyliumrich
StarflowerBorago officinalisrich
Common Mallow / ButtonweedMalva neglectarich
Catchweed / Sticky WillyGalium aparinerich
European Black NightshadeSolanum nigrumrich

Phosphorus

Phosphorus is a vital nutrient for plants. These are some of the functions phosphorus contributes to or does on it’s own for plants:

  • photosynthesis
  • strong root growth
  • sugar metabolism
  • cell development
  • genetic information transfer
  • enhances fruit, vegetable, and grain quality
  • vital to seed formation
  • augments the efficiency of nitrogen, and other nutrients, in plants
  • increases disease resistance
  • improves flower formation and seed production

Signs of phosphorus deficiencies as indicators

After hours and hours of research, I’ve found no specific plant that thrives in phosphorus-poor soils. If you know of any please let me know so I can update this.
The best option is to study the plants and look for these indication symptoms in your plants, but do be aware that these signs can also be symptoms for more than one problem with your plants and their environment.

  • overall stunted growth
  • smaller leaf sizes
  • fewer leaves
  • darkening of leaves
  • dark, purplish coloring on the leaves
Example of purplish and darken leaves.
Example of purplish and darken leaves.

Do note that the only highly accurate method is to do a soil test.
This will naturally not be free, but many might find the cost makes sense.


Moist and wet plant indicators

plantlatinsoil types
GoldenrodSolidago sppmoist
Sow ThistleSonchus arvensismoist, fertile
Dwarf MallowMalva neglectamoist
MossesBryophytawet
Speedwell IvyVeronica hederifoliamoist
Corn SpurreySpergula arvensismoist
Purple Dead NettleLamium purpureummoist
Field HorsetailEquisetum arvensewet, standing water
Common KnotgrassPolygonum avicularemoist, wet
ColtsfootTussilago farfarawet, clay
Lesser Swine CressCoronopus didymusmoist
Cabbage ThistleCirsium oleraceumwet lowlands
Marsh FoxtailAlopecurus geniculatusmoist
Examples of moist and wet plant indicators

Indicator trees

Certain trees are a bit selective as to where they will grow, and if you see any of these and they are fully mature then that is also a good suggestion that the soil have been fairly stable in the soil where the tree grows.

If your aim is to grow certain trees in an area then observing what trees already thrive in an area will suggest which trees one can successfully grow.

treelatintarget
Black AlderAlnus glutinosawet
WillowSalixwet
MapleAcermoist to very wet
BirchBetulacool, moist
Black Gum (Eucalyptus)Eucalyptus aggregataswampy, wet
Eastern RedcedarJuniperus virginianadry, limestone soils
Bur OakQuercus macrocarpadry

Geobotanical prospecting (mineral plant indicators)

Geobotanical prospecting is the act of searching for mineral deposits by observing certain plants that either have a higher tolerance of heavy metals or actually require these metals to survive. These plants are called metallophyte.

plantlatintarget
Alpine PennycressThlaspi caerulescenscadmium, zinc
Zinc VioletViola calaminariazinc, lead
Spring SandwortMinuartia vernalead, copper
Sea ThriftArmeria maritimacopper
Scurvy-grassCochleariasalt
Common BentAgrostis capillariscopper, zinc, lead, nickel, arsenic
PlantainPlantago lanceolatachlorine, magnesium, potassium, and sodium
Examples of geobotanical prospecting plants

Some of these plants may be used to aid cleaning up contaminated soil (Phytoremediation)


Over to you

This is by far the whole list of indication plants, they’re just for inspiration. I hope this can change your questions from “How can I get rid of this plant?” to “Why is this plant here?”

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