Are Earthworms Good For Gardens?
The term Earthworm refers to a specific group of invertebrates within the taxonomic phylum Annelida.
Earthworms belong to the Oligochaetes — which means 'few bristles.' Worldwide there are over 3,000 species
described. Earthworms are, as their name suggests, terrestrial, their skin is permeable and they need a moist
environment so they don't dry out.
The different species of earthworms have individual requirements just as a
dog has from a cat, some earthworm species live in compost, some live in permanent burrows deep
down in the soil, others are content with the middle ground and make complex networks of tunnels as they
explore the earth.
Earthworms and nightcrawlers are terms that are often used interchangeably. In reality, the two worms differ
in appearance, function and when and where they are seen. Nightcrawlers, otherwise known as dew worms
or garden worms, appear at night when the dew is on the grass or sidewalks.
An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a
tube-within-a-tube body plan, are externally segmented with corresponding internal segmentation. To learn more
about earthworm anatomy, click the button below.
Earthworms are hermaphrodites, meaning they have both male and female reproductive organs.
When two earthworms are ready to mate they adopt a head-to-tail position, cover themselves in a layer of
mucus, and exchange sperm. The saddle produces a mucous tube which detaches and moves forward along
the body, collecting on the way the earthworm’s own eggs and the sperm received from its partner.
Fertilization occurs in the mucous tube which is shed from the front end of the earthworm. This dries in the soil
to become an egg capsule, from which one or more young earthworms will eventually hatch. Many species can
reproduce several times a year.
We all know that a healthy compost heap is writhing with worms, working their way through the kitchen and
garden waste to help create organic matter to feed our soil—these are mostly brandling worms, recognized by
their red, stripy appearance—but I’m talking about the worms which, if your soil is good, you’ll find in large
numbers throughout your vegetable plot. How useful are they?
Well, first, they’ll be munching on leaf fragments and other plant debris that accumulate on the soil surface.
Research indicates that, having passed through worms, these bits and pieces are broken down more easily by
microbes. Result? Humus! That black gold beloved of all organic gardeners is produced more quickly with the
help of worms than by microbes alone.
Earthworms play a key role in modifying the physical structure of soils by producing new aggregates and
pores, which improves soil tilth, aeration, infiltration, and drainage. Earthworms produce binding agents
responsible for the formation of water-stable macro-aggregates.
With worms having little to do but eat, they shift a surprising amount of material. One study on forests came to
the conclusion that worms buried over 90% of the plant debris left on the soil surface. Another indicated that an
impressive forty tons of soil per hectare were brought to the surface in worm casts in the course of a year.
This is great news as worm casts tend to be very fertile, holding more calcium, iron and phosphorous than the
soil around them. They also contain significant amounts of nitrogen and help to stabilize carbon, reducing the
release of carbon dioxide from the soil. The small particle size in the casts means they can even hold more
water.
Of course, worms also burrow, and not all in the same direction. Epigeic species burrow in the litter on the soil
surface; Anecic species live in vertical burrows that can descend several feet. Endogeic species burrow below
the surface in horizontal or random lines. Aristotle described worms as "the intestines of the soil", which is a
good way to regard the passages they create. Burrows open up the soil to air, bringing oxygen to plant roots,
and create pathways for water and roots. The presence of earthworms has been shown to have a notable
effect on the amount of water a soil can hold and on how fast it sinks in.
Red Wrigglers are earthworms too and they have many properties which make them ideal for the compost bin.
Of all the worms suitable for composting, the Red Wriggler is the most adaptable and hearty. They can withstand
a wide range of environmental conditions and changes that would kill most other earthworms. The Red Wriggler's
voracious appetite makes it the champion of the compost bin and a virtual worm casting (worm
poop) machine.
Red Wrigglers are fairly small, generally getting no larger than 5 inches. They like to live in colonies, often
congregating into a writhing mass around a food source. Maintaining this close contact makes them prolific
breeders.
They can withstand a wide range of temperature extremes. Typically Red Wigglers thrive in temperatures
between 65° F and 80° F. When the temperature dips, Red Wrigglers need to be protected from freezing
weather.
There are thousands of earthworm species worldwide. These species are placed into one of three groups
based on their feeding and burrowing habits. Each group contributes to soil health in different ways, but they
all have a couple of things in common.
This group is composed of species that primarily consume plant litter and organic matter at the soil surface
and make nutrients more rapidly available for plant uptake. Epigeics are small worms with short life spans
(3 to 4 years) and high reproductive rates. During winter, they bury themselves under the litter layer to
survive, but if the top foot of soil freezes, they die.
Members of this group create continuous horizontal burrows within the soil profile and feed on subsurface
soil enriched with organic matter. These earthworms help aerate the soil as they burrow. Endogeics have
lower reproductive rates than epigeics, are larger, and have longer life spans (up to 6 years). In winter, they
burrow deeply in the ground and fall into a dormant state.
This group of earthworms, which feed on surface litter and create deep vertical burrows, incorporates organic
material into the mineral portion of the soil (below the surface) by pulling surface litter into their burrows. The
vertical burrows also improve water flow through the soil. Anecics have the lowest reproductive rate but are
the largest earthworms with the longest life span (4 to 10 years). They retreat deeper into the soil and go
dormant when cold temperatures arrive.
There are over 100 known native earthworm species in the United States. They primarily reside in
undisturbed native habitats (forests and grasslands) that were not glaciated.
Exotic earthworm species were accidentally brought over on colonial ship ballasts and in potted
plants during the 1700s through the 1940s. They thrive in and dominate large stretches of land that
have been disturbed (agricultural and urban areas).
Earthworms can process around 0.08 to 0.3 grams of soil in one day, depending on the species.
Earthworms are hermaphrodites (have both male and female reproductive organs). When two
earthworms mate, both transfer sperm to one another and grow egg cocoons in the clitellum (the
engorged region close to the middle of a mature earthworm).
Earthworm eggs hatch in about 3 weeks under favorable soil conditions.
Earthworms can live from three to ten years, depending on the species and soil conditions.
Given their activities, it’s a good idea to encourage as many as possible to hang out in your soil, so my first
suggestion is to dump the rototiller. Agricultural research has shown that the more soil is tilled, the fewer the
worms are in it and, despite rumors to the contrary, worms cut in half do not grow into two worms. It’s true
that, in some circumstances, one half might survive, but on the whole you can assume that a bisected worm
is a dead worm.
Digging also presents a clear and present danger to them, though on nothing like the scale of a rototiller,
and worm safety is one of the reasons I’m a great advocate of No-Dig beds which leave soil undisturbed. If
you must dig, using a garden fork rather than a spade is less likely to damage worms.
Bear in mind, too, that worms eat organic matter and, if supplies drop off, they will vote with their contractile
tissue and move elsewhere. They also need moisture, which is why in dry weather you’re likely to find them
in the damp soil under stones or wound into a tight ball. It’s their way of protecting themselves from heat and
dryness until conditions improve.
The best answer to keeping worms happy is to look after your soil.
Ensure that it’s well fed with compost and
spread with mulch, and don’t dig unless you have to. Not only will it be good for your plants, but the organic
matter and improved water retention will make the worms want to call your garden home.
Bilateral Symmetry
If you cut an earthworm down the centre, you would find that the left and the right
sides of its body are identical or symmetrical.
Clitellum
The clitellum is a swelling of the skin and can only be seen in earthworms that are ready
to reproduce. It may be white, orange-red or reddish-brown in colour. Earthworms are
ready to mate when their clitellum is orange. Most of the material secreted to form earthworm
cocoons is produced within the clitellum.
Pharynx
Earthworms push the pharynx out of their mouths to grasp hold of organic matter. They pull the
food back into their mouths and then wet it with saliva.
Gizzard
Earthworms do not have teeth, so they use the strong muscles of the gizzard (and grains
of sand and soil).
Intestine
Once the food is ground up, it moves into the intestine where digestive fluids continue to
break down the food so it can be absorbed.
Aortic Arches
Most earthworm species have five aortic arches or ‘hearts’ that move blood around the
body. The number of aortic arches varies amongst species.
Dorsal and Ventral Blood Vessels
The dorsal blood vessel runs along the length of the upper side of the earthworm. It
contracts and pumps blood to the aortic arches. The ventral blood vessel runs along
the lower length of the earthworm.
Cerebral Ganglion
The earthworm’s ‘brain’. Earthworms have a simple nervous system. The cerebral
ganglion is connected to a ventral nerve cord that runs the length of the body. Each
segment is connected to this cord, allowing earthworms to move and respond to light,
touch, chemicals, vibrations and more.
Circular Muscles
Circular muscles surround each segment. When earthworms squeeze these muscles,
they become long and thin. Circular muscles alternate with longitudinal muscles to help
earthworms move.
Longitudinal Muscles
Longitudinal muscles run the length of the earthworm. When these muscles contract,
earthworms become short and fat. Longitudinal muscles alternate with circular muscles
to help earthworms move.