Fern Fronds Have Spores on their Undersides. What Is Their Purpose?

by | May 18, 2023 | 1 comment

Fern spores are small “spots” on the underside of the fronds. Have you ever noticed them and thought your fern had a bad case of scale? Unfortunately, scale does love ferns as a rule. See the pictures below.

Fern fronds with scale

The pictures below are of a bird’s nest fern that I allowed to get a horrible case of scale. By the time I discovered it, it was too far gone to save. Then, I may have allowed it to get worse for pictures. Yikes.

As you can see, the scale are everywhere on the FRONT of these fronds. There are mature ones, (the large brown ones), some that are less mature, and crawlers are present, too. Crawlers are scale that are still moving before they find the place they want to settle down on your plant. Then they attach, make that covering, and start sucking the juices out of your plant.

Check your fern fronds

If you check your fronds often, you can detect the scale early and get it under control. I use Bonide Systemic Houseplant Insect Control. I sprinkle it on the potting mix according to the label directions and water it in.

Fern spores are in a distinct pattern on the BACK of the fronds and scale are willy nilly wherever they want to be.

What are fern spores?

Fern spores are the means by which a fern sexually reproduces and are simple, one-celled, microscopic structures. They present themselves on the BACK side of the frond. They usually are in a pattern of sorts in comparison to the scale insects shown above.

Spores are in clusters, called sori (singular-sorus). The clusters of spore cases are sporangia, and these can be different shapes, such as round, oblong, kidney, or linear-shaped.

Until these spore cases are ripe and ready to release their spores, an indusium covers them, but not all ferns have these.

What is an indusium? Is it on a fern frond?

An indusium is a membrane that covers the spores and protects them. The ones I have circled below are still somewhat protecting the spore cases. The brownish, wrinkly ones to the left are old. The spores have already shed. All the spores contained on one leaf may ripen at different times.

Fern spores with indusium
Notice the covering over the spores which is an indusium

Fern spores life cycle

First, we start with the fern frond and the plant it comes from. It produces the sporangium on the underside of the frond. Some of these sporangium have a row of specialized cells around them called the annulus.

When the spores mature, the indusium shrinks, and the annulus contracts. The spores eject out into the world to make more ferns.

Do all fern fronds have spores. No. All fern fronds do not have spores. You may have a fern plant with spores on some leaves and no spores on other leaves.


When the fern spores germinate, a small heart-shaped “plant” grows, called a prothallus. This contains the archegonium and the antheridium on its underside. The archegonium contains an egg and the antheridium contains the sperm.

The key to the fertilizaion process is water. If there is no water present, the sperm cannot swim to the egg for fertilization. If it has enough water, the sperm and egg unite and a new plant can grow.

My ZZ plant below resides below a bear’s paw fern and the spores from the fern fronds covered the leaves. When I discovered it, I took it to the shower for a rinse. Imagine how well it can photosynthesize now.

fern fronds with spores
The spore covered frond above my ZZ plant below

Fern fronds appear where they want to

I have an asparagus fern in the same room with my bear’s paw fern. One day, I found that there was one growing in among the asparagus fern’s stems. I didn’t plant it there.

Spores can travel on the wind great distances outside. Inside my house, it traveled a few feet to this asparagus fern. It may have been transfered on the fur of Henry the kitty, or it may have been nearer to the bear’s paw than I can remember. Either way, now a bear’s paw fern is growing in my asparagus fern.

True fern in asparagus fern pot
This bear’s paw fern (Aglaomorpha) grew in a pot of asparagus fern that was in the same room

This holly fern, Cyrtomium falcatum below is growing at a local green house, Graye’s Greenhouse. It is growing in a crack between a wooden bench and an upturned bucket. Every time I go there, I am amazed by it.

Immature fern spores on a fern frond

This holly fern is in a small 4″ pot, and one of the fronds is beginning to develop spores. Notice the light color of the spores. As spores mature, they turn black or brown. There are some ferns with yellow spores, but I do know holly ferns turn brown.

holly fern spores
These are immature holly fern spores. Notice the color is light .

Sporeling or first fern frond

A sporeling is the first frond that emerges from the prothallus. The frond does not resemble the mature fronds of the parent plant. As the plant grows and matures, the prothallus disintegrates. The fern will begin to look like its parent plant as it sends out new fronds.


Am I going to talk about collecting spores and starting your own plants? No. If you decide that is something you want to do, I’m sure there is plenty of information about that out there. I’m going to leave that to the pros for now. Maybe someday.

I have separated a fern to propagate it, like the one below.

fern propagation
Separating a baby fern from the mother plant

I hope you learned something about fern fronds and spores today. Check your fronds often, because it could be scale, but if the spots are only on the backside, you are safe. Unless they are also on the front side ….. Just be aware there is a difference and keep your plants clean and healthy.

I hope you have a great week, plant friends!


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1 Comment

  1. Labijo

    when i was child, i usually takes anything in their undersides. Some fern also have in the upperside.


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