The Q&A Archives: How do you grow ferns from spores?

Question: Growing ferns from spores and thier care during growth

Answer: Here's how it's done:
Collect spores
Developing sporangia on the underside of mature fern fronds start out green and gradually turn brown, black, or yellow. After ripening and releasing their spores, the sporangia look fuzzy and slightly pale. Ripe sporangia have the most intense color and look firm or very slightly fuzzy. Place a portion of the fern frond having ripe sporangia in a loosely closed white paper mailing envelope and let dry at room temperature for about 2 days. Species with green spores must be sown within a day or two of being shed. Spores of most other species are viable for a year or more. Their viability can be extended by keeping the spores in the refrigerator (about 40?F).

Prepare soil
I use a 50-50 mixture, by dry uncompressed volume, of Canadian peat moss (sometimes sieved through 1/8th inch screen) and washed sand having 1/16th inch grain size. Crushed perlite of the same size can be substituted for sand. Perlite has some advantages: it holds water, it reflects light, it might have less contaminants than sand and it usually crushes instead of denting or scratching. Sometimes I add 1/16 teaspoon of ground dolomite per cup of dry peat moss to partly counteract its acidity. After adding tap water (about 50% of total dry volume), 'sterilize' using microwave (4 minutes per cup of mixture in a 800 watt oven) or pressure cooker (25 minutes at 15 psig). Pouring boiling water through the soil also works but leaves the soil a little too wet. Other media I have tried include commercial potting mix, milled sphagnum, vermiculite, crushed terra-cotta, filter paper, rockwool, agar, and weak fertilizer solutions. The peat moss/sand mixture is hard to beat.

Wash sowing containers and all utensils
I use 2.75 inch diameter by 1 inch high clear plastic fishing lure containers with stacking screw-on lids. K-mart and Pay-Less sell them. Instead of stacking, I just use the top one as a clear lid. I suppose you could save room by using them stacked with side lighting. Any plastic or glass container covered with plastic wrap would also work well. I've tried using 20mm diameter by 60mm high borosilicate glass vials with rubber lined screw-on lids but growth was slow and early transplanting was required. The obvious advantage of glass is that the soil can be sterilized while in the container. All equipment should be disinfected using either heat or clorine bleach. If it can withstand heat (fishing lure containers cannot), boil for 5 minutes. If not, soak in 10% bleach solution for 30 minutes and rinse once or twice with previously boiled water.

Transfer soil into sowing containers
Fill each container with about 1/4 inch deep of soil. One cup of soil is enough to fill 15 to 18 fishing lure containers. If the soil is warm while you transfer it to the sowing container, rising air currents help prevent contamination. It's best to do this, and actual sowing of spores, in a room that is unlikely to have airborne mold spores. My success rate seems to vary most due to this factor alone. Someday I hope to build or buy a positive pressure hood for this work; or maybe I'll disinfect the interior of a walk-in closet and use it strictly for fern sowing.

Set up sowing area
Prepare a table for sowing spores by wiping it down with 10% bleach solution. I place the following items at hand: #2 natural bristle artist's paint brush, 3 inch artist's spatula, clean smooth white paper (childrens finger painting paper) cut into 2 inch squares, and heat gun or blow dryer.

Sow spores
Separate spores from chaff by putting the shed mixture on a paper square and tapping while tilting the paper; the spores stay behind while the chaff bounces forward. Better separation can be had by sifting the mixture through a fine stainless steel mesh (40um openings). Use the spatula or brush to pick up or brush off spores into sowing container. Sow thinly so that crowding does not require transplanting (drop from about 2 inches a volume of spores equivalent to approximately 1/64th of an inch cubed). Be sure not to open the sowing container for more than a few seconds. Use the hot air gun (set to 250?F) or the hair dryer to heat the spatula and/or brush between sowings.

Place spore containers in warmth and light
I keep temperatures between 60? and 80?F by placing them about one foot below two 40 watt 'cool' or 'warm' fluorescent tubes with a white painted reflector. These can be left on 24 hours a day or turned off during warm hours to keep the temperature down.

Wait for ferns to grow
If the sowing container is sealed, no watering is necessary. The fishing lure containers are almost air-tight and only need watering about every 2 months. If watering is necessary, mist with distilled water; or if growth is yellow, add 1/4 strength nitrogen rich fertilizer. Gametophytes (prothallia) first appear usually in 2 to 4 weeks (6 months maximum for some species). The first fronds (sporophytes) appear about 6 to 12 weeks later (up to a year later for some species). Since fern fertilization requires liquid water, I usually mist with distilled water when the gametophytes are about 1/8th inch across.

Transplant baby ferns
When fronds have reached about 3/8 to 3/4 inch high, transplant using tweezers into small pots of peat moss or potting soil and keep in a mostly covered container. I transplant into .75 inch square seedling pots and place in a "mini-greenhouse" --11 x 22 x 2.5 inch plastic trays with snug-fitting clear covers.

Mold and algae growth can be controlled by lightly misting with a solution of 1/4 teaspoon Physan per gallon distilled water. Physan-20 is benzalkonium chloride; other brand names are Physan, Green-Shield, Barquat MB-50, BTC, Roccal, Zephiran, Consan, and RD-20. I usually transplant individual ferns instead of fern clumps in order to observe the growth characteristics of the individual plants. Growth and survival rates are similar with either method.

Start hardening them by removing some of the cover so that relative humidity drops to 60%-80%. Aseptic technique is less important after they are transplanted. During the first year or so, as the ferns begin to mature, they gradually start requiring the environmental conditions of the full grown plant. For unfamiliar ferns, I always transplant about 2 or 3 times as many as needed to cover losses during the first 2 years. By the end of the first year, most medium-sized ferns fill 2.5 inch pots and have fronds 4 to 12 inches long.

Save unused contents of sowing container
Most left over gametophytes and ferns can be kept indefinitely (8 years and counting) if kept in a cool moist dimly lit place. I put all leftovers in the same sealed clear plastic tackle box that is kept on a shelf about 6 feet from a window in our basement. Temperatures range from 40? to 65?F. No attempt is made to use sterile techniques. Although the plants do not grow, most stay healthy and can be transplanted at any time.

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