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Propagation Using Seeds

by Larry Vierheilig

Collection and Storage

Seeds of California native species are not always easy to obtain, however this is changing. As landscape interest grows, seeds are slowly becoming more readily available. I do not recommend collecting seed from the wild for a variety of reasons that I will not discuss here. If you have access to friends or neighbors who have native plants you want to propagate, collect the seed from them.

Seeds that are wind dispersed or borne in fruits that discharge the seeds upon drying should be harvested when the fruit is almost mature. The seed can then be dried in loosely covered containers or open paper bags. California poppy and lupine are two examples of plants that discharge their seeds as is California Lilac (Ceanothus).

Fleshy fruits such as Nightshade and Gooseberry should be gently crushed and the pulp containing the seeds thinly spread on paper to dry. The seeds can then be readily separated from the pulp.

If storing seed for future use, they should be thoroughly dried for several weeks. Storing at 85 degrees Fahrenheit (F) day temperature with protection from dew is recommended. After cleaning, place in envelopes or bags and label. Place the envelopes or bags in a sealed container with several moth balls for a day or two. This will kill seed-eating insects and their eggs.

Shelf-life of California native seeds is highly variable. It can vary from a few hours for Willow to several decades for Lupine. Moderate to long-lived seeds are best stored in cold, dry conditions. Most other species can be store in a cool, dry room for two or three years.

Dormancy and Treatment

Germination requires moisture, proper temperature, and oxygen. While the germination time varies with the species, most annuals wildflowers will germinate within one to two weeks. Most of the longer-lived perennials will require three to six weeks, with some requiring up to several years. Since slow and sporadic germination is usually the result of seed dormancy, treatments to break the dormancy just prior to sowing are often used.

The two types of dormancy are seed coat and embryo. Seed coat dormancy is due to the impermeable nature of the coat to water and/or oxygen. In nature, these types would remain in the soil until the seed coat weathered enough to allow penetration of water and oxygen. Some seeds require being subjected to fire before their dormancy is broken. California Lilac, manzanita, and members of the legume family (Lupine) exhibit seed coat dormancy. Methods used to break seed coat dormancy include scarification, hot water, dry heat, fire, acid, mulch, cold and warm stratification, and light.

Internal or embryo dormancy generally encompasses a number of physiological conditions. This is referred to as an after-ripening period. For some species, this may just be a period of dry storage after the seed matures. For others it requires moisture combined with high or low temperatures or both. The more common method of breaking internal dormancy is through the use of cold stratification.

In those seeds which have both types of dormancy, sequences of cold and warm stratification are required. The simulation of several years of seasons may be necessary for these types. Under natural conditions, these seeds would not germinate until several years after maturing had passed.

Scarification

This is mechanically accomplished by rupturing the seed coat using sandpaper, file, knife, or even a rock tumbler. The point is to break through the seed coat and allow moisture and oxygen to penetrate. Lupine seeds require this type of treatment.

Hot Water

This is the preferred method for small to medium-sized seeds rather than scarification. For this treatment the seeds should be dropped into about six times their volume of water at a temperature of 180 to 200 degrees F. Leave to cool for 12 to 24 hours before sowing. Do not use softened water or aluminum containers for this treatment. Both are toxic to the seeds.

For seeds with especially hard or thick coats, place the seeds in boiling water for the specific length of time prescribed for the species. Immediately place them into cold water and allow to cool. Sow immediately.

Dry Heat

Usually, this method is not compatible for use with an oven. The temperatures commonly used are in the 180 to 212 degrees F range. The seeds should be spread in shallow containers (glass) and placed into a preheated incubator or oven. Specific times and temperatures are species dependent. After the treatment, the seeds should be immediately cooled and sown.

Fire

This is a fun treatment for all of us fire-bugs. Some genera have very thick, tough seed coats and will only germinate after exposure to the heat of fire. For this treatment, the seed should be sown in the fall in a slightly moist medium. Do not water. Place a layer of pine needles, four to six inches deep, over the top of the seedbed. If using a flat, do not use plastic. For a wooden flat, place a strip of aluminum foil over the edges to prevent burning. Ignite the material; do this outside the house or garage or other enclosed area on a calm day. After the material has cooled, thoroughly water and treat as any batch of sown seeds. Even using this treatment, manzanita seeds will require at least two months to germinate. Patience is certainly required. For those seeds that require several seasons to germinate, continuing watering the seedbed until June. Then let dry through the summer. Watering can then be restarted in mid-October through the winter with germination happening, hopefully, prior to the spring.

This treatment is not exact. Variables such as amount of heat reaching the seed, and duration are subject to several factors.

Acid

This method is used to break down especially thick impermeable seed coats, such as manzanita, and gives us a chance to play mad scientist. Where is Igor when you need him? The use of concentrated sulfuric acid [H2SO4] is employed with this treatment. Seeds, if left in this solution too long, will turn to charcoal; therefore the temperature and length of time the seeds are soaked are critical. The acid needs to be used at room temperature with the period of soaking varying from a few minutes to several hours. The variability in the soak time is dependent on the genus. The solution and seeds must also be stirred occasionally using a glass rod. Do not stir too often as this will raise the temperature of the acid and may damage the seeds.

At the completion of the soak time, remove the seeds immediately and wash thoroughly in several changes of water. When the washing is complete, the seeds can be sown immediately or dried and stored for up to three months.

Since sulfuric acid is very caustic and can be dangerous to handle, do so with the use of proper clothing and gloves. Alternatives to this method are fire, hot water, and scarification.

Mulch

In this treatment, it is the microbial bacteria that breaks down or softens the seed coating. While it is slow, this is one of the methods used by nature. To employ this method, use a six to eight inch deep container and fill half way with seedbed medium. Good seed starter mixes are available at your local nursery. Lightly mist the soil medium. Sow the seeds and cover with a one inch layer of well-rotted mulch. If not available, a one inch layer of composted wood shavings can be used. However, do not use redwood or cedar shavings. If this is not available, a three inch layer of wood shavings soaked in a water/compost starter solution for three hours can be used. Keep the mulch/shaving moist but not wet. Seeds sown in the early spring or early summer and kept moist throughout the summer will require three to four months to germinate.

Cold Stratification

This method simulates the cold winter conditions. For small quantities of seed, mix at a ratio of one-part seed to three-parts moist peat moss of vermiculite and place in a sealed polyethylene bag or glass jar and place in the refrigerator at a temperature of 35 to 41 degrees Fahrenheit. For those species requiring freezing temperatures, store at 28 to 32 degrees Fahrenheit. For large quantities of seed, soak the seed for a three hours in water and place wet in a sealed container. For both methods the seed must be kept moist during the treatment. In your periodic check for moisture content, also check to see if root tips are forming. If so, remove the whole batch and sow immediately. Root tips that have become too long will be easily broken during the sowing process and reduce the survival rate.

Lengthening the stratification process will do no harm if the root tips are still very short or not showing at all. Shortening the process by even a few days may be detrimental since primary dormancy may not be broken.

Depending on the plant species, cold stratification usually lasts from a few days to three months with one to three months being the most common. After stratification is complete, remove the seeds and sow immediately before they have a chance to dry out.

Warm Stratification

This method exposes seed to moist, warm conditions at room temperature or above. The temperature employed is usually 65 degrees Fahrenheit or above. As in cold stratification, the seed should be mixed with moist peat moss or vermiculite and sealed in a polyethylene bag or glass jar. Using your imagination for warm places, these could include the top of the refrigerator, top of a VCR, kitchen cupboard near the oven, desktop, or other suitable place. Keep out of direct sunlight.

Light (Photochemical Dormancy)

Some species are light-sensitive and must receive light to germinate. The intensity and duration of light received, by the seed photoreceptors, interact with available moisture and temperature to control germination.

If germinating the seed indoors, use a cool, white fluorescent light source of 75 to 125 foot candles for eight hours a day. Do not cover the seed after sowing; just water-in. Cover the container with a glass plate or plastic to help in maintaining a moist atmosphere.

Time to Sow

Any seed treatment, including stratification, must be accomplished prior to the sowing date. Here are some approximate sowing dates:

As always there are exceptions; Dudleya and buckwheat should be sown by the end of June, Bush monkeyflower by early July, Lupine by early October, and manzanita by mid-October. The above dates will produce plants of sufficient size for fall planting and the beginning of the winter rainy season.

Germination Media

Most California native species have no unusual requirements for media. My favorite mix is equal parts of garden compost (from my compost pile), builders sand, and moist peat moss. Do not use beach sand as there are too many soluble salts contained in it. To moisten the peat moss, soak for two to three hours in water. Squeeze out the excess moisture prior to use.

To pasteurize the seed mix, spread onto a shallow pan and place in the oven at 140 degrees Fahrenheit for 30 minutes. If you spouse objects to this use of your appliances and the accompanying odor, you can lightly dust the mix with a fungicide such as “Cooke’s Fungicide” or similar product. The object is to kill the fungus that causes dampening-off.

If sowing in containers and pots that have been used before, thoroughly scrub them to remove all roots, dirt, and other foreign material. Then soak in a solution of one-part bleach to nine-parts water. For clay and wooden pots, soak for 30 minutes. For plastic, soak for three minutes. After soaking, thoroughly rinse the pots and containers to remove all bleach residue.

Sowing

This method applies to seeds sown in containers and flats. Seed germination depends, to a large degree, on proper preparation of the seed bed as well as depth seeds are covered, proper moisture maintenance, and appropriate temperatures. In preparing the seedbed, select a container(s) or flat(s) with drainage holes in the bottom. Fill with moist seed mix to within one-half inch of the rim making sure the mix is level. Firm the mix lightly with a large-surface flat object and water thoroughly. Following watering, verify that the surface is still level. Why all this level stuff? If the surface is not level, there is an excellent chance that the seed, with each watering, will migrate to the lowest spot in the container/flat. When germination occurs, all the plants will be in one, densely-packed location. This presents a real transplanting problem.

Sow the seeds in rows or broadcast evenly over the surface. The seeds can then be covered with more seed mix, washed sand, or milled sphagnum moss (sphagnum moss put through a 1/4-inch mesh sieve). Sphagnum moss is an excellent seed cover due to it’s slight acidity which helps prevent dampening-off fungus. After sowing, lightly mist the container/flat and place in a sheltered location out of direct sunlight.

Depth of sowing or thickness of seed covering is critical to the success of the germination. Too deep and the developing shoots will die before reaching the surface. Too shallow and the seeds and young shoots may die from lack of moisture. As a general rule, seeds should be covered to twice their smallest diameter. Very small seeds can just be watered-in after sowing. For protection against drying out, a piece of polyethylene film or glass can be placed over the container with a single sheet of newspaper on top to prevent heat build-up. As the seeds start to germinate, remove the plastic film/glass and newspaper. Continue to maintain the seedbed just moist. Germination for most species takes from one to four weeks.

Field Sowing

Sowing directly on the open ground usually gives the best results of any method. Moisten the soil, till in a two to three-inch layer of compost, and level. Large seeds such as lupine, sea-dahlia, manzanita, and lemonadeberry should be sown first and lightly raked in. Next sow the smaller seeds on top and moisten the area. This method will give excellent results even when the seed is sown as late as early December.

Seeds and Recommended Treatments

The above is only a small portion of a table on seeds and treatments from Seed Propagation of Native California Plants by Dara E. Emery published by the Santa Barbara Botanic Garden.

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