Native Hawaiian Garden (7722)

Plants from Seed

Before describing the methods I use to grow native plants from seed, I should describe myself. I am an impatient person. Perhaps, this is why I only became interested in plants later in life. I was a zoologist by education and training; a person more interested in moving things than stationary ones. I have the greatest admiration and amazement for people like my friend, Anna Palomino, and others who can wait month after month for a tray of seeds to sprout. I cannot. This is why I so frequently use techniques such as scarification to induce germination. And, while these methods are frequently effective, one should not begin to assume they are natural. In fact, the research of Carol & Jerry Baskin (https://bio.as.uky.edu/users/ccbask0) and others suggest scarification and other shortcuts are not the natural means of germination for many of the seeds that respond to these methods. So, why use germination shortcuts? I’m impatient.

Why grow from seed?

There are three reasons to prefer growing your native plants from seed. First, only seedlings, not cuttings or air-layers, develop a taproot. A taproot can be important for firmly securing the plant in the ground and for accessing deep moisture sources. Tree species, in particular, are far less likely to topple if they are grown from seed than from cuttings. Out in Kalaeloa where it is very dry and the water table is about ten feet below the ground, I discovered that naio and naupaka grown from seed survived much better than those grown from cuttings if they were not watered after their initial planting; cutting-derived plants required several months of supplemental irrigation to match the seed-derived plant survival percentage. Second, only with seed is each plant a unique genetic individual. In a garden setting this may not be important, maybe, not even desirable. But, for a field restoration project, genetic diversity is all-important in order to maximize the chances of the new colony surviving the myriad of environmental variables. You can use cuttings from many genetically distinct parent plants – if you have them. But, often you don’t. You may have only a handful of parent plants to start the new colony. Growing from seed (along with the cuttings from the parent plants you do have) will yield the greatest genetic diversity possible. Third, in Hawai‘i, most native plants naturally propagate themselves via seed. By propagating your own native plants from seed, you can learn about this natural process. Perhaps, even learning something that will prove crucial to understanding why the plant is becoming rare in nature or how we can maximize our success in germinating a limited seed supply from a now-dead plant or extinct colony.

There is more than one correct way to grow native plants from seed.

Nearly every native plant horticulturist has his or her favorite techniques for growing native Hawaiian plants. Some prefer to place the seeds in a tray of media under a mist system. Others prefer agar-filled petri-dishes. Still others plant each seed in its own individual pot and hand-water until germination. Below, I describe how I prepare seeds prior to planting, my three preferred methods of sowing seeds, and the germination shortcut treatments I often use.

Preparing seeds for sowing

Depending upon the plant, I may have to wash away the fleshy fruit from a seed(s) or extract the seed(s) from a dry fruit capsule or chamber. Regardless, I never sow fruits, with two exceptions: (1) plants with a stony endocarp like naio, in which case, I treat the endocarp as a seed, and (2) plants like Achyranthes and hinahina kū kahakai with very small seeds that are very difficult to separate from a surrounding thin dry fruit. After cleaning the seeds with water, I sterilize their outer surface by soaking the seeds in a 10% bleach solution for approximately 15 minutes. (Be certain to stir the solution several times so all the seeds’ surfaces are exposed to the bleach.) I then rinse the seeds several times in water to remove the bleach and allow the seeds to air dry. If I plan on storing the seeds, I place them in a labeled (i.e., species, date, collection site, number of parent plants, number of seeds) paper envelope and place the envelope in a refrigerator. If I intend to sow them, I use a variety of methods to speed or enhance their germination (see below). Which method I use is based on previous experience or readings, or is the beginning of an experiment. For specific species, you should go to that species webpage to discover which method I prefer.

Method One

For species I’ve never grown before, rare or limited seeds, seeds that germinate slowly, or seeds highly susceptible to bacteria or fungus attack, I prefer to use small clear plastic berry containers filled with vermiculite and kept under a florescent light inside my home. This way, I can keep a close eye on them, and being inside limits their exposure to harmful pests, diseases, etc. I use new berry containers or those washed and soaked in 10% bleach for about one hour. After experimenting, I’ve found vermiculite to be the best media for these situations; it is practically sterile, soft (to avoid root damage when transferring the seedlings) and holds a lot of water. If I know the seeds are particularly susceptible to fungi, I will add a quarter teaspoon of sulfur to the vermiculite (Note: Too much sulfur will cause abnormal root development.) and/or cover the seeds with a layer of moist sphagnum moss (which is known to suppress the growth of some fungi). I check the containers each day and spray them with tapwater about once a week to keep the vermiculite moist. Depending upon the size of the seed, I will sow anywhere from nine to 36 seeds per container in a regular pattern so I know exactly where each seed is sown. I sow the seeds shallow with only a few millimeters of vermiculite covering them. Only after a germinating seed has shed its seedcoat and fully developed its seed-leaves (cotyledons) do I very gently, using a small spatula, transfer the seedling to a dibble tube (see below for media mix). The seedling then stays under the fluorescent lights for a least one week to recover from the transplant before I move it to a shadehouse. At the shadehouse, I keep the seedling for another week under 75% shade before moving it to benches at 50% shade. In the shadehouse, the seedling is watered daily.

Clear plastic berry containers with and without sphagnum moss.

Method Two

For seeds I have germinated before, germinate quickly, or are not overly sensitive to harmful bacteria and fungi, I plant 1-3 seeds directly into a dibble tube filled with an approximately equal mix of clean perlite, vermiculite and peat moss (and, occasionally small cinder). These tubes are then placed in a shadehouse (50% shade) and watered daily. I prefer dibble tubes over other types of containers because: (1) they force the roots to grow deep, (2) have a small cross-section that cuts down on space needed and shadehouse weeding, (3) are easier to plant in rocky substrate, and (4) maintain most plants for a very long time (years, if necessary) without the need for repotting; the tubes slow root and stem growth once the plants reach a certain size (usually about 10 inches) and the plant exists in, essentially, a stasis phase. I purchase my dibble tubes (cone-tainers) from Steuwe & Sons, Inc. Steuwe & Sons (www.stuewe.com) has a wide line of useful nursery products.

Dibble tubes in racks.

Method Three

Two problems with dibble tubes are: (1) they can dry out very quickly if there’s a failure with the irrigation system, and (2) they are not easily purchased except through mail-order. Therefore, recently, I’ve been switching over to more traditional containers for sprouting seeds. One of my favorites is a rectangular pot, 3½-in X 3½-in X 5-in tall (see photograph). These have a number of desirable features. First, they are square – not round! Short round containers may look nice but they are notorious for causing the roots to spiral around the bottom of the pot and strangling the plant. Second, they have internal ridges that further help prevent spiraling of the roots. Third, they are deeper than they are wide. (Actually, I’d be even happier if they were 6 inches tall.) And, fourth, you can buy form-fitting trays that prevent these pots from tipping over with the wind (or the wanderings of unwanted cats in the shadehouse).

Generally, the above pots are appropriate for germinating large seeds such as those from wiliwili or lonomea. However, for smaller seeds, I prefer a smaller rectangular pot, 2¼-in X 2¼-in X 3¼-in tall (see photograph), called a ROSEPOT. Unfortunately, these do not have internal ridges. But, like the larger pots, you can buy form-fitting trays to neatly organize your seedlings and keep them from falling over. The roots of your seedling can quickly fill these small pots, so, don’t make the mistake of keeping the seedling in them too long. As soon as the plant’s roots start peeking out the holes in the bottom, you should consider transplanting it to a larger pot.

Large and small square pots for Method Three.

Soaking

Prior to sowing any seed, I soak it in tapwater for at least a day. The water should just cover the seed in order to maximize gas exchange with the air above; submerging the seed in several inches of static water could potentially suffocate the embryo. Lately, I’ve been soaking seeds for periods longer than a day, sometimes, with surprising results. For example, the four ‘akoko species I’ve grown (Chamaesyce celastroides, C. degeneri, C. kuwaleana, C. skottsbergii) all begin to germinate in the shallow water after a few days (see ‘Ewa Plains ‘Akoko for photograph of germinating seeds). Similarly, species such as maua (Xylosma hawaiiense) germinate quicker and in greater synchrony if I soak them for about one week before sowing. If you decide to soak your seeds for longer than a day, be sure to gently wash the seeds and change the water at least once every day. This will deter the growth of water-loving fungi that might harm the seeds. The longest I’ve ever soaked seeds before sowing is two weeks.

Scarification

Scarification, in its many forms, is simply a means of penetrating the seedcoat so water can easily enter the seed. For many seeds, this is all they need to get germination going. For example, plants in the family Fabaceae (e.g., koa, ‘ohai, uhiuhi, wiliwili) germinate following scarification. You can scar the seedcoat physically using sandpaper, nail-clippers or a knife. You can also scar the seedcoat by immersing the seeds in hot water. I generally soak the seeds in 180oF water and leave the seeds in the cooling water overnight to see if it had the desired effect (i.e., swollen seeds). Other native plant horticulturists I know use lower or higher temperatures with success; one horticulturist pours boiling water over the seeds held in a strainer which has the added advantage of sterilizing the seedcoats.

Scarification of wiliwili seed with nailclipper and ma‘o seed with sandpaper. Note that the scar was made on the side furthest away from the embryo (i.e., hilum).

Gibberellic Acid

Today there’s lots of information readily available about gibberellic acid (GA) on the Internet so I will not go into details of its discovery, history in research and horticulture, etc. Suffice to say, GA is a naturally-occurring chemical (actually, many different types of gibberellins have been discovered) found in plants that, in low concentrations, promotes seed germination in some species.

My interest in using GA began with experiments to quicken and synchronize the germination of ‘iliahi (sandalwood). Later, I discovered it was also effective on Scaevola species (naupaka). Based on earlier studies by others and my experiments, I settled on treating seeds with a 0.05% - 0.1% solution of GA (i.e., 50 - 100 mg per 100 mL). I normally soak the seeds in a shallow GA solution in the same manner as I soak seeds in tapwater (see above) for five or more days. After 1-3 days of soaking, I discard the old solution, gently wash the seeds, and place the seeds back in a fresh GA solution. Since GA affects embryo metabolism, you want to make sure the GA is actually getting to the embryo. Therefore, for seeds with a thick seedcoat or surrounding endocarp such as ‘iliahi, I scar the seeds before soaking.

You can buy GA online from several suppliers. I generally buy a small amount (i.e., 1-10 g) of highly concentrated powder (e.g., 90%) and store it in my freezer rather than buying premade solutions that have a limited shelf-life. Because GA is effective at very low concentrations, making GA solutions requires a sensitive scale to accurately weigh out the GA. Fortunately, metric scales accurate to 1 mg (0.001 g) are much cheaper today ($100 or less) than they were back when I started my GA experiments. Unfortunately, this decrease in price is most likely due to the illegal drug market. (It’s probably best to just smile and laugh when visitors to your home or nursery see the scale and think you might be supplementing your income by selling drugs.) The GA normally takes some time and stirring to dissolve completely. You can hasten this by first dissolving the GA in a very small volume of alcohol before adding water to achieve the correct concentration of solution.

Embryo Extraction

Sometimes the seedcoat either physically prevents the seed’s embryo from absorbing moisture or contains germination-inhibiting chemicals. One way to get around this is to remove the seedcoat and sow just the embryo. Of course, for this shortcut to be successful, you have to be very careful not to damage the embryo during extraction, and sow the embryo in sterile or pathogen-free media. Applying a light coating to or dipping the embryo in bactericide, fungicide and/or sulfur can protect it before and while it is germinating. Of course, this shortcut requires some surgical practice and considerable time, so it’s not really practical for large numbers of seeds. I have used embryo extraction with good success for Nesoluma and Sapindus.

Ā ā Ē ē Ī ī Ō ō Ū ū

Steps in embryo extraction of Lonomea (Sapindus oahuensis) embryo (Top to Bottom): scar & soak seed, remove most of outer leathery seedcoat, remove most of inner papery seedcoat, remove embryo from remaining seedcoats.