Native Plants of Hawai‘i

The difference between soil and media

Historians argue about the exact number, but they generally agree that there were anywhere between 250,000 and 1,000,000 indigenous inhabitants of the Hawaiian Islands when James Cook first visited in 1778. Shortly thereafter, thousands and thousands of Hawaiians began dying of introduced diseases like tuberculosis, yellow fever, influenza, measles, and many others; diseases, to be sure, deadly to Europeans and Asians, but not nearly to the degree that they killed Hawaiians. Why — why were these diseases so fatal for Hawaiians? If you answered, "Because Hawaiians had no inherited genetic immunity to these pathogens, unlike Europeans and Asians, who, over centuries, had developed a limited genetic resistance, thereby reducing the worst symptoms of these diseases," you would be correct!

So, what the heck does this history lesson have to do with native Hawaiian plants, soil, and planting media? Well, when Cook and all the other foreigners arrived in Hawai‘i, they didn't just bring human disease pathogens, they also unknowingly brought hundreds, perhaps, thousands, of other alien microbes, some of which are harmful to plants. And, just like the Hawaiian people, the native plants of Hawai‘i, isolated for thousands and thousands of years, do not possess the genetic immunity or defenses against many of these new harmful pathogens. Today, much of the soil in Hawai‘i is contaminated with these foreign microbes, particularly in human-occupied places. Therefore, whenever you place a native Hawaiian plant in the ground, you risk exposing it to these harmful microbes. Sometimes, the plant does just fine because of its natural physiological defenses or environmental conditions that favor the plant over the microbe. But, sometimes, that's not the case, and an alien soil microbe invades the plant causing disease or death.

When it comes to planting, we can't instantly make the plant develop immunities that sometimes take hundreds or thousands of years to be acquired. So, the best we can do is provide the plant with the best environmental conditions (e.g., proper light and water, well-draining soil) so it has its best chance of fighting off any microbial attack. Alternatively, we can avoid the risk altogether by keeping our plants out of the ground and, instead, growing them in a container filled with media free of these harmful microbes. In the following section, we describe the types of (usually) microbe-free media we have used and prefer.

Types of media we like

Media (: plural; medium: singular) is material used to fill containers for plants. Media can vary from one to several material mixes. Most growers have found that using a combination of two, three, or more media has greater benefits than using a single medium, which is often reserved for seeds, cuttings, or plants with unique environmental needs.

Media can be divided into two large groups, organic and inorganic. An organic medium contains ingredients made of large carbon-based molecules like cellulose that decompose over time into smaller molecules like carbon dioxide. Examples include: sphagnum peat moss, coconut coir, and compose. An inorganic medium contains ingredients made of usually small non-carbon-based molecules that do not decompose into even smaller molecules; their chemistry remains pretty much the same over time. Examples include: perlite, vermiculite, and volcanic cinder. Both organic and inorganic media have their advantages and disadvantages, which we will elaborate on in the descriptions below.

Sphagnum Peat Moss

Sphagnum peat moss is sphagnum moss collected from peatlands, bogs, mires, moors, or muskegs, primarily in Canada and Northern Europe. It is then dried, milled, and pasteurized after harvesting. Pasteurization kills any harmful microbes that might be in the peat moss.

In addition to being nearly sterile, sphagnum peat moss has many desirable characteristics, such as:

It can absorb and hold up to 20 times its weight in water.
It contains phenol, an antiseptic that kills bacteria.
It has a cation exchange capacity of 90–140 meq/100g, which permits it to hold onto mineral nutrients for later release.
Its fibrous texture allows plant roots to spread easily, promoting strong root development.
It creates air pockets, which improve drainage and allow plant roots access to oxygen and water.
It is lightweight.
It is resistant to decomposition, thereby slowing nitrogen depletion.

It is often claimed that amending with sphagnum peat moss can decrease soil pH. However, most peat moss sold in garden centers is neutral or only slightly acidic. Only Canadian sphagnum peat moss has a low pH of 3.0 to 4.5 and will effectively reduce soil pH. However, peat moss will not acidify soil containing minerals like limestone (i.e., alkaline soil). For alkaline soil, sulfur is the best amendment for acidifying.

Sphagnum peat moss is often the main ingredient in commercial mixed media such as SunGro Sunshine #4®, a mix of peat moss and perlite. Avoid buying store brands of "peat" without first reading the ingredients list. Many are not pure sphagnum peat moss or have additives such as fertilizers, water retainers, etc. It is better to add amendments yourself in order to produce the results you want.

Peat versus peat moss versus sphagnum peat moss: Peat is a more general term for materials harvested from bogs which can include things like dead tree leaves, stems, etc., in addition to moss. In contrast, peat moss is generally synonymous with sphagnum peat moss (although you should always check the product label) and is peat made almost entirely from sphagnum moss.

Peat, including sphagnum peat moss, is currently being harvested at a non-renewable rate, and, therefore, some gardeners have switched to other organic media similar to peat such as coconut coir.

Coconut Coir

In the past, when coconuts were harvested, the husk was considered a waste product to be discarded until people realized it had numerous applications in gardening and home products. Today, coconut coir (also called coco coir) is a popular medium for hydroponics, container gardening, and soil amendment.

There are two types of coconut coir. Brown coir comes from mature ripe coconuts — it is strong but relatively inflexible. White coir comes from pre-ripe coconuts — it is much more flexible, but not nearly as strong as brown coir. Nearly all the coconut coir used for hydroponics and container gardening is brown coir.

The process of making coconut coir involves multiple steps that include soaking the husks in water, then drying them for over a year, followed by chopping and processing the coir into pith, fiber, or chips. The soaking can be done either in tidal waters or freshwater. However, if done in tidal waters, the coir takes up a lot of salt that needs to be flushed out by the manufacturer at a later stage.

There are several benefits when using coconut coir for container plants:

It can absorb and hold up to ten times its weight in water.
It has a neutral pH.
The fibers create air pockets that promote strong root growth.
It has antifungal properties.
Unlike peat moss, which breaks down, coir breaks down very slowly and can be used more than once.
Many insect pests will not "make a home" in coconut coir.
It is a repurposed waste product from a renewable resource, unlike peat moss.

Coconut coir also has its disadvantages:

It is inert, or, in other words, it contains no nutrients. Therefore, you will need to fertilize your plants more often than with some other types of media.
It is most often sold as dry compressed bricks. Therefore, you need to rehydrate the coir before you can use it. However, this is not difficult!
It behaves like a sponge, so it can appear wet even when it does not hold enough water for the plant. Overwatering can also happen if the coir is watered too frequently.
The quality of coconut coir can vary a lot. It needs to be aged properly, kept free of pathogens, and thoroughly leached of salt if it was soaked in tidal water. Choose a manufacturer that has a trusted reputation.

While both of us have used coconut coir on occasion, we prefer sphagnum peat moss for container plants. The research is mixed as to whether seedlings sprout and grow better in a peat moss mix compared to a coir mix; we just do not have enough experience with coir to give our opinion. We don't like the fact that it is harder to tell when coir is dry compared to peat moss, and this makes watering a bit trickier. Some gardeners believe coconut coir really does behave just like a sponge and can pull water away from a plant's roots, which, of course, could be bad. Lastly, we have no counter to the non-sustainability argument against sphagnum peat moss and for coconut coir, but can only hope harvest methods change for the better (like they did for timber in Canada, Europe, and the USA).

Compost

Compost is a mix of ingredients used to improve the physical, chemical, and biological properties of soil. It is very, very rarely used for container plants. Compost is normally made by decomposing plant and food waste, as well as other organic materials such as manure. The resulting mix is rich in plant nutrients and beneficial organisms such as bacteria, protozoa, nematodes, and fungi which compete and prey upon harmful organisms and microbes. It also improves soil structure by increasing air spaces and water retention. By improving soil fertility, compost can reduce the dependency on commercial chemical fertilizers.

Sphagnum peat moss. Coconut coir block (Photograph courtesy of D-Kuru - Own work, CC BY-SA 3.0). Rehydrated coconut coir (Photograph courtesy of Bander Al-Shakhouri - Bander Al-Shakhouri, CC BY-SA 2.5).

Perlite

Perlite is made from a type of obsidian volcanic glass that is, gray to black, dense, and heavy at its early stage of creation. The glass is heated to an extremely high temperature (1,560 to 1,650°F; 850 to 900°C), which causes the water trapped inside to vaporize and escape. This, in turn, causes the glass to expand up to 20 times its original volume, while the remaining trapped air changes its color to a brilliant white. (Perlite is sometimes described as "volcanic popcorn," a shorthand description for this creation process.) The new material, perlite, is much lighter in weight (and color) than the volcanic glass it was made from and has numerous crevices and cavities that can hold tiny water droplets. While perlite can be easily crushed between your fingers, it does not crumble under the light pressure exerted on it by other medium or soil particles. It also does not decompose or shrink and is clean and sterile. The exact chemical composition of perlite varies slightly, depending on the source of the original volcanic glass; however, it is usually composed of about 75% silicon dioxide.

Perlite has numerous horticultural benefits:

It improves drainage. Perlite performs like tiny rocks in a media mix or the soil, helping water drain through it.
It improves aeration (i.e., oxygen), which is good not only for the plant's roots but also for beneficial nematodes, bacteria, etc.
It prevents media or soil compaction, resulting in better root development, as well as a reduced chance of waterlogging which can lead to root rot.
It retains water in its tiny pores that roots can access.
It has a neutral pH of 6.6 to 7.5.
Its non-organic composition does not provide a favorable environment for most pests or diseases.
It is durable and does not degrade over time.

And, a few disadvantages:

It does not hold onto nutrients very well, requiring more frequent fertilization.
It can be dusty when handled dry. While perlite is safe to use, it is best to wear a dust or particle mask when working with dry perlite as the tiniest particles may cause respiratory problems. Many gardeners will initially wet the perlite before handling it extensively to reduce this problem.
It is made from a non-renewable resource.

You can buy perlite in different grades (i.e., particle sizes), from Fine to Super-coarse. There are also different types of perlite:

Horticultural perlite, usually sold in small quantities, is made primarily for propagation and container plants.
Agricultural, usually sold in large quantities, is used for amending soil.
Industrial perlite is: used to make lightweight plaster, cement, and ceramics; for insulation and commercial filtration; added to cosmetics and cat litter.

Whenever purchasing perlite be sure to buy the proper type. Horticultural perlite is nearly always clearly labeled as "Horticultural Perlite."

We routinely use perlite.

We use either 100% perlite or perlite mixed with another sterile or pasteurized medium such as vermiculite or peat moss to germinate seeds. For tiny seeds, use a fine-grade perlite; a medium or coarse grade is good for larger seeds.
We use either 100% perlite or perlite mixed with vermiculite to root cuttings.
We use medium- or coarse-grade perlite as part of a media mix for container plants to increase drainage. Depending on the plant, the mix can be anywhere from 25% to 50% perlite.

Vermiculite

Vermiculite is a naturally occurring magnesium-aluminum-iron layered silicate with water held between the layers. It is mined in countries around the world, including the USA, South Africa, Australia, Russia, and Brazil. Its name comes from the Latin "vermiculare," which means to breed worms, a reference to how it is made. After separation and sorting, the silicate is heated to an extremely high temperature (about 1,000°C), a process called exfoliation, causing it to expand into long strands resembling worms. It is then graded by particle size (Extra-fine to Coarse) to be used in horticultural, agricultural, construction, packaging, and auto manufacturing.

Vermiculite has numerous horticultural benefits:

It improves aeration in a media mix or soil, providing more oxygen to a plant's roots.
It can absorb and hold 3 to 4 times its weight in water, increasing moisture retention in a media mix or soil.
It holds onto essential nutrients such as potassium, magnesium, and calcium, thereby reducing fertilizer leaching.
It has a near-neutral pH.
It is inorganic, non-toxic, sterile, and deteriorates very slowly.
It is great for seed germination or rooting because the seedling or rooted cutting can be pulled from the vermiculite with minimal damage to its root system when transplanting.
It is very lightweight.

And, a few disadvantages:

It can hold too much moisture for some plants.
It can foster alkaline reactions that raise the pH in the root zone.
It is a nonrenewable resource.

Perlite versus vermiculite?

The biggest difference between these two is that perlite primarily helps increase drainage, while vermiculite primarily increases water retention. Therefore, use more perlite in your mix for plants that do better with a media that dries out between waterings, and more vermiculite when you are growing plants that prefer an always moist mix.

Is vermiculite safe?

Many years ago there was an isolated case of a mine where asbestos had formed alongside the vermiculite and contaminated it. This mine, located in Libby, Montana, USA, was closed in 1990. Today, the vermiculite sold is asbestos-free. However, when handling vermiculite, particularly the finer grades, it is smart to wear a dust or particle mask because the tiniest particles may cause respiratory problems. Many gardeners will initially wet vermiculite before handling it extensively to reduce this problem.

We routinely use vermiculite.

We use either 100% vermiculite or vermiculite mixed with another sterile or pasteurized medium such as perlite or peat moss to germinate seeds. For tiny seeds, use a fine grade vermiculite; a medium or coarse grade is good for larger seeds.
We use either 100% vermiculite or vermiculite mixed with perlite to root cuttings.
We use medium- or coarse-grade vermiculite as part of a media mix for container plants to increase water retention. Depending on the plant, the mix can be anywhere from 15% to 33% vermiculite.

Volcanic Cinder

Volcanic cinder is created when gas bubbles within molten lava rapidly burst and cool upon exposure to air during an eruption, giving the resulting lightweight porous rock, often called "scoria" or cinder, its characteristic texture and appearance. Unlike here in Hawai‘i, volcanic cinder is not commonly sold in mainland garden shops. In Hawai‘i, it is often sorted by size and color and sold: to decorate landscapes (primarily using red cinders), for horticultural use (primarily black cinders; aka Dark Cinder), or for soil augmentation (primarily unsorted cinders). Cinder is also sometimes used for pathways, fire pits, and leach fields.

It should not surprise the reader that volcanic cinder is one of our favorite container media for native Hawaiian plants, given that the majority of native plants, particularly those growing on Maui and Hawai‘i Island, are adapted to survive in lava-based substrate.

Volcanic cinder has other properties that make it a good medium for containers:

It improves drainage.
It increases aeration (i.e., oxygen for roots).
The tiny pores retain moisture that the plant's roots can access.
It is heavier than other similar inorganic media like perlite, which helps prevent containers from easily tipping over. And, most cinder does not float or blow away.
It contains essential minerals for plant growth, such as potassium, iron, calcium, and magnesium.

Volcanic cinder also has a few disadvantages.

It is not sterile or pasteurized like perlite, vermiculite, or sphagnum peat moss. Therefore, there is the potential of introducing harmful microbes into the media mix. To minimize this risk, never repurpose cinder that has been used as groundcover, etc., as a medium for container plants.
It is a rare material almost exclusively harvested from Hawai‘i Island and is slowly being depleted.

Red versus black cinder?

Overall, red cinder is heavier than black and has fewer pores and air pockets. Therefore, red cinder may not retain moisture quite as well as black cinder. However, it is important to remember that cinder exists on a continuum of color and density; it is possible to find light porous red cinder, just as it is possible to find less porous heavy black cinder. There are numerous anecdotal claims that black cinder is better than red cinder for container plants because of chemistry (e.g., "Red cinder contains too much iron.") We have not been brave enough to scientifically test these claims, and have always opted to use black cinder for all our horticultural needs. If anyone knows of a scientific study that answers this question, we would be very grateful if you contacted us (EickhoffandKoebele@gmail.com) so we can share it with everyone.

Sand & Crushed Coral

We only occasionally use coral sand or crushed coral as a medium for coastal native plants that appear to benefit from the chemistry of these media (e.g., alkaline pH, high calcium and magnesium). Other growers, particularly those outside Hawai‘i, will sometimes use silica-based sand to improve the drainage and aeration in a media mix or the soil. Here in Hawai‘i, we have better alternatives, like black cinder or perlite.

If you decide to use coral sand or crushed coral, be sure you buy it from a local garden or home improvement store. While it may be tempting to collect sand or coral rubble from the shoreline, it is illegal to take sand or coral from any beach or shoreline in the State of Hawai‘i [Hawaii Revised Statutes].

Course and medium grades of perlite. Extra-coarse vermiculite. Mixed grade black cinder. You can buy different grades (particle sizes) of perlite and vermiculite, anywhere from Fine to Super- or Extra-Coarse. Black cinder is also sold in different sizes and types. However, it is difficult to buy a select grade of black cinder in small quantities such as a ¾ cubic foot bag.

Native Soils

We rarely use soil directly from the ground as a media component in our plant containers because of the possibility of introducing harmful microbes, and because many lowland soils contain very little organic matter and drain water poorly. However, we do have a few exceptions to this "rule."

Some native plants establish a symbiotic relationship with certain soil microbes. For example, many Fabaceae, such as koa, have a symbiotic relationship within the nodules on their roots with nitrogen-fixing bacteria in the soil called rhizobia. Also, many, many plants improve their water and nutrient intake by having roots connected to underground fungi called mycorrhizae.

For native Fabaceae, as well as other native plants we feel are doing poorly in their containers, we sometimes go to where a wild (or occasionally cultivated) plant is growing in a native forest or along a native coastline and collect a handful of soil directly beneath the wild plant. Back at home, we then sprinkle or mix some of this soil on/into the media inside the container hosting our native plant with the hope of inoculating the media with beneficial rhizobia or mycorrhizae. The inoculation doesn't always work to improve the health of the plant, but, in our experience, it works often enough to be worth the effort.

One of us (Eickhoff) is fortunate enough to have a long-term successful residential native Hawaiian garden where fallen leaves, twigs, flowers, and fruits are not raked up and removed but rather permitted to naturally decompose into a native compost. This compost naturally recycles nutrients back to the native plants, improves water absorption and retention, and promotes the growth of mycorrhizal fungi that further help the native plants take up water and nutrients and compete with harmful microbes in the soil [Koko 2021]. To date (2025), using soil from this "natural" native garden as a media component for container plants has proven to be equally beneficial for all the same reasons (i.e., fertilization, water absorption and retention, promoting mycorrhizae).

Ectomycorrhizal mycelium (white strands) associated to Picea glauca (larger brown) roots. Photograph courtesy of André-Ph. D. Picard CC BY-SA 3.0.