Backyard Imu, Northwest Style

A jealous BBQ looks on as salmon roast over coals and the imu is unveiled.

When my kama'aina* daughter turned one, part of what fed the party was pork, taro, and sweet potato cooked in an imu, the Hawaiian word for an underground oven. In a little community garden on the flank of Pu'owaina (Punchbowl) in the middle of Honolulu, a group of Rotuman and Tongan guys helped me through the process, and the results were delicious.

The imu begins with a shallow bowl dug into the ground. I cannot explain the physics of it, but I'd bet that this shape turns out to be a very efficient one in terms of both heat loss and storing heat in the ground. Also, this shape can accomodate a few pieces of taro or a whole hog without a lot of adjustment.

But let's not get side-tracked. And really, what self-respecting Hawaiian would make an imu without counting on feeding a bunch of people?

An earth oven is a way to go whole hog, to cook hundreds of pounds of food if need be. You need a shovel, enough rocks to cook the food, and enough fire-wood to get those rocks red-hot. Besides the food, you need leaves or foil to wrap it, more vegetation to create the steam chamber, and some coffee sacks to cover that before the dirt caps it all.

The labor requirements are no more complicated than the materials. One person can handle a small imu (I just did a 65-pund pig solo), but a crew helps. Somebody has to know what's going on with the fire and the rocks, so that when it's time to cook, the rocks are hot and the coals are minimal. Somebody needs to know how much vegetation is needed and whether water should be added. Enough people have to be aroubnd at cooking time to assemble and bury the oven quickly, minimizing heat loss. Someone has to stick around long enough to spot escaping steam and ad more dirt.

In Hawaii, banana trees solve most of the vegetation issues. Smashed sections of trunk keep food off the rocks and provide a lot of moisture for the steam; meanwhile the leaves can wrap individual food packets or cover the entire feast. Ti leaves make even better food wrappers, and give a flavor that distinguishes anything I could cook there from anything I can cook here.


In the Northwest, people also cooked with earth ovens for thousands of years, back to when the ancestors of Hawaiians were sitting around their proto-Polynesians imus mulling over the prospect of open-ocean exploration. I've never been party to a purely traditional local earth oven, but did help do one to cook "black moss," a Plateau delicacy. Then this year a bunch of local archaeologists decided we would like to replicate an earth oven.

Our effort was hybrid. We cooked camas and taro (along with sweet potato and blue Peruvian potatoes), and of course pork. No bear meat was available, but we did roast some salmon outside the imu. We used some techinues I'd learned in Hawai'i, and some from here. But was this some post-Modern mash-up? Not really, plenty of Hawaiians came to the Northwest starting in the 1810's, meaning that we were about 200 years too late to be  doing anything unique.

One thing that makes it easy to cook underground in parts of the Northwest as well as some remote flecks of land in the Pacific is the presence of vesicular basalt in both places. I've seen other types of rock used here, but where basalt cobbles full of small air bubbles can be had, it's what people use first; I've even heard from Plateau tribes that areas with particularly good stone were special resources, and oven stones were traded to outsiders. Sweat lodges also use this rock. The vesicles help it withstand heating and cooling with fewer fractures and far less shrapnel than other kinds of stone.

Vegetation is a more difficult translation. Even here in the Salish Sea's balmy south, banana trees are not an option. In fact, there's no analog I can think of for the non-woody, water-filled, neutral-tasting banana stump. The largest leaves around come from the skunk cabbage, which I'm reluctant to use without knowing what it does to flavor. So, I looked at various books and in an ethnography of the Twano people (most of their descendants are now enrolled in the Skokomish Tribe, based not far from where I live), it mentioned big leaf maple as a staple of oven wrapping. This is what goes closest to the food; the big leaves sort out into layers laid immediately below and above the food. Below that: sword ferns. They don't have much moisture, but they make a cushion, a tangle-weave of springy midribs and leaflets that let steam circulate and hold the food above the charring coals and red rocks.

But that's once the fire has done its thing and the food is being placed in the oven. Before that, you've got to heat the rocks, which is more or less the same procedure anywhere on earth: burn hard wood until the rocks are red and hot. In Hawai'i, almost everyone now uses the exotic tree Kiawe, a big-ass mesquite introduced by a trader two centuries ago--so goddam BTUs packed into logs you cannot believe. Here, maple and its more flavorful counterpart alder are just fine. Burn your oven with evergreens, and your camas will taste a little like turpentine.

Building the fire, constructing a heap of rock and stone, requires forethought. The first thing I do is plant a pole in the bottom center of the imu. Then I lay a log or couple of two-by-fours from the base of the pole to the edge of the bowl. Then, stack kindling, covered by logs and rocks and rocks and logs until there's a heap around the pole. When the time is ripe to ignite, pull the pole and the log, put in balled up paper or your accelerant of choice, and fire it up. Air flows in from the rim and rises up the pole-void chimney. Pretty soon, you have a jet of flame shooting up and out of the wood-and-stone volcano.

Every hour or so, push the coolest rocks into the flame, and add fuel. Add wood that will fuel the flame, wood that will insulate the most exposed rock, wood that will further the fire. An hour later, you do it again. Eventually, you feed more to the fire than you used to set it up in the first place. After three hours, you make sure to toss on the largest pieces. An hour before you end the fire, start feeding in the last fuel, tosing sticks onto the pile to keep rocks from losing heat. Your goal is to end up with a maximum of red rocks and a minimum of unburnt fuel. I think 5 hours is typical.

But you'll never get rid of all the coals. Shovel out what you can. Redistribute the rocks across the bottom of the pit, making as even a layer as possible. Because there will still be some coals, and because the rocks will be hot as hell, toss on some dirt. Seems like an inch give or take is enough; the goal is largely to keep from igniting the ferns you are about to toss on.

But before you do that, go find the four stout (2"-diameter if you can find them) ironwood sticks, and lay them at four "corners" of your pit, with one end touching bottom and the other extending out from the rim. This will be important to the Northwest no-banana technique you'll use later.

Sticks in place, you put in the ferns half a foot thick or more, spray them a little with the hose, and add maple leaves so they're few thick over the ferns. Then comes the food: a pig in foil-covered chicken wire (if the wire is new, burn it over the fire before this, to off-gas the galvanic coating or whatever other crap may be adhering), foil or leaf-wrapped packets of food, and whatever big root foods you may have handy. Biggest stuff in the hot center, smallest stuff on the periphery. Then, more mape leaves, and finally more ferns. If you got 'em, cedar boughs help weigh down the ferns, but if not, jump straight to wet coffee bags. Usually a half dozen will be plenty--they help make sure that dirt does not penetrate to the food.

Then shovel on a ton of dirt. No vegetation, if you can help it, include no stems or leaves that create channels for escaping steam. A foot or soil would be great, to insulate the oven beneath. Yank out any fronds or leaves sticking out of the pit. Tamp down the edges, especially around the four ironwood sticks emerging from the edge for the pit. Then, one by one, pull out those sticks, and pour a half-gallon of water down each hole. Shovel some dirt in the hole to seal it up, and move on to the next. If you can, put the stick back in the last hole and tamp it down instead of back-filling it; that way you can add water or monitor the sound/smell/temp of thelower oven if needed needed.

Then again, I've never added water or done much to monitor. So you could just seal it entirely, like I did last time.

When is it done?

I dunno, you might as well ask me how many kilos of rock or wood I used. Enough. I suspect that the cooking all happens in the first 10 hours, after which the oven just sits there being warm. All I can say is that is makes no sense to open the oven before you're ready to eat. Ten or twelve hours into it, the soil surface will feel warm, but the hottest of the heat has dissipated. If you had adequate vegetation and moisture, and if you managed to cover the oven so air cannot sneak in, nothing can burn. It just stays warm for a ridiculously long time until you are ready to eat.

Early in the summer, I cooked maybe 40 pounds of pork shoulder and root foods for 12 hours, and it was all well-cooked. Just the other day, with more rocks, I cooked a 65-pound pig for 16 hours, and it was well-cooked. I'm starting to get over the panic of wondering whether a couple dozen peopel will stand in judgment as I open a pit of undercooked pork.

In the aftermath of an earth oven, you have several interesting by-products. The dirt-lid you just shoveled off the pit has been steamed for hours; the part closest to the fire will have been sterilized, which makes for a good component of potting soil. There will be a fair amount of steamed/smoked leaves and ferns, all fodder for compost. And remember how you had to scoop out coals from the final fire phase? Well, you could partially bury those in back-dirt and let them slowly turn into charcoal, or you could let them burn into ash, or whatever. Whatever path, at some point you end up with

Kama'aina is a child of the land, a person born on the island. Since ancient times, the first birthday was the biggest celebration, which meant throwing by far the biggest party we new haole parents had ever attempted.