Hydroponics is generally done in either a trough or a tub. Here on Earth, gravity demands that hydroponic troughs for Nutrient Film Troughs (NFT) are laid on a slope of 40:1 to get enough of a flow rate. On Luna this translates to a slope of 7:1 or more which will make lunar hydroponics equipment look very different, probably depending more on the use of rockwool, tubs and pots in troughs than pure NFT .
The next additions were two roughly cubic, 27 cm square, 15 litre tubs of aggregate. Later systems abandoned the large, open tubs and concentrated on suspending the 74mm baskets in light-tight tubs, allowing the nutrient to trickle down the root systems that hung out from the basket. Larger versions were produced using 150mm baskets originally intended as waste paper baskets. In both cases, the bottom of the basket is suspended well clear of the bottom of the light-tight tub and approximately 2cm of liquid is left in the bottom by the drainage tube to allow particulates to settle out.
To simulate locally available (lunar) materials for filling tubs and the small pots used in troughs to hold the seedling still, Apollo Lunar Regolith Simulant samples were obtained from JSC and proved to be non-toxic but far too fine to use for a hydroponic medium, forcing the root systems to exit the surface of the medium. Locally obtained granite chips were used for aggregate beds rather than the usual river gravel, approximating larger lunar rock fragments in the 3-5 mm range. For the larger baskets containing perenials, rough river pebbles were used to satisfy domestic requirements, again from volcainic rock. The larger pebbles surrounded a core of smaller chips and some similarly-sized pummice fragments.
Also used were expanded, porous ceramic spheres in the 3-7 mm range which it may be possible to produce locally or import due to their low mass and ease of storage - they are durable and can be sterilized at incinerator temperatures. The majority of plants were grown in standard pumice/vermiculite "mix" which again may prove suitable for local manufacture or packaging but is not so durable. Pumice would perform almost identically but is prone to developing algae internally - not a problem if we have a solar furnace on hand to sterilize the growing medium but this facility is not currently available.
Polystyrene beads were briefly considered, but these are hard to sterilize and too opaque to light, and so allow nutrient-sapping algae to flourish within.
The crops were all planted in the same system and given a 16/8 hr water on/off cycle to ensure adequate root oxygenation. At lower pressures the period will need to be split into shorter segments to prevent the roots drying out. This is a simpler system than bubbling air through the water to oxygenate it, and "waterfall" oxygenation may prove difficult under 1/6 gravity.
The entire circulatory system initially held a maximum of 20 litres including all piping, reservoirs and pumps. The water was replaced every 14 days to avoid the use of expensive nutrient dosing equipment. 1 kg of nutrient feeds the basic system for nearly 10 months on a subsistence level.
The capacity was later increased to 25 litres as additional tubs were installed. As the open tubs tended to hold large quantities of nutrient solution back, additional space was needed for their outflow when the pump was turned off. Expensive nutrient monitoring equipment was subsequently purchased and revealed that the nutrient levels assumed were below optimal levels; top-ups became more frequent. The system had increased in biomass by approximately 60%, and experience allowed more crops to be grown. Currently the system uses 1kg of nutrient approximately every 3 months at optimal production levels, and "dumping" of used nutrient is carried out every month.
Yields here may not appear large, especially considering the prolific quantities normally produced by hydroponic systems. But it must be bourne in mind that these plants were being grown in conditions in which soil-based crops perished and that would not be experienced for more than 2 weeks continuously in the Lunar environment - the lack of weather makes this a dead cert. Species successfully grown in these conditions are listed first, and the rest follow.
Broad beans grew very well indeed in NFT troughs, although early emergence of the flowers led to many not being fertilised. Each play put out 5-6 main branches, each branch bearing in excess of 25 bean pods. Stakeing or tying up is necessary to prevent the beans smothering nearby plants or from pulling their own root system out of the trough when they lean over.
Broccoli did very well, and despite its notoriety amongst youngsters as being inedible produced heads of good flavour - particularly when stir-fried with soy sauce, the stems being sliced thinly across the grain. It grows well in either small NFT tubes, or in 74mm baskets of agregate.
Buttercup lettuce matured slowly, reaching a small but harvestable size (17 cm tall x 12 cm) in 10 weeks. They got bitter if left to grow larger, but would have easily packed in at 12 plants per 2.5m row of an NFT system.
Cauliflowers would not grow in the darkest parts of the system, but elsewhere the plants have developed to 20 cm high, healthy seedlings within 2 months. Time to flowering is 4 months, which was the first time we'd actually managed to grow a cauliflower at all! They'll need another month before being of a useful size. Best results were obtained in vermiculite/perlite mix in the tubes rather than small baskets.
Chervil is a herb popularly used with fish and used in herbal salads that superficially resembles parsley. It took very well to this environment and, grown in mix, turned from 10 mm seedling to 19 cm x 15 cm bush in just over a month and continued to grow prolifically. If fish are involved in lunar agriculture, this salad herb will be most useful.
Chinese broccoli does not resemble normal broccoli beyond being a brassica. It is a type of sprouting which more closely resembles mustard greens. It grew to form narrow, single shoots approx. 20 cm tall in 8 weeks when the whole plant could be harvested. We found the stems to be too tough to reasonably handle, though the leaves were a welcome addition to stir-fry. When the shooting tip and some leaves are removed, the plant regenerates new shoots on a weekly basis and it is this harvesting method that we recommend. It fared well in mix or ceramic beads, however this crop does not match the yield of Japanese greens and has nothing that makes it preferable to that crop.
Coriander can be grown for the seeds that are well known in indian cooking, or for the greens which are used extensively in balti dishes or salads but are relatively little used in the West. Resembling parsley, the plant grew over 9 weeks to a height of 20 cm and then slowed down. This was grown in beads.
Fijian chilies are a small, bushy chili producing nearly spherical fruit approx. 9 x 12 mm that is relatively spicy. A single plant 25 cm tall consistently produced 3 chilies per week. When the lighting levels were increased (by mother nature) chili production increased substantially. These were grown in ceramic beads, but are probably better suited to tubs.
Habañero chilies are approximately the same dimensions as cherry tomatoes and are extremely hot. They were slow to ripen, producing one ripe chili per week, but produced a continuous supply of 3 green chilies per week per plant. The plants are substantial, being 70 cm tall and 30 cm across. Again, these were grown in ceramic beads, but are probably better suited to tubs.
Japanese greens resemble spinach with more rounded leaves and a distinctive lemon flavour. Our best success story for winter hydroponics. We use them in soups, stews and stir-frys, and they go well with fish. They are extremely fast growing, plants producing a dozen leaves 15 cm or more long in 8 weeks. They can be picked continuously and pack in at 12 plants in a 2.5m run, growing in any medium. They do not like too much sun; they wilt and go to flower rapidly, but this is an easily rectified problem.
Lollo Rosso lettuce is a red-tinged, flatish lettuce with a stronger flavour - not unpleasantly so - than the usual green stuff. It grows relatively flat, much faster than the buttercup variety but requiring slightly more room. It is our lettuce of choice for winter growing, maturing in 8-10 weeks when it is 27 cm across and 15 cm high which makes it a tight fit for 10 plants per row. It may be continuously picked without killing the plant, so keeping the size down while producing very fresh sandwich fillings!
Parsley is renowned for its vitamin C content and rapid uptake of trace minerals. Used mostly as a herb and to counteract garlic breath, it is not going to be a huge crop but aesthetically people find parsley garnish very pleasing. Morale cannot be underestimated (apologies if I sound like Neelix). Parsley is relatively slow growing, taking 3 months to reach 18 cm in height and not regenerating rapidly.
Paw paw was initially unsuccessful as our cuttings would not take. By propagating a 10cm tall runner from a mature plant however, the paw paw thrived and grew to 50cm over winter. No fruit as yet but the plant is heathy and vigorous with an impressive root system. It is growing in a 150mm tub of aggregate and pebbles, shown on the right of the image to the left. The other tub contains younger sugarbeets and assorted herbs.
Runner beans grew very well in clay beads, cropping prolifically and having a useful annual vine that could be used to fill unused higher spaces, perhaps shading tomatos. The beans also dry well when mature.
Strawberries seem to work well in aggregate baskets or troughs. We grew them in aggregate one year and subsequently used the aggregate bed as a nursery for strawberry plants. As the plants produce runners, we anchored them in the aggregate and produced many more new plants for the next year. These were planted in small aggregate baskets which they appear to prefer to the open tubs, suffering less from rot. We are using a variety called Pajaro but by using different varieties it may be possible to have fruiting plants continuously. Some of the alpine varieties may survive the lunar night well.
Tomatos grew prolifically in adequate sunlight using pummice and aggregate filles containers in NFT. We determined that the best yeild was to be had by allowing the plant to hang down from the run rather than trying to stake it up. Both beefsteak and cherry tomatos responded well.
Red Silverbeet or chard grew very slowly. It was selected over normal green silverbeet on the naive assumption that red lettuce did better than green lettuce and the same might apply for chard. Wrong. Green silverbeet and spinach will be tried next year.
Snow Peas are the local name for those peas which are grown for their edible pods, which were grown in open aggregate tubs. They produced copious amounts of foliage but took over 100 days to produce very few pods. The vine - growing to 1.5m tall in some specimens - has an annoying tendency to collapse under its own weight.
However, after sufficient sunlight became available, the plants produced pods prolifically: 7 plants produce 100g of pods every 3 days. If sufficient light can be found, these plants are a welcome treat to the vegetarian diet and produce pea seeds that can be sprouted for salads in the lunar night.
Tabasco Chilies performed poorly, producing few fruit, which were fairly small by Tabasco standards. Results were so disappointing that a different variety will be tried next year. Chili seeds in general have problems sprouting in the cold and dark. Other chilli varieties performed well, but getting the seeds to sprout is a problem.
Advocado is being grown from a stone and is shown as the rightmost plant in the twin 150mm basket tub on the left. The stone took more than a month to germinate in an aggregate trough and was moved to the basket when approximately half the size shown in the picture.
Chickory Lettuce is an unusual variety that can be cut and regrown like chickory. We have seed, but no plants to test this on as yet. It is said to be possible to propagate from broken-off parts of the plant.
Chives and onion chives are being grown. The onion variety seems to be doing best. They are growing in small baskets.
Passionfruit is being grown as the "large black" variety, shown as the leftmost plant on the picture on the left. A trellis has since been added to allow it to climb and be trained as a vine. No fruit yet, but tendrils are being put out and the plant is healthy.
Pepino is also known as the "tree melon". A native of Peru, it comes in many varieties. This one has a vine-like habit and will hopefully share the trellis with the passionfruit. As yet we have only managed to get cuttings to take root using vermiculite in tubes, but have high hopes.
Sugarbeet is coming up, but has an unexpectedly long growing season. The large leaves between the strawberries on the left are a single sugarbeet plant which has now been growing for over a year. The beet at its base is over 7cm in diameter and shows no sign of sprouting.
All-year-round lettuce was a dismal failure. It didn't want to even germinate.
Garlic seemed to respond well initially, grown in aggregate
under the snowpeas or strawberries. After 4 weeks we had healthy shoots
15 cm tall, coexisting happily with the snowpeas at a lower level, although
they prefered the company of strawberries. However, as the peas and strawberries
grew up, the garlic became lost underneath it - even under the strawberries.
No garlic survived, and it was a real pain finding the hidden garlic bulbs
when the chips were recycled.
A different variety known as "elephant garlic" is being tried, and seems to be doing well in a small NFT tub of vermiculite. It has also been suggested that garlic and other bulbs fare better with intermittent irrigation which will be attempted in follow-on projects.
Apples have a distinct advantage if grown in hydroponic systems and I plan to grow them in tubs of aggregate. Normally, productive varieties need to be grafted onto a hardy rootstock. With hydroponics, root systems have to do far less work so grafting should be unnecessary.
Babacco is similar to paw paw, much like a cross between a yucca plant and a melon. It is claimed to be possible to grow from cutting to fruit in 90 days, and I will be testing this together with performance from seed when I get hold of it.
Wheat is presenting import problems. Due to the high nutrient requirement for producing grain, it is probably best to minimise the nutrients "lost" in the straw by using dwarf varieties. We are currently awaiting a decision by the NZ agriculture department on whether to allow the import of Apogee wheat for hydroponic trials, and bureaucracy is set against the individual. Apparently if I were a multinational and wanted to trial genetically modified crops I wouldn't have a problem, but non-GM wheat for an individual can be ignored by the ministry.
This actually needs to be done to recover the elements locked in the roots, the nutirients locked in the inevitable soil bacteria, and of course, the water in the growing medium. It is also the best way of sterilizing the medium, if it is sufficiently robust.
We have only had two minor problems with pests: Some small beetles that set up a fiery home inside some chilies, and an aphid infestation on the snowpeas that was cured with flyspray. For lunar insecticide self-sufficiency, we should probably add pyrethum-producing plants and tobacco to our hydroponic garden.
Nevertheless, coarse filters became necessary to stop organic matter and dead insects clogging up the finer tubing.
One adjustment made that won't be necessary on Luna was to protect it from the gentle Waitakere rains. And the torrential downpours in this part of the rainforest!
In short, we need to demonstrate that the mass invested in the system is worthwhile over their design life, compared with the mass needed to import an equivalent amount of dehydrated foodstuffs.
This document is ongoing.