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On a world I am building, the day and night cycle lasts 9 years, resulting in the majority of animals migrate across the planet. One of the problems that my sapient races (one on the out going night side and one on the up coming night side) suffer is that they cannot stay in one place long enough to reliably farm.
How can a species that never finds itself in one place manage to farm?
If an entire race of sapient creatures are moving, they need to take their animals, clothes, tools, houses with them. Their settlements are nomadic caravans, and the few times they get ahead of the dawn/dusk line is when they circle up and rest.
They also need to carry their supplies with them. It means having tank carts with water to cross a desert/arid region, and finally, agriculture on wheels.
Your races developed hydroponics or movable soil planters. The crops are not planted on the soil, they are on planters mounted on carts, and those carts are carried along with the migrating caravan.
The plants are also not our regular earthen plants. These plants have evolved in this planet, and have been selectively bred by the nomads for centuries to be in this moving farm.
Of course plants need more sunlight to grow. So the farming carts need to stay on the back of the caravan or the front, depending if you are follwing the dawn or dusk, respectively. The plants on the dawn side would benefit from a cool environment (as the sun is rising over frozen places) and the dusk plants would enjoy more heat (as the caravan is going into lands that were under the sun for 9 years).
P.S.: This assumes they are nomadic and are always on the move. The actual time spent walking can vary depending on how close to the equator the caravan moves (credits to ckersch in the question comments.)
On a planet with a 9-year day/night cycle, some plants will adapt to that cycle. They will lay down roots in the day cycle, and then die off for four and a half years, only to spring back to life when the sun comes back. Just look at how any plant copes with a cold winter environment (trees lose leaves, perenials die to the root and grow back from a bulb, etc)
With trees, farming would involve cultivating and pruning trees in the sunlight, waiting for them to go dormant and lose their leaves, nomad-ing off for half a decade, then coming back when the sun returns to burst into bloom and fruit.
Other plants could be grown like tulips. Tulips bulbs are planted in the fall about a month before hard frost. They stay dormant through the winter, then start growing again in the spring. I imagine a plant on your world kind of like a potato, where you plant you cuttings in a plowed field right before frost, then when you and the sun get back the plant starts growing again.
If you stay in one place to protect your crops until the sun leaves, you could pick two places on opposite sides of the earth and migrate between the two every 4.5 years. I imagine that there isn't much to disturb your frozen gardens in the winter (any large destructive wildlife would have to hibernate or migrate too) so there wouldn't be much damage while you were gone.
To stay in the light, groups would farm collectively
With a 9 year day/night cycle, a single group would not be able to settle down and farm a plot of land for more than a few days. Living near the equator, on an Earth-sized planet, with an Earth-length year, the nomads would need to cover an average of 7 miles/day. If they can travel 21 miles in a day of constant motion, they can stop for two days if they spend a day traveling, for for two days traveling, and so on.
However, that doesn't mean that static farms can't exist, merely that a single caravan wouldn't be the sole proprietors of one. Instead, a long line of caravans would stretch out across the day side of the planet, each travelling and stopping on the same schedule. A group would spend, for example, twelve days on a farm, then pack up and travel for six days to the next farm.
Each group would always do the exact same thing on each farm. Since groups would live at a certain time of day, a caravan would be responsible for a single task such as clearing land, planting crops, nurturing plants and chasing off wild animals, or harvesting crops. The harvesting groups would cache the crops they harvested, which would then go on to nourish the leading caravans the next year. Crop stores would be rationed among all of the caravans on a specific farming route, so that all groups would have food and could continue their jobs on the farms.
Entire permanent villages would probably be constructed over time, with caravans moving between villages. These villages would contain store houses for crops, smithys for maintaining tools, as well as structures like schools and libraries. All of these would be public, of course, since each caravan would only live in a village for six weeks before moving on to the next. The libraries would be particularly important, since the lead caravan and the tail caravan would have no other contact with each other. Cultures, traditions, and news would be shared by writing them down and leaving them in the libraries for the other groups to read. Things like crop reports or details on how to continue things like building projects would be critically important for caravans to pass along to their successors, so all caravans would likely have dedicated librarians whose jobs it would be to read everything that groups have written down in the previous nine years and to compile reports of what their caravan did while staying at the village, before moving on.
If this planet has 4.5 years of nighttime, I can only imagine the bizarre adaptations present in its plantlife. You need to take some liberties with what the local flora is like on this planet, since you offer no details that really limit what it can look like.
You mention your two groups of nomads live on the dusk line and one on the dawn line. The dusk line nomads could plant an annual crop that lies dormant on the night side and grows very vigorously on the day side (imagine really big megaflora that yields a crop worth planting 9 years in advance - it has 4.5 years of energy to grow on). They would plant these crops at their planned settlement sites along the way, advancing to the next one once night approached. I imagine the species on the day side are more active and numerous, so maybe these crops have robust defensive traits as well: huge fields of bramble surrounding the flowering part, etc. so that your nomads can be reasonably sure their crop will remain unmolested.
Your dawn-line nomads have a problem: Anything they plant is going to start growing immediately and will either be consumed by wildlife during the day or, depending on your planet's temperature, will have been dormant for 4.5 years by the time they see them again. For them, you will have to establish what kind of plant life can endure the nights. Fungi, conifers, tundra vegetables, perennials with hardy rootstock that reliably fruit early in the daytime. Without further details, your dawn nomads are more likely foragers.
Some problems explaining a "roaming" agricultural subsistence strategy:
Agriculture takes a lot of time to develop -- or at least it did in Earth's history. Nomadic lifestyle is simply not conducive to its development. If your planet's inhabitants had already developed agriculture before the planet went into its prolonged day and night cycle, or some plant species somehow naturally lend themselves to effectively being domesticated, that would be different, I imagine.
People don't just decide to develop agriculture for the heck of it. (Sure, here on Earth people may have been experimenting around for millennia planting little things in gardens and whatnot but it wouldn't really have been "agriculture.") Nomadic subsistence patterns work for nomadic groups in no small part because of the nature of the size of the group. Those strategies don't work for larger populations that want to stay sedentary, and non-sedentary populations have no motivation to attempt them if they don't need to. Agriculture is hard.
Some (hopefully helpful) ideas:
If you're envisioning that some of your native plant species require sunlight to actively thrive and others that don't, then that might make it easier to explain plant life cycles than trying to devise species that are adapted to growing during both day and night conditions. An analogy here is how plants on Earth adapted to winter conditions go dormant (the botanical equivalent of hibernating). Essentially plants would be dormant for half the cycle while others would be growing in the same location. They're not competing for nutrients because they're not active at the same time. There could even be some symbiotic relationships. Like maybe in the night cycle there's some cool nocturnal vines that grow up the trunks of the daytime-loving trees, and in the day cycle the roots or just dormant and the trees are the ones growing.
There are already things like cicadas here on Earth with very long life cycles. This Wikipedia page mentions species having life cycles from 2 to 17 years long. So I suppose very protracted hibernation periods for animal/insect/etc. life could certainly be on the table. The advantage here is that you can evolve species suited to either portion of the cycle without having to hand-wave things like complicated thermoregulatory systems so much.
In the end, focus on the planet in terms of itself. The things living on your world evolved under the conditions there, so perhaps the way agriculture is performed there is radically different than what we conceptualize as agriculture here.
It's not that hard, really.
If people are migrating across the world, let's say it takes them 6 months. This means they can move across the entire world in 6 months, then have 4 straight years to farm. You need to find a way for this to be possible for your people to move across the world.
This might be living closer to the poles (resulting in a much shorter lateral circumference than the equator) or some sort of technology (ships, trains, etc) or a smaller world. The Earth's circumference is about 24k miles, so if your people migrate they need to move ~140 miles/day if they go halfway around in six months. Even on earth this drops fairly significantly the further north you go.
Similar to snowbirds who travel south for winters, your people will travel across the world.
This means your people will be settled for ~4 years. Or more if you want them to have more advanced technology to allow them faster traveling.
Growing certain plants in constant sunlight is really crazy and causes some earth plants to have ridiculously better yields. Some don't, however, obviously those that depend on a sunlight cycle. Depending on how earth-like you want the plants to be you could mimic earth plants behavior under constant sun or you could invent specific fauna for your world.
Is the planet roughly Earth sized?
Can people circumnavigate the globe on land or are there any oceans in the way?
How fast can they move? With even simple railroads, they could cover the distance between terminators in a few weeks, and, more importantly, bring a lot of harvest or farm equipment with them.
How hard does the surface freeze over a 4.5-year night? This defines how close they can get to the dawn terminator before encountering ground too frozen to farm.
How fast can they raise a crop? A party arriving at the frost terminator (aka thaw line, just behind the dawn terminator) would need enough food on hand to last until the first crop came in.
The simplest strategy seems to be for each family or village to settle down at the frost terminator and raise as much food as possible during the nearly 4.5 years of sunlight. When overtaken by sunset, they would up stakes, migrate all the way to the frost terminator, and start over. Given about 4 years, they certainly should be able to preserve and store enough food to last through the migration and ripening of the first crop.
To this simple strategy we add trade. As people spread out across the globe, the arable land becomes filled with farms and farmers, plus a constant trickle of migrants passing through on their way West from the sunset terminator to the thaw line. Now, instead of everyone individually saving food for 4 years, the farmers could help feed the migrants on the way, and stake them to enough supplies to bring in their first crops.
The settled farmers would also maintain the railroads and other infrastructure, both for their own use, and to speed the migrants' passage.
All this would require a great deal of trust, a sophisticated market and accounting system, and universal faith that the migrants would, in turn, become settled farmers who would work off their passage debt by maintaining the railroads and hostels, and feeding all the new migrants.
When nationalism finally did arise, the territory of each country would look like two stripes, running north and south, on opposite sides of the globe.
The most common aphorism for an insane, ill-advised, doomed venture would be "migrating East". The best stories written about this world will probably be about protagonists who are, in one way or another, migrating East.
Not sure if this was mentioned, but you might be able to use the plant life-cycle as a motivator for migration.
Some plants don't produce a harvestable yield the first year. Fruit trees, for example, can take several years in order to grow to a size large enough to produce a crop. Perhaps your nomadic peoples have already planted (or identified existing) vegetation that is on a similar cycle as your year. They then pick and eat some, and plant others that will be ready to consume their next time around. As for the "well then why don't they just stay where the mature crops are?" question, perhaps their most fruitful production years are limited? It's your world. Everything is adjustable. Good luck!
You are describing something quite similar to transhumance, which involves seasonal migration to different territories, based upon weather conditions and the needs of crops and herd animals.
It's simple what we do: After we harvest our crops, we pack up and travel towards the setting sun. As we travel, we stop at other farms, paying for our stay with some of our harvested crop. Eventually, the sun is over head, and later behind us. When the sun is almost below the horizon behind us, we begin our next year's crop.
Of course, that's what we do. It's a long march - we must cover over half of the globe each season in order to be able to plant at the start of the growing season. There is another tribe that travels in the other direction, through the darkness until the sun rises again. They travel less far, but they face greater dangers.
Not really an answer, just thinking about your funny planet:
It would have to be similar to Venus, rotating against its orbital motion. Venus has a temperate climate (no extreme day/night temperature differences), it's only slightly too warm. The huge jetstream in the upper atmosphere levels out day and night temperatures.
Question is if a planet that has an ambient temp. of 25 (35?) degrees could maintain this jetstream and an extremely dense cloud layer and a humid, oxygen containing atmosphere.
Agriculture would start off accidental. The nomads would pick what they could as they travel and eat it on the way. As they eat it they would drop seeds, spreading those crops along their path. Containers for grain might leak, leaving a scattering of seeds behind them. As time goes on, there would be better and better food supply along one nomad's track.
At some point, they may notice this and go it on purpose. They could spread or plant the seeds in the more favorable locations as they go.
It is likely that morning crops and afternoon crops will be different (Noon too, if it isn't too hot). If the morning and afternoon crops are the same, then you could have two groups of nomads in the same track and that would double the benefits.
Also, be aware that if there are any oceans that stretch North and South, your nomads are screwed.
With a day/night cycle of 9 years, having two cultures migrating at rotational speed, each culture is making a full circumnavigation of the globe in 9 years. Unless the planet is incredibly uniform, or the nomadic tribes incredibly new, it seems likely that there would be a kind of path which we assume both tribes would follow. A visible line of settlements. This would happen naturally due to water and shelter availability, and then over time, those would become outposts of a sort.
In that scenario, each point on the trail is visited once each 4.5 years by one of the two migrating tribes.
Certain plants, such as fruiting trees, could be planted by tribe A and then in 4.5 years, harvested by tribe B. That could be the basis for an interesting symbiosis. The tribes might depend on each other for survival. Each harvests for themselves, and plants for the next. Initially that could be accidental, eating apples then leaving the seeds which happen to become new trees. Later it could become intentional.
Depending on the size of the races, they might not need to, as the naturally growing plant life will already provide the food they need without farming more.
If the day night cycle is 9 years (ignoring the thermodynamic problems that adds, particularly for natural plants), than they could harvest the seeds from crops and replant when they reach their next resting point. This requires that they can sit down for a year at a time to grow things. If they have to take their farming with them, the energy to move the crops will outweigh the energy they provide, allowing them to only keep small plants with them (like medical herbs).
Of course, if these plants are strong enough to survive multiple 9 year revolutions, they could raise a 'wild' plot and move on, harvesting and up-keeping it when they return to that spot 9 years later. Making a plot at every rest point in their migration.
