While I think a lot of people realize that open farms are extremely powerful in the late game, I believe it is less obvious that you don't need that much terraforming to make them work. So I wanted to share a strategy I have been tinkering with, complete with some observations about open farms that were new to me. The idea is to skip regular farms and go from hydroponic farms straight to open farms. I am going to assume you play with the "hunger" rule and want to avoid exploiting trading with rival colonies, and thus need your own food production early game. The key is that altough open farms only become really powerful after hitting 30% temperature, the combination of hydroponic + open farm is already powerful after hitting 20% temperature.
Phase 1: I don't like sand (Temp 0-20, Water 0-10)
Your early game will be coarse and rough and irritating, and it will get everywhere. You want to collect the follow things ASAP: 20% temperature, 10% water, 5-10 seeds, an open farm, and ~5 water surplus to run that farm. So you will want to run 2-4 GHG factories for 20-40 sols and complete 2 "capture ice asteroids" projects, which will each occupy a rocket for 3 sols 18 hours. GHG factories only consume 0.5 fuel each, and you can run 2 GHG factories continously with 6 large solar panels + 1 battery (=20 power). But you need 200 extra fuel for the projects, plus a lot of extra machine parts. So I recommend going for at least 2 fuel refineries early. The commander profile "Geo Engineer" will give you 2 GHG factories for free, and make importing the seeds cheaper -- although it essentially becomes pointless after this phase. "Oligarch" is a nice choice for the extra fuel.
You will need the techs "Martian vegetation", "greenhouse mars", "Interplanetary Projects" and "Domeless Farming". The latter two normally cost 6k and 14k respectively. This will likely be the hardest bottleneck. I generally recommend aggressively spamming sensor towers to get research freebies. There are also some sponsors that fit this strategy much better than others. Terraforming Initiative is the obvious choice, since it give interplanetary projects for free and cuts the cost of domeless farming in half. The maybe less obvious is Japan, which gives a lot of free early research for scanning and gives you an incentive to build a machine parts factory early on for sponsor goals. Since you will need a lot of machine parts very early, building at least a small one can pay off.
Third best choice is Europe IMO, since it generally enables a heavy research focus. Either way, you will only want the absolute essentials in other tech trees before you are sure that you have all necessary prerequisites when temperature hits 20%.
Before you can start open farming, you need a small patch of land with positive soil quality. You can build a forestation plant in the middle of the space where your open farm will be, reduce its radius to the minimum, and run it on lichen for a short while. 2 seeds will be more than enough to get a small patch of yellow, and you don't need more. Afterwards, salvage the plant and build the open farm on the patch of fertile land.
Phase 2: This is where the fun begins (Temp 20-30, Water 10-15)
Until you hit 30% temperature, you need botanists to grow seeds. GHG factories become 80% less efficient after 25% temperature, so that will take a while. The good news is that hydroponic farms are bad at growing food but really good at growing seeds. A hydroponic farm will output 1 seed per sol -- same as a regular farm, but with half the workforce. Using an open farm that plants herbs, you can convert 1 seed to 10 food -- so one hydroponics farm can feed 50 colonists. An open farm will seed 50 crops per sol, and a herb costs 0.1 seeds, so one open herb farm + 5 hydroponic farms can output 50 food, or feed 250 colonists. Obviously, you can also just import seeds.
The second good news is that you only need to run the open farm when planting seeds, but not while they grow. So to feed e.g. 100 colonists, you only need to run your open farm less than half of the time, drastically reducing its effective water need. Overall water efficiency for hydroponic + herbs is pretty similar to regular farms -- 50 food for 13 water -- but you already save half your workforce).
Cold waves will make crops whither, and in this phase you do not want to waste seeds or food. I would recommend stopping the farm 1 sol before the cold wave hits, and all herbs will have been collected by the time the cold hits. Same with toxic rains if you decide to use the "import greenhouse gasses" project. A subsurface heater will also cover most of the area of an open farm.
It is also noteworthy that farm seeders will plant crops on the unoccupied hexes with highest soil quality, but they will also do that when that soil quality is below the minimum requirement for the crop, wasting the seed. So if you have only a small patch of viable land, it can be a good idea to seed some herbs, stop the farm and wait for them to grow and improve the soil, and only then turn the farm on again.
Phase 3: Unlimited power (Temp 30+, Water 15+)
At 30% temperature and 15% water, you can run cover crops. An open farm running 2 slots of cover crops and 1 slot of potatoes needs only water to output ~3,33 seeds and ~116,67 food each sol, while increasing soil quality. So it can feed about 680 colonists without worker input. One strange thing I noticed is that a farm that only has 1 potato + 1 cover crop plant type seems to only plant cover crops, so be sure you actually use 2 cover crops and 1 potato for the plant types. You also need 2 times as many cover crops as potatoes to harvest more seeds than you use.
You likely won't need more food for a long time, and I recommend setting up rover transport routes to feed eventual outposts.
If you want to take the open farm to even higher levels, you can research bushes: A single plant and a small patch of bushes will be enough to sustain a farm, and you can replace the cover crops with herbs, putting food output to ~150 food. A second step is to build 2 large lakes directly adjacent to the farm on each side. That will be enough to raise soil level to 20%+ for the whole farm. Soil level from lakes does not decrease (fast enough), so a "watered" open farm with external seeds can run only potatoes and output an incredible 50*7= 350 food each sol. At this point your limiting factor is likely storage and transport. The late-game upper limit for an open farm is one that runs only wheat -- 50*12=600 food per sol, enough to feed 3000 colonists.
Overall, you sacrifice quite a bit in the early game, which is the main tricky part. But as soon as you hit 20% temperature 10% water, you can cut your botanist workforce in half, and as soon as you hit 30% temp 15% water, you won't need any botanists anymore and have an almost unlimited food output with a single open farm. I am still not quite sure how optimal this strategy is, but it is definitely fun to pull off if you are bored of standard dome farms and are looking for a challenge.
Phase 1: I don't like sand (Temp 0-20, Water 0-10)
Your early game will be coarse and rough and irritating, and it will get everywhere. You want to collect the follow things ASAP: 20% temperature, 10% water, 5-10 seeds, an open farm, and ~5 water surplus to run that farm. So you will want to run 2-4 GHG factories for 20-40 sols and complete 2 "capture ice asteroids" projects, which will each occupy a rocket for 3 sols 18 hours. GHG factories only consume 0.5 fuel each, and you can run 2 GHG factories continously with 6 large solar panels + 1 battery (=20 power). But you need 200 extra fuel for the projects, plus a lot of extra machine parts. So I recommend going for at least 2 fuel refineries early. The commander profile "Geo Engineer" will give you 2 GHG factories for free, and make importing the seeds cheaper -- although it essentially becomes pointless after this phase. "Oligarch" is a nice choice for the extra fuel.
You will need the techs "Martian vegetation", "greenhouse mars", "Interplanetary Projects" and "Domeless Farming". The latter two normally cost 6k and 14k respectively. This will likely be the hardest bottleneck. I generally recommend aggressively spamming sensor towers to get research freebies. There are also some sponsors that fit this strategy much better than others. Terraforming Initiative is the obvious choice, since it give interplanetary projects for free and cuts the cost of domeless farming in half. The maybe less obvious is Japan, which gives a lot of free early research for scanning and gives you an incentive to build a machine parts factory early on for sponsor goals. Since you will need a lot of machine parts very early, building at least a small one can pay off.
Third best choice is Europe IMO, since it generally enables a heavy research focus. Either way, you will only want the absolute essentials in other tech trees before you are sure that you have all necessary prerequisites when temperature hits 20%.
Before you can start open farming, you need a small patch of land with positive soil quality. You can build a forestation plant in the middle of the space where your open farm will be, reduce its radius to the minimum, and run it on lichen for a short while. 2 seeds will be more than enough to get a small patch of yellow, and you don't need more. Afterwards, salvage the plant and build the open farm on the patch of fertile land.
Phase 2: This is where the fun begins (Temp 20-30, Water 10-15)
Until you hit 30% temperature, you need botanists to grow seeds. GHG factories become 80% less efficient after 25% temperature, so that will take a while. The good news is that hydroponic farms are bad at growing food but really good at growing seeds. A hydroponic farm will output 1 seed per sol -- same as a regular farm, but with half the workforce. Using an open farm that plants herbs, you can convert 1 seed to 10 food -- so one hydroponics farm can feed 50 colonists. An open farm will seed 50 crops per sol, and a herb costs 0.1 seeds, so one open herb farm + 5 hydroponic farms can output 50 food, or feed 250 colonists. Obviously, you can also just import seeds.
The second good news is that you only need to run the open farm when planting seeds, but not while they grow. So to feed e.g. 100 colonists, you only need to run your open farm less than half of the time, drastically reducing its effective water need. Overall water efficiency for hydroponic + herbs is pretty similar to regular farms -- 50 food for 13 water -- but you already save half your workforce).
Cold waves will make crops whither, and in this phase you do not want to waste seeds or food. I would recommend stopping the farm 1 sol before the cold wave hits, and all herbs will have been collected by the time the cold hits. Same with toxic rains if you decide to use the "import greenhouse gasses" project. A subsurface heater will also cover most of the area of an open farm.
It is also noteworthy that farm seeders will plant crops on the unoccupied hexes with highest soil quality, but they will also do that when that soil quality is below the minimum requirement for the crop, wasting the seed. So if you have only a small patch of viable land, it can be a good idea to seed some herbs, stop the farm and wait for them to grow and improve the soil, and only then turn the farm on again.
Phase 3: Unlimited power (Temp 30+, Water 15+)
At 30% temperature and 15% water, you can run cover crops. An open farm running 2 slots of cover crops and 1 slot of potatoes needs only water to output ~3,33 seeds and ~116,67 food each sol, while increasing soil quality. So it can feed about 680 colonists without worker input. One strange thing I noticed is that a farm that only has 1 potato + 1 cover crop plant type seems to only plant cover crops, so be sure you actually use 2 cover crops and 1 potato for the plant types. You also need 2 times as many cover crops as potatoes to harvest more seeds than you use.
You likely won't need more food for a long time, and I recommend setting up rover transport routes to feed eventual outposts.
If you want to take the open farm to even higher levels, you can research bushes: A single plant and a small patch of bushes will be enough to sustain a farm, and you can replace the cover crops with herbs, putting food output to ~150 food. A second step is to build 2 large lakes directly adjacent to the farm on each side. That will be enough to raise soil level to 20%+ for the whole farm. Soil level from lakes does not decrease (fast enough), so a "watered" open farm with external seeds can run only potatoes and output an incredible 50*7= 350 food each sol. At this point your limiting factor is likely storage and transport. The late-game upper limit for an open farm is one that runs only wheat -- 50*12=600 food per sol, enough to feed 3000 colonists.
Overall, you sacrifice quite a bit in the early game, which is the main tricky part. But as soon as you hit 20% temperature 10% water, you can cut your botanist workforce in half, and as soon as you hit 30% temp 15% water, you won't need any botanists anymore and have an almost unlimited food output with a single open farm. I am still not quite sure how optimal this strategy is, but it is definitely fun to pull off if you are bored of standard dome farms and are looking for a challenge.
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