You need to construct towns and then have them build the grain boost buildings Things like Grocer etc each you build will boost all ur cities' grain output and thus increase maximum cap a city can grow to. I've gotten a fortress in 1 grain tile up to level 5 before Just needed plenty of towns.

As the others have said, this is most likely a food problem. However, if you built a Garden and you are sure that this did not result in any growth of the city population (note that a Garden might not be sufficient - I don't recall exactly how much food a city hub gives you, but adding a Garden to a 3-grain city only increases the food supply in the city by 45, so unless you had 155 or more population in the city before adding the Garden, you wouldn't be able to go from level 3 to level 4 just with the Garden), there is something else at play, as that should have provided food for at least a short period of population growth. Check the improvements you've built in the city and see if there are any that carry a growth penalty (the only such building I can think of off the top of my head is the Sacrificial Altar, an Empire-only building that I think upgrades from the Cleric, so I don't think you'd have it, but it won't hurt to check to see if you have something like it). Also open the city details window and check the tool-tips for food and population growth, and if you have a save from before you added the garden, open that up, check the current population at that stage of the game, and compare it to the current population in your current save.

If the lack of growth is due to growth penalties from structures within the city, you can consider adding outposts and upgrading them with Consulates, although this will require you to research Economics, if you have not already done so, and you will need to ensure that the outposts you build to try to boost your Fortress's growth are closer to the Fortress than to any other city you own. This is easy enough to check after the outpost has been built, but once the outpost has been built you don't have any way to relocate it if it was improperly placed, and unless you're playing with Arcane Monoliths, that new outpost cost some city 30 population. If you're playing with a Kingdom faction and have a Life spellcaster, you can alternatively apply the Sovereign's Call city enchantment if you have an essence slot, as this applies a flat +2 growth bonus to the city. Other possible magical solutions include Gentle Rain from a Mage champion with the Hydromancy trait, or Nature's Bounty from Sorcery in the Magic branch of the tech tree; both of these increase the amount of food available in the city, so if these don't push you into the next higher food surplus level, they won't help your city grow if the issue is that population growth penalties are zeroing out your population growth (on a two essence tile with 3 grain, Gentle Rain is better than Nature's Bounty, as Gentle Rain gives +25% total food production per essence whereas Nature's Bounty gives +1 grain to the city, and so Gentle Rain is a 50% food bonus compared to the 33% bonus of Nature's Bounty in this case, so if you're willing to spend a city enchantment on either of these and you have both available, go for Gentle Rain; if you're willing to spend both enchantment slots on food spells, both will obviously be better, as Gentle Rain + Nature's Bounty will effectively turn your 3-grain tile into a 6-grain tile, while Gentle Rain on its own will effectively turn it into a 4.5-grain tile, and Nature's Bounty on its own turns it into a 4-grain tile).

EDIT3: Found it!, elementalDefs from the Hugemaps mod didn't have the growth modifiers in it, thus disabling normal growth.

Ahwell, now to find out why some mounts get shown 2x when mounted

I am also having issues with growth,

Now i played this game a long time ago, when it wasn't legendary heroes and i remember from then that surplus food should give me growth even without +growth buildings, right? like, i checked on google, pretty much everywhere it says that.

Now about a year orso ago (probably more then a year actually now that i think about it), when LH 1.6 was the newest version i got Legendary heroes and even back then i had the problem that no matter how much food i have left, i dont get growth without growth buildings, now back then i figured i'll just mod some buildings to also have a growth modifier, me being under the assumption it was a bug which'll get fixed in time.

Now i came back to the game after getting patch 1.7 and the dlcs... AND STILL, no growth, right now in a game, 315 food, and either 30 or 0 pop (depending on wether i que up a pioneer), yet i still don't get any growth and i think iirc first city upgrade to town/conclave/fortress comes at 100ish people? (also, when i hover the growth stat, it shows no modifiers at all, just Growth 0 and the explanation that surplus food gives growth... which it obviously doesn't)

Is my game just bugged? or could it be because of the mods i use? (Children of Storm/Champion Bonanza/Black Market Bazaar/Snaking/Bridgemod/Hugemaps/Mythic Buildings/Heroic Pursuits) or is this normal and from LH onwards you specifically need growth buildings and enchantments (even though that would make light even better then dark once more since they get the growth spell + it gimps the AI completely, since they also never get much if any growth going on either)

So ye, could someone please enlighten me?

edits: oh i also got cheatmode on, because i wanted to check if it still had the growth thingy, so i figured let's spam grain buildings, please don't tell me cheatmode makes growth not exist, right? couldn't find anywhere online that said it would, so i assume not, but you never know (also even though i could ofcourse use the COS races i decided to check on a default race first, in this case Capitar)

edits2:!!! i checked the playthrough sticky, the guy playing there founded a city and INSTANTLY on it's first turn it's got a growth of +1, without anything having been built... wtf...

EDIT3: Found it!, elementalDefs from the Hugemaps mod didn't have the growth modifiers in it, thus disabling normal growth.

Ahwell, now to find out why some mounts get shown 2x when mounted

N/A