The Garden has five branches of upgrades, only one of these is nessecarily significant though all are rather helpful to some extent
Seeds Availible - 34 for Turnips, 31 Million for Salty Crackers, and 95 Billion for "Mystery Seed"
Boxes Availible - 62 Million ~ (20 upgrades)
Carrot Grow Speed - 50 Million ~ (20 upgrades)
Turnip Grow Speed - 62 Million ~ (20 upgrades)
Cracker Grow Speed - 50 Million ~ (20 upgrades)
Seeds available is the "Nessecarily significant" upgrade, it allows you to access each selection of seed at the top of the Garden, and without the seeds being unlocked you will be unable to boost your DPS multiplier or prestige, it is to be noted that Carrots are unlocked at the start for free, and the price of the Turnips makes them practically free.
Boxes available affects the amount of boxes that you are able to grow your crops in, unnessecary for creation of the final seed, as you only need a single crop. It is still significant for creating high percentiles of Cracker DPS bonus and Animal chances.
Carrot growth speed is arguably the least useful upgrade above. Due to the already high basic production of carrots you hardly need any upgrades past the first five, and the fact the average player never uses the carrots in the first place.
Turnip growth speed is rather significant if you plan to grow cows, as turnips grow at a rather slow pace. Upgrading to the maximum, however, (though not worth the DPS that the cows produce) will result in turnips growing at a decent enough pace that you can farm enough to max cow percentiles within a few minutes.
Cracker growth speed is the most sought after of the grow speeds, many players only grow crackers for the majority of their gameplay as crackers can increase your DPS to 3x the original amount, they often take a decent amount of time to grow, making grow speed a nessecity if you wish to max out the multiplier.
It is to be noted that all crop effects are not permanent and will slowly decay over time (With exception of the Mystery Seed)
The Carrot, the most basic crop generates a bonus +0.5% to your chances of spawning a pig instead of a sheep, making it necessary to have farmed at least 100 carrots to cap your chances of pigs out at 50%, the Pigs give more coins upon death compared to the Sheep, giving 5 coins base.
The Turnip, the second most basic crop generates a +1% to your chances of spawning a cow instead of a sheep, making it necessary to have farmed at least 50 Turnips to produce the maximum chance of having a Cow spawned, 50%, the Cows give coins upon death larger than Sheep and Pigs, giving roughly 50 coins on death.
The Salty Cracker, the second most advanced crop, often the most farmed generates +5% (roughly) to your DPS multiplier, increasing the output of every single board, capping at 300% efficency (3x), making it nessecary to farm several full boards of crackers to get a full multiplier.
The Mystery Seed ("Starrot"), whose effects can be read more in depth on the article "Prestige" is the ultimate goal of every game of Idleplex, allowing you to reach a new game, a clear board, and lettting you play the game all over again with a permanent +20% DPS boost.