[Updated 15/12/2022]
The aquarium-keeping hobby is full of debates, some of them simply due to the amount of misinformation in the hobby and aquarists not understanding the science or logic behind certain topics. Other times, the debate is rightful.
One debate in particular - whether plants should be added during or after the cycle, falls in the latter category, and rightfully so. After all - it seemed like aquarists had success, and failures, with both cases. So what gives?
The purpose of this article is to give perspectives from both sides - to ultimately help you decide if you in particular want to add plants during the cycle or after (assuming you do want to add plants to your aquarium).
To preface the discussion, I must make the statement that the purpose of this article is not to discuss the benefits or issues caused by adding plants to an aquarium. That is a whole other topic.
So, let's talk about when to add plants to an aquarium. To do this, we need to first define what cycling even is, or what a cycled aquarium entails. There are two dominant, and really truly accepted definitions nowadays, as stated here. Both definitions entail ensuring ammonia and nitrite remain at zero, by ensuring that at least 1ppm ammonia (and 2.74ppm nitrite) to be consumed or simply not produced at all each day. The first definition specifies that this must be done via nitrification, the second is simply by whatever means possible.
The crux of the debate lies in the realization that plants also consume ammonia, and therefore by definition are competitors of nitrifiers (for ammonia). Let's address each definition in turn with this in mind.
Cycling: establishing enough nitrifiers that 1ppm ammonia can be fully oxidized to nitrate (via nitrification) each day.
In response to this definition, there is zero debate. Plants, given that they compete with nitrifiers for ammonia, would certainly slow down the growth of nitrifiers or prevent them from growing to a large enough population to consume 1ppm ammonia a day.
This is so clear that many Walstad aquarists (google what Walstad aquariums are) go as far to entirely avoid even calling what they do 'cycling', using terms such as 'establishing' or 'conditioning' an aquarium to avoid any confusions of what they are doing - which is to establish whatever organism to ensure the aquarium is safe for live stock, caring not at times even to measure ammonia or any other parameter, relying on the health of fish and other organisms (plants, specifically, but sometimes also algae and so on) as indicator of aquarium health. I want to stress that Walstad aquariums are extremely diverse, as are aquarists who keep them and their perceptions, so I am not making a blanket statement of what they are or do, but rather just briefly describe some of what is involved.
So, is it so bad that one relies on organisms such as plants to consume ammonia rather than nitrifiers?
Thus leads into the debate surrounding the second definition of cycling.
Cycling: establishing an ecosystem whereby 1ppm ammonia can be consumed, or simply not produced each day.
Here is where the debate rages.
Arguments for adding plants during the cycle:
The plants themselves will consume ammonia, i.e. helping with cycling, or indeed one can consider it 'cycling with plants'. Especially if one wants the plants to dominantly consume ammonia in the future anyways, this can be especially preferable.
There is only one wait time for an aquarium to be 'established'. Rather than cycling, then adding plants and waiting again for the plants to root, substrates to settle down, etc., before adding live stock, both processes happen at the same time.
The full 'ecosystem' sans live stock is established from the get go, so parameters may be more stable earlier. Adding plants along with live stock at once can see parameters change, sometimes enough to cause harm (or even death) to the live stock. Of course, this won't be an issue if plants and live stock are not added at once, though it is not always preferable.
Arguments against adding plants during the cycle:
A robust population of nitrifiers can be established first, to ensure they are able to consume ammonia when nothing else can. Nitrifiers are very resilient and can go ammonia-starved for weeks to months before going dormant, let alone die off, so they can act as a 'buffer' for a very long time. Establishing a robust colony of nitrifiers prior to adding plants can therefore be highly beneficial. Nitrifiers are able to perpetually consume ammonia without needing to grow/reproduce, while other forms of ammonia consumption is as a nitrogen source, i.e. to grow/reproduce, therefore requiring other nutrients too. So if there are nutrient deficiencies, plants and other organisms may not be able to consume ammonia, and if this happens when live stock is already present in the aquarium, then it can be bad. The 'buffering' nitrifiers can then handle the slack when this occurs, but only if they were already previously established. Not everything goes to plan either, and plants can also die off, causing ammonia spikes that way. Again, nitrifiers can help consume the ammonia from the plant die off, preventing harm to live stock. But again, only if there is already a robust colony of nitrifiers to begin with.
A robust population of nitrifiers can in fact help plants better establish in an aquarium. Even the most careful of aquarists cannot always avoid unhealthy plants, especially when plant health is not always in their control. The process of transport can cause harm to plants, and at the beginning when the plants are recovering and unable to consume ammonia effectively (or when the aquarist has yet to get everything right), ammonia can build up and of course eventually become harmful to the plants, which can snowball a cycle of more damage, more ammonia, again and again. This can be especially prominent if one tries to use something like a deli shrimp to cycle an aquarium, which can release A LOT of ammonia, enough to actually harm plants. If nitrifiers are already established, then they can at least take care of the initial ammonia so that it won't become toxic to the plants. Of course, different plants are impacted by different concentrations of ammonia differently, and usually ammonia does have to be very high (from our point of view) to cause harm, so it may never reach that point. But it is a consideration.
Plants can consume nitrates, making it hard to track the progress of ammonia production and consumption.
Plants can die off, making numbers be way off or be unable to track progress. For example, some times one keeps on getting ammonia measurements - and they have no idea if this is because the ammonia they added is not being consumed, or if it is consumed, just ammonia is produced at the same time.
So ultimately, it generally boils down to whether one wants to ensure ammonia can be established first as a robust method of ammonia consumption, or if one finds it to be unnecessary, and in fact that advantages offered by planting immediately outweighs the disadvantages. And this is not something that is right or wrong - at least not generally.
Generally, the perception is that for beginners, it is better to add plants after the cycle. They would benefit from the robust nitrification initially established, especially given beginners may be more prone to killing off their plants (and thus the buffer offered by a robust nitrifier colony can be highly preferable). For those that know what they are doing, the nitrification buffer can be never needed and therefore not justifiable to establish.
So - it is absolutely up to you what you want to do. There are advantages and drawbacks in both cases.