How deep into flower you can prune will be dependent on how healthy your plants are. Ideally, the bulk of your branches are pruned off before or during the flower stretch. Though sometimes I can still be too optimistic about branches reaching the canopy, only to end up chopping them off in a few weeks due to not reaching the canopy as I was hoping. The only pruning I do past the flower stretch is mostly those little budlets, but sometimes branches still get pruned in weeks 3-4.
The best part is that those shitty branches make for amazing clones. Rather than being hopelessly optimistic about a mediocre branch reaching the canopy, chop it off and clone it. Instead of that shitty branch yielding a couple grams of budlets, now you have a 1ft+ tall clone you can prep for your next grow.
Even in flower, a healthy plant is capable of taking a great deal of stress. Take a look at what the monsoon winds here did to this plant.
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Those were some big branches I lost, one got ripped off 5-6 weeks ago, and I lost the other one a few weeks ago. Those branches were part of the canopy and were between 2-3 ft tall. 4-6 ounces in weight I'll lose come harvest, but I'm just thankful that is the only casualty for the year because I've had much worse.
Definitely worth saving branches that make it to the canopy, but with a caveat. Those branches in question are usually pretty skinny, however the very top part of those skinny bottom branches can occasionally make it to the canopy itself.
When this happens, I only save the very top of the branch and prune everything else off.
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I'll get a more recent picture that is more up close than this, but hopefully this 2 week old photo illustrates the point.
Notice the branch on the very far right of the plant, almost looks like its floating now that I think about it. You can see just how much of that branch was removed, and how much was left on the branch itself. The branch had tons of little budlets and some leaves, notice how they stripped branch is mostly covered by the canopy? I stripped everything on that branch that was below the canopy, the top part of the branch that actually became a part of the canopy was left untouched.
As other posters have mentioned, removing everything on a skinny branch except for what is part of the canopy will result in better growth overall.
Think of your branches as a highway for a bus full of nutrients to "drive" on, and every bud/budlet as a "bus stop". Instead of nutrients stopping and being absorbed by every budlet on the branch, now only the top part of the branch is receiving any nutrients. This enables the top of a skinny branch to actually grow into the canopy, and produce buds thick enough to warrant being a part of the canopy.
Now, instead of that one branch being filled with dozens of tiny budlets, it has one large single cola/bud sight on it. As a result, the branch will definitely have more weight to it. Look again at the branch in question, the bud on it looks like a top nug now, despite the fact it is one of the lower bottom branches.
That's partly true, actually. Those lower branches can in fact reach the canopy in some cases, but only if the budlets forming on those lower branches are consistently being pruned/removed. By doing this, we're effectively redirecting the flow of energy in that particular branch. Instead of all the energy being directed to the myriad of budlets on said branch, the energy is now directed on the top most part of the branch. Now that the branch doesn't have to worry about those little budlets, and can now dedicate 100% of the energy toward reaching the canopy.
The leaves also correspond to specific branches and/or bud sights. The leaves are providing energy to what they are connected to.
Ultimately, whether sunlight or artificial light, it still cannot penetrate through leaves. Leaves absorb light, so even with the power of the sun, those budlets will never amount to anything. The instant light comes into contact with leaves, we now have energy converting as the leaves convert the light instantly.
This is why trees cool down yards, and why fuller spectrum lights lead to cooler grow rooms. Trees don't "block" light, they absorb it. So you literally do not have heat, how can you? The light was converted into energy for the plant, as opposed to heat. Similar to DE HPS/LED vs SE HPS; single bulbs have little usable spectrum, where as DE HPS is somewhat close to LED in terms of usable spectrum. The result? A single DE produces less heat in a grow room than 2 SE lights, simply because the plants are capable of absorbing more of the DE/LED light than SE light. More light converted into plant energy = less light converted to heat.
I'd much rather the light hit one of the huge fan leaves on my canopy, than hit all of those little budlets on the bottom and hope for the best. Even with unrestricted access to light, those budlets will never get very big. The reason the plant consistently makes attempts to produce budlets is for going to seed. Think about all the places you find bananas, usually on the bottom half of the plant right?
Consider the surface area of all of those tiny little budlets vs. the surface area of a large fan leaf. The fan leaf absorbing all of that light and transferring energy to the canopy will always be better than removing the fan leaf.
Not only will the budlets be incapable of absorbing the same amount of light as that fan leaf would have, but now because that light isn't being absorbed it is now becoming heat. Indoors or outdoors, the plant and light itself work the same.
We can either add some decimals in weight to those budlets, or remove the budlets and add some grams in weight to the top buds.
Exactly this, and you can see where and how it happened in the photo I posted above. Energy
has to go somewhere, so if it isn't going to those budlets anymore, it can only go to the top of the branch. As a result, the branch catches up with the canopy and becomes like that of a top bud.
This isn't just true of marijuana, but pretty much all plants. Tomatoes grow into tangled nightmares when not properly pruned, melons/cucumbers can very quickly take over your garden/yard, peppers won't grow as tall without pruning, etc.
Proper pruning is a very standard and crucial process of any garden. It promotes better quality fruits, and in greater abundance with consistency to boot. It also drastically reduces your chances of an infestation, disease, or mold/etc.
It is very easy and understandable to believe the pruning is counterintuitive, however it is crucial to the success of any garden.
Plants that aren't pruned will typically opt to preserve their genetics as opposed to producing substantial fruits. A wild tomato plant isn't going to produce fatty eye catching tomatoes; it'll grow as few low hanging fruits as it possibly can in the hopes of furthering its lineage.
