When I read about pruning the proper season to do it is often specified, and differs between plants. Why should the season matter? Are there actual risks to pruning at the wrong time, or is it just a matter of the time a plant takes to to start sending new growth around the cut parts?
You should also consider the possibility of diseases as to when to prune. For example, in northern North America we have oak wilt in many localities, which means that oaks must ALWAYS be pruned during the winter, before March 1st, in those places. This is because the wilt is vectored (transmitted) by an insect that is attracted to oak sap. When it feeds on the sap, it passes the disease to the tree. Pruning in winter means that the tree doesn't bleed sap - both after pruning and in the spring when the sap rises.
As noted in the other comments, you should research each of the trees that you intend on pruning to make sure that they're pruned correctly.
Depending on the plant or tree there are really two distinct reasons and in some cases both.
1) you can prevent the plant from doing what it is you planted it for.
2)you can interrupt the plant’s cycle, prevent it from doing something that it needs to for its own health and survival.
For instance, most temperate fruiting trees (including flower selected cultivars) set up buds in winter, you therefore usually want to prune these trees late fall to early winter so the tree can setup buds in the wood you have left behind. If you prune too late such as in the spring you have reduced the number of buds set which will lead to a year of weak growth or over vigorous growth from fewer buds, that weakens the tree not only this year but potentially next year too because this year is when it’s gathering energy to prepare for winter and set new buds. Also you are affected if you expected something out of it such as apples or a beautiful display of cherry blossoms.
In the case of forsythia, you prune after the plant has dropped its flowers, flowers bloom on 1st year wood, so in this case if you prune too early, you lose all the work the plant did to give you beautiful stalks of flowers.
Follow the pruning instructions for each of your plants, there is 100s and in some cases 1000s of years of wisdom and experience that causes those words to be written for you.
There are some risks to pruning at the wrong time for some plants. Some examples:- Acer should be pruned when the tree is more or less dormant, partly so as to see its shape clearly when the leaves are not present and not destroy the natural form by pruning, but also because, if you prune it in spring, let's say May, when the sap is up, it will bleed; prune it between Septemher and January and it doesn't bleed (though in the UK, they don't bleed from end of July till the following early winter). In cooler areas, for permanent plants, pruning at the end of summer might encourage new growth which does not have time to harden off before the winter cold arrives, and it dies back. Fruit trees in the genus Prunus, such as plums, should not be pruned in winter in some areas (UK for instance) to reduce the risk of silverleaf disease, so pruning for those is recommended in spring or midsummer, depending on the age of the tree. There are many other examples with other plants where risk of infection will be higher if they are pruned at the wrong time - what sort of infection can vary between regions, according to weather.
Already mentioned is pruning at the right time so as not to cut off wood which will produce flowers the following year, so generally, shrubs which bloom in spring are pruned immediately after flowering; the subsequent new growth is what will flower the following year. Pruning at the wrong time won't kill the plant, but it won't have any flowers. When to prune for flowering may even vary by variety of plant; Hydrangea macrophylla varieties should not be pruned back hard for they won't flower if you do, but Hydrangea paniculata can be pruned back hard and will still flower again. It's largely down to whether a plant needs to keep growth that is either one or two years old in order to produce flowers, because those are the shoots that are programmed for flowering.
It's best to check the pruning rules for your area for whatever kind of plant, because there are good reasons for the recommendations.
There are various reasons, and nature is never so precise, so you may tweak the "rules" for your region/climate (and for specific cases).
If you want fruits, you should prune so that plant will flower, and it can keep the flowers/fruits. Note: if you care about flowers, it is the same: you prune for flowers, not for the form/green parts.
Some plants want to create fruits, so if you prune flowers (when they are bad, end of period), you force the plant to do new flowering period. So you do not make the plant to make fruits (which take them a lot of energies).
If you prune too late, you will get probably a disproportion of vegetable buds, vs flower buds.
Note: many gardeners prune all years. E.g. in winter for fruits, then to equilibrate more the fruiting, then the suckers, etc. etc. And always ready to do emergency pruning, when strong wind or snow broke branches.
Where the fruits buds are located, depends on the species (often just family). On grapevine we should prepare branches for next year pruning. On peach trees we should carefully prune (but strong): the plant will lose less energetic buds (near trunk), so if you prune too late or not enough, you will have a tall tree, without leaves and fruits on bottom. Not so nice to harvest, nor to look at it.
Note: climate affect also when to prune. i think it is essential to have the big books about gardening: from time to time you need to look how/when to prune (and how to multiply) a specific plant. unless you are a professional with much memory ...get a book. Generic rules are not so useful (but to help speed up the "try and error", which look bad in a garden, but it is a good way to start conversations).
Pruning can be done ANY TIME of the year. I have never run into a plant that shouldn't be pruned any time of the year. There are plants that need to have old wood pruned out or other more finely tuned pruning. Definitely when unfamiliar with a plant before pruning that someone should go learn about that plant, FIRST.
The only rules are to use bypass pruners or pruning saws, cleaned with alcohol...don't leave stumps of branches, don't cut off more than 1/3 of the plant at one time (to include lawns), SHARP pruners. There should be no growth from the cut EVER. Cut whatever branch that goes against the form such as angling towards the middle of the tree, dead or damaged or dying branches all the way back to the main branch or trunk it was growing from! Leave no stumps!
Do not try to cover the cut with anything. Trees need to be thinned, if you are trying to go against the natural form of the tree you'd better know what you are doing or just don't do it.
I have had the spouses of a couple take the pruners and HIDE them from the pruner nut of the family! I LOVE pruning and luckily I KNOW what I am doing. It is addictive. Hey, pulling weeds can be addictive.
You need to know how to help your plant BE the plant you want or better or healthier, yet before you plant that plant just learning all about that plant is critical so that pruning is not necessary to that degree. Ugh, did that make any sense?
Pruning too late in the season is just like fertilizing plants late in the season. All that energy from whatever you cut off going into the plant will cause new growth. New growth is going to be killed during the oncoming cold season. Or pruning too late just like fertilizing too late will make the plants far more susceptible to fungus and disease and insects!
A good rule of thumb is, according to ISA:
As a rule, growth and wound closure are maximized if pruning takes place before the spring growth flush
We may ask why?
This useful simple rule is based on studies about trees Physiology and tree Phenology.
An important physiological activity of trees is the detachment of foliage:
- trees spend energy in order to detach their foliage
- Before a complete detach of foliage a tree is able to take back the energy that was spent in foliage formation. This Energy is then stored in the wooden part of the tree as carbohydrates
The main model on tree Phenology is Askenasy Curve.
These rules are based on scientifical studies, not opinions.
The first rule we should follow is the National or Local law and regulations. Usually, a large period is given among which you may choose a week or a month according to tree Physiology and tree Phenology studies.