Kiwi's are vines not trees. They grow very fast and large and if your arbor is designed properly they should be prunned. If not they should still be prunned, but it may be difficult. I would not recommend prunning them once they begin leafing out, it could lead to damage. Usually there is one male vine for 4 female vines, again depending on what you have as an arbor. The best time to prune is at the last part of winter while the plants are dormant. Here it is about mid February.

If they are blooming, then you should not prune now if you want any fruit this year. After bloom, any males can be pruned back really hard, up to 90% removal. If you have females with fruit, then it would be best to wait until the vines are dormant next winter. If no fruit sets, then go ahead and prune them back hard as well, then re-train the new growth. Any bleeding from the cuts will not harm them.

~kiwinut

Kiwi growth may become excessive. Pruining to achieve a strong stem with a few strong canes should be prioritized.
Start now and you will be glad later.

Thank you for all your replies!

There seems to be a disagreement b/w george_23 and kiwinut about whether pruning kiwi once they begin to leaf out, unless I've misunderstood. I did notice that it bled a lot when I cut it, and that's what made me worry. Is it a difference of timing b/w george_23 and kiwinut's answers? For example, george_23, do you think its okay to prune the kiwis after they have flowered?

I'm not worried about fruit growth (they haven't yet had any). I would prefer to obtain a strong stem as grantman suggests.

There is always a chance of damage when you prune after the plants leaf out, but that does not mean it will always cause damage. I have pruned many trees vines when I shouldn't and no harm came. Vines will bleed a lot, but will stop. Mother nature often prunes plants with wind, hail, animals and they still make it. I sometimes prune my grape vines after they have set grapes to add size to the grapes. I prune the vines in the winter anyway and a lot of what grows is going to be cut off anyway.

You said you have plants that look a couple of years old. Does that mean you don't know for sure or maybe someone else planted them and you bought the land or something. If they have never made fruit, then maybe ther is no male vine.

I would agree with the desired vine form. All mine are straight up to at least 6 feet with a single vine. Then I prune in the winter to control shape, growth and vine size. Just like many plants the growth can be to either make plant or fruit depending on how you prune.

If the vines are a couple of years old they should be at least 1 1/2 inches in diameter, and already have a single main trunk. If not you have a mess and should try to get a single trunk. I assumed that already was the case and did not feel it worth prunning until winter.

In fact, this is my first year taking care of the kiwi plants. I think they have been underpruned in past years. They are around 5 years old, but the main trunk is only 1 inch thick. They are between 5-8 tall.

There are 3 females and 1 male, but only the male has ever flowered. That was last year, and it didn't flower much. It looks like only the male will flower again this year. They are all growing against a wall.

As for the form, 3 have a vertical trunk, but the horizontal branches are crazy and some are very long (e.g. two metres). One of them has 4 trunks. I don't know at all what I should do about this one.

Kiwis are long lived plants so yours are still in their infancy they need you to shape them for the long term. Hopefully they have good soil and some sort of structure against the wall to climb on. Do you think someone intended an "Espalier" style of growth? Most people in USA think of an overhead trellis like grapes but bigger.

You should be seeing baby fruit now (if you had any flowers) and can cut off any new wild growth, and get some fruit this year. Obviously make sure you keep them well weeded and mulched, and watered until you pick the fruit and they slow down.

Then look at them in January and decide how you want to shape it and be brutal. Decide where your main stems are going and the biggest fresh ones from last year (at least a foot apart) and cut off the rest. Tough love!

Maybe post a picture so we can see what you are working with, especially the 4 trunk one.

Then give them a good layer of compost in early spring and plenty of water as they start to grow again. Try to keep the branches spread out for sunlight and air.

That's my 2 cents... Good Luck