What will happen if I wash(in cold water) a merino sweater that says "dry clean"?
Will it pill? Or shrink?

IDK, Woolite(tm) cold water stuff is made to wash wool at cold water temperatures.

sheep dont shrink after a rainstorm..

Better to hand wash with Woolite, then blot with a towel to remove most moisture, then air dry flat.

I wash all of my Icebreaker merino wools on cold tap water, gentle cycle, set to low spin, hang air dry only. I wash about once every 8 weeks. I have not noticed any shrinking or destruction of the fabric.

Tip: be careful wearing your merino wool on public transit. If a bedbug decides to hitch a ride, how will you know? The only way to be safe is to wash it on hot and also throw it into the drier on high; thus ruining your wool. Your best bet is never to bring merino where nasty critters are likely to be found.

According to my wife, knitting and yarn stores sell various wool-washing products that are better than Woolite.

Yep they do

Look!

Before and after a rainstorm

https://www.nikwax-usa.com/en-us/prod...-1&fabricid=-1

I use Nikwax's product and it works well. Its pricey, but it'll last a long time.

That's enough proof for me.

I wash my wool like I wash my hair, hand hot using a detergent. I find that this works better than a soap, even a gentle woolmark soap. A few drops of washing up liquid or shampoo seem to work well in a gentle wash cycle.
Wash wool on its own, spin at low revs (600 on my machine) and dry flat on a towel away from direct heat and don't stretch it out wet.
If you hand wash, don't agitate too violently, just let it soak for a while, and never wring. I squeeze a balled up item very gently.

The official care pages for my brand of Merino is a good read.

Woolite is not good, nasty chemicals, soap is bad BUT detergent is perfect(Dawn). Hot water is OK, rubbing it when wet is the problem. Here is a link that I have followed for many years. Drying wool is a PIA but I usually do my wool sweaters at the end of the winter season. For wool Ts I use the washing machine and they do not hold up very well, lots of holes. I do not recommend using the machine My sweaters are many years old and look very good, the Ts are only 1-2 years old.

https://www.fuzzygalore.biz/articles/wash_sweater.shtml

'Eucalan' brand wool wash. my wife the crazy knitter uses it for all high end wools including qiviut (super expensive musk ox yarn). swish it in a sink, squeeze and air dry.

me? with my Smartwool baselayer i put it in a sweater bag, minimal detergent and tumble it on no heat/air only/fluff setting.

here is a link to Smartwool's care page.
https://www.smartwool.com/care

I'll echo the vote for Eucalan. My wife, my mother, my sister, and my aunt are all crazy knitters so we have plenty of experience with Eucalan. I handwash all my merino "dry clean only" stuff in Eucalan, follow the directions on the bottle, and air dry. No problems.

I don't trust myself, so I follow the tags instructions. My marino sweaters that say "dry clean only" get dry cleaned. My ibex balaclava says it can be washed, so it gets washed.

Thanks, everyone. I hate dry cleaning. Too many chemicals, too expensive.
This makes me wonder... can I wash wool suits by hand also?

You wash your sweaters with flea shampoo??

Actually, dryer heat kills them. So if you're worried, put the clothes in a dryer on high heat without washing first. No shrinkage.

Ask your mother!

Yep! When I can bring them back home because they run fast.

It's dangerous for them outside. Especially during the hunting season.

wash? you're supposed to wash them?

3 winters now of commuting using smart wool and icebreaker Marino wool mid weight shirt and long johns??? Well anyway wash in front loader with other clothes with standard liquid detergent and warm water moderate spin and hang dry. Hate using driers when I have the sun. If no sun the house has clothes hangin to dry. adds moisture to the dry inside of house in the winter. The wool clothes are three plus years old washed weekly as stated above and still in great condition.

I have some merino wool clothing that is 30 years old. It's getting pretty ratty, but I consider merino wool to be one of nature's best inventions. I wash it in the machine and hang dry it.

True, sheep don't shrink, but they can grow their hair/fur continuously. Sweaters don't grow.

No. Not because of the wool but because of the haircloth padding in the chest and shoulders. (Friend is a custom tailor)

Handwash with whatever surfactant suits you and lay flat on top of the washer or dryer to dry for a couple of days. Do not try to hand launder your expensive Savile Row four button suit!