Hello,
I have owned euphorbia tirucalli for a couple years. It is a winter growing euphorbia. I believe new growth is only from the terminal branch tips. No "back-budding". Hence, I do not think it will "fill out" in the way you desire, and pruning it back will not get you the desired results. Sorry.

Disclaimer: As I've only had this plant a few years, if anyone thinks the above it not right, please correct me, I'd like to learn new things and don't want to spread mis-information. :)

Here is a picture of the plant, if that helps!

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here are pics of my tirucalli in composition groupings. not very good, but oh well....

opposites attract:
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the three amigos:
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yours looks alot bigger and older than mine! :)

I've had a E. tirucalli (plus cuttings) for almost 10 years now, mostly kept inside in Kansas and Minnesota (it had those two halcyon years in Phoenix where it lived outside). Yours is actually more filled out than mine is usually.

To keep mine from getting too huge I've trimmed it several times, including fairly drastically. It does sometimes resprout from cut-off points, but not predictably. In this, I agree that it won't "fill out" the way you want.

I now put mine out for the summer, where the growth in the sun is more compact and thicker. However, I have had all sorts of trouble with sunburn early in the season. After three summers in Minnesota, which ought to be relevant to Seattle/Massachusetts, I keep my plant inside until we're done with frost (i.e., May). It has started growing inside during the spring, somewhat sparsely. But then the growth stops because it is usually too cool in May - so I leave the plant in the shade to avoid sunburn. I then wait until consistent summer warmth (mid-late June), and more new growth, to put it out in the sun again. It stays there until October, or serious frosts.

I have also been experimenting with the best location to keep a plant inside overwinter. I would like to prevent my succulents from growing early in the spring if the light is not sufficient for dense growth. Thus, this winter I am leaving some small E. tirucalli in my basement, which stays in the 50s-60s well into June. So far this winter I have a large plant in relative cool and dark, near only one not particularly bright window, and things look OK so far.

Finally, if you like these plants you need sometime to see a real full-sized one. In Phoenix there is one in the Desert Botanical Garden which is the size of an orchard apple tree. I'm sure there are also big ones all over southern California, where some people tire of them because of their size, growth and sap.

Thanks for the follow up guys! Unfortunately, I live in an apartment in downtown Boston with no outdoor space. No patio, no balcony, no nothing. And to make it worse, as if we don't already have bad weather here, I have North facing windows. As a matter of fact, I am surprised that my plants live.

I am planning on moving to Austin in a few years, and am hoping to plant it in the ground there, so I am assuming it will grow to some size there!

The neighbor's specimen is close to 20' tall, planted in the 1970's(?). I need to get a picture of it--it's magnificent. The first 5' is just a big trunk. The neighbor got out there one year and hacked off some side branches and had to be taken to the emergency room due to the fumes from the toxic sap. Here's my little baby cutting courtesy of the neighbor's giant. It's about 3' after 4 years:

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Mine started out very sparse and lanky as yours in the pictures. I did nothing at all to it--it filled in and became bushier by itself. HOWEVER, I have Mediterranean conditions here with plenty of heat and sun, just what it likes, so perhaps that is a factor as well.

My pencil tree is several years old and is now about eleven feet tall and six feet wide. But mine is a green color, I would have rather had the red color. One year we had a hard frost that killed most of the small branches but not the big thick ones. The tree resprouted small branches eventually. I know the sap will cause a rash if it gets on your skin or blindness if it gets in your eyes, but this is the first time I heard of the fumes being toxic.

but this is the first time I heard of the fumes being toxic.

Yes, first time I'd heard of that as well. It was just such a big specimen--how many of them are out there that large? Mine has been very slow growing--it is in a super-dry spot.

I came on line to see if anyone else has had problems with fumes from pencil bushes. I live in Florida and have several growing in yard. The one in the corner of patio was getting taller than house, hitting the soffets and bushing out into the walk way. I knew past trimming rooted well so I cut it up in 3 pieces. Immediately my eyes had a slight irritation within half hour i experienced the worst pain have Ever felt. My heart was racing, etc. Being stubborn I did not call 911, which I know was stupid. After 2 houts and alot of flushing it passed. I hope no one ever goes through that again, so be careful.

There are reports of fumes being generated from Euphorbia latex, especially in enclosed places or under warm conditions. The latex of this particular species tends to be especially irritating to the eyes and you want to be careful to avoid any chance of the latex actually getting into the eyes. It can cause blindness.

It is important to remember that sensitivity to this type of plant toxicity will vary individually. Some will be easily affected; others not at all. I seem to be pretty resistant to Euphorbia issues, which is a good thing as I am very fond of them, both the succulent forms and hardy garden euphorbias (I've trimmed my Fire Sticks pencil cactus several times with no ill effects).

I will echo the concerns with pencil cactus, please be careful, it is highly dangerous!!! It is not worth experimenting if you are affected by it or not. I recommend not even owning but but If you insist try to wear gloves and goggles,and wash your hands anyway when you are done handling it, and god forbid DONT TOUCH YOUR EYES!!!! I learned this the hard way, the sap almost blinded me!!! I was trimming someones plant for them...