I have a single citrus plant (a donor plant, but he did not know what kind of citrus it was). It is full of blossoms. Do I need to do anything for this plant to form fruit? It is in my greenhouse. It is only a single plant and there are no other citrus trees anywhere around me as far as I know.

It would help if you knew what kind and how old it is. Apparently, the day of citrus growing for the backyard gardener won't be for many more years as Florida now has that fatal disease "greening". Citrus greening is one of the most serious citrus plant diseases in the world. It is also known as Huanglongbing (HLB) or yellow dragon disease. Once a tree is infected, there is no cure. Unlike the canker that hit our citrus some years ago; this makes the fruit unpalatable and also kills the tree.
I have one citrus tree growing in the ground; a Meyer lemon. Those lemons are huge and juicy. However all the fruit is growing on the lower limbs of the tree - nothing on the top branches. I don't know why and neither do my gardening friends. Very odd.
I have a Key Lime tree in a huge pot, and it's loaded with blossoms at the moment (for the second year); also a Persian lime (also in a huge pot) which produced quite a bit of fruit. I actually made a few key lime pies this past year - something I'd never done before; just wonderful. I had some English friends staying with me last weekend and served it for dessert -- they raved about the pie.
Back to your citrus tree; it would be good if you could identify it -- I'm pretty sure you don't need another citrus tree to encourage blooming and fruiting (as you would with Papaya). Just get a good citrus fertilizer and apply at the right time for your area. I'm sure you won't have to worry about greening for a few years yet.

sorry i'm a month late.....just arrived to this site 3 days ago
regarding your citrus "plant" you don't need multiple plants for it to produce i had a meyer lemon for 20 plus years (still have it) and never owned another citrus it produced very well
to me the meyer lemon is one of the most fragrant plants and it is a bee and butterfly magnet......

Until/if I get fruit, I have no clue what sort of citrus I have. The guy that donated it to me didn't know either.

I know I'm late to the party but I'm growing a Meyer lemon here. It's outdoors in the warmer weather and indoors under lights in the colder weather. Citrus does like a lot of compost but also free draining. If it blooms when it's indoors, I use a clean paintbrush to carry pollen from one flower to another. It's pretty successful but I get more fruits when it's outdoors being pollinated by flying creatures.

welcome @zippi1!

you are so right about the fragrance of the Meyer lemon fragrance, it's definitely one of my favorites. I've got two trees in containers that I really enjoy.

What color are the flowers? My Improved Meyer's Lemon is in bloom and has pink and white blooms. My Satsuma orange will bloom a little later, but sometimes it blooms early. It has VERY fragrant white flowers. The branches have thorns on them. Here is a poor picture of the lemon tree to give you an idea. Sorry, it's dark outside but this will give you something to compare to.

My Meyer lemons have been huge and juicy; the size of a good orange. I'll have to take photos next time. I'm pleased to see blossoms higher up the tree finally. The last two seasons all the fruit were on the lower branches. Rather strange. I still don't understand it; but I did a little pruning and that seemed to have knocked some sense into it !

My citrus tree is a calamondin tree, grown in a container. From what I have seen with my tree, it started to make more profuse blooms on the third year and then followed by the fruits, usually around late Spring to whole of summer when it is much warmer here and it is getting more direct sun. I have observed it really likes lots of watering here, there was a year it delayed the fruiting and I realized I may have been underwatering it. But once I gave it more watering, especially during our very dry warm periods, it has resumed making the fruits as soon as the blooms are gone.

Right now it is making new little blooms again, and some branches are having some new fruits, and some older branches still have some old fruits I have not harvested yet. Our temps are rising fast now, in the mid 70's to low 80's, and with the city trees still asleep, it is getting more sun.

It really is a tree for the outdoors, so just wait till your outdoor conditions improve, and give it all the sun, water and air you can give to improve its growth.

I can't wait to see what my potted Improved Meyer Lemon, Satsuma Orange, and Meiwa Kumquat do this year. We had our front yard tree uplifted so that more sunlight will be let in. More sun=MORE FRUIT!!

What's the difference in your "Improved Meyer Lemon" tree compared to the normal Meyer lemon. The one I had decades ago had large, great-tasting lemons, but were chock-full of seeds.

I asked that, too. "Improved" against viruses, I think?

I, too, have no Idea what the difference is between an "improved" or regular Meyer lemon tree. I don't recall that the label on mine stated anything. however, I can tell you that there are very few seeds in mine and the amount of juice is incredible. I shared some with neighbors and friends and they are so impressed.

Here is the explanation.
http://homeguides.sfgate.com/c...

Who has tried it out in temps down to 20F without protection? (From the homeguides.sfgat link)

Keep my babies out in the cold?! Whaaaat?? Any threat of freezing or close to it and it's under the porch they go!

Well, I was brought one as a gift last summer. It's still in that container for now. I've left it outside enough that it got unintentionally exposed to overnight frosts and temps down to about 30F briefly. Those didn't seem to harm it, but were enough to kill and severely damage a couple of other plants left out at the same time. That link Larry provided said it can withstand temps down to 20F, but after that it would need protection. That sounds as if it would be a plant that could be in the ground the year round in Kingwood, doesn't it? There would be an occasional winter here that would work, but I think generally we'd have at least one cold spell that exceeded the cold tolerance. Even so, that kind of knowledge is useful for winter handling. It clearly likes more sun than it gets when I bring it in, so the more time it can spend outside without that being a death sentence, the better. However, I don't always assume that kind of information is necessarily correct, so I was wondering if anyone was growing it where it would have been exposed to temps that cold.

My calamondin has been exposed to 21F two years ago, apart from some dropped leaves it endured well. I gave the root zone good watering a day or so ahead of the freeze warning to help it.

Sometimes if you crush a leaf you can tell what it is.... a lemon vs an orange
Sequoia in California

@drdawg I think that when the original Meyer Lemon was brought from China to California early in the 1930-1940s, it had a virus that wasn't noticeable. Killed lots of citrus and sometime in the 1950's someone in California came out with a Meyer Lemon variety that did not carry the virus and wasn't susceptible to it and they name it the Improved Meyer Lemon. I think that was what I was told when I bought mine last year.