Actually though most of the stories say it is the first flower to bloom in space, and Scott Kelly tweeted it as such, it turns out that there have been several flowers grown in space before, most recently in 2012, but the first such was way back in 1982. It does seem to be the first Zinnia to flower in space.

Here is Scott Kelly's tweet

"First ever flower grown in space makes its debut! #SpaceFlower #zinnia#YearInSpace"

Here is the CNN news story about it, where Lin Taylor mentions an earlier flower in space. Astronauts grow first zinnia flower in space -

Another photo of the flowering Zinnia

The variety is the Zinnia 'profusion' orange.

Earlier flowers in space

Here are some photos of the previous plant to flower in space, a sunflower, grown by Don Pettit in 2012.

from: Letters to Earth: Astronaut Don Pettit

And after it had gone to seed

Letters to Earth: Astronaut Don Pettit

He did it as his own personal experiment in his free time.

First flower in space

But that's not the first flower in space either. According to the Guiness Book of Records, the first flower in space was Arabidopsis which was grown on Salyut 7 in 1982. More details here. They came to flower and then they set seed so completed the full cycle from seed back to seed again. This is a rather unprepossessing plant with small flowers. 

"The largest plant was 65 millimeters tall, had 12 green leaves, three tiny pink flowers about 2 millimeters across and a pod 3 millimeters long. When the pods ripened, they burst open to reveal the seeds within. It was great news for the biologists who were keen to receive seeds from a plant grown in space". The story of Space Station MIR page 380.

Salyut 7 as seen before its dramatic rescue in 1985. The systems are down, it's frozen, and not working properly as you can see by the misaligned solar panels. Anyway - this is the station where the cosmonauts grew the first flower in space, Thale Cress, in 1982.

I can't find any photographs of the plant that show the flowers themselves (there are some black and white photos here with the petals removed for scientific purposes). But this is the species of plant that was first to flower in space.

Arabidopsis thaliana - also known as Thale Cress

Why the Zinnia flowered - astronaut autonomy

Interestingly the reason the orange zinnia flowered is because Scott Kelly was given permission to water it when it seemed to need watering rather than on a strict scientifically set out schedule. After a promising start the plants got mouldy from too much water

Eventually they ended up like this, Scott Kelly on Twitter :

"Our plants aren't looking too good. Would be a problem on Mars. I'm going to have to channel my inner Mark Watney."

He was advised to increase the fan rate to dry them out, and cut off all the mouldy leaves.

Some of the plants died but this helped the others.

But then he said they seemed too dry now. He asked for permission to water them on Christmas Eve, but was told by mission control he had to follow the procedures of the scientific experiment. That meant he couldn't water them until the next scheduled watering on 27th December.

But he argued with the ground staff for his case, to be able to water them when needed, rather than according to the experiment protocols.

"At this point, Kelly very politely lost his temper. “I think that would be too late,” he argued with the ground staff. “You know, I think if we’re going to Mars, and we were growing stuff, we would be responsible for deciding when the stuff needed water. Kind of like in my backyard, I look at it and say ‘Oh, maybe I should water the grass today.’ I think this is how this should be handled.”

"Instead of being annoyed at having his project taken over, Smith was ecstatic. He had a volunteer-astronaut willing to care for his plants personally instead of blindly following a pre-set chore list? Best Science Christmas Ever!

"The Veggie team scrapped their book of detailed procedures, and created a streamlined “The Zinnia Care Guide for the On-Orbit Gardener.” Instead of set schedules and procedures, it was a concise set of basic guidelines to help astronaut-gardeners use their human judgement."

From: Astronauts Almost Killed Their First Flowers (Article on Gizmodo by Mika McKinnon).

So, in response he was given more autonomy to water them when he thought they needed watering. And the big complex list of procedures to follow for cultivating the plants, including when exactly to water them got reduced to a single page “The Zinnia Care Guide for the On-Orbit Gardener.” giving him autonomy to do gardening in space basically.

And just over a couple of weeks after that the ones that had survived flowered.

Trend towards increasing astronaut autonomy - starting with Skylab 4 "mutiny"

Back in the days of Skylab then astronauts had almost every minute of their day timetabled, they were told exactly what to do and in what order. They had the idea that time in space was so precious, that it had to be strictly timetabled and that any time wasted was like a huge waste and cost.

But then with Skylab 4 the so called "mutiny" lead to new ideas, that astronauts in space like any humans need some free time, if they are there for long periods of time rather than just a day or two as in the earlier missions. Time to relax, to contemplate, think, or for their hobbies.

Skylab 4 where the so-called "Mutiny" happened - the transition from the previous Skylab missions where everything was timetabled pretty much down to the minute, and the present situation where astronauts have considerable discretion in the order in which they perform non time critical tasks and also have free time which they can use for photography, contemplating the Earth through the Cupola, playing music, reading, watching movies, hobbies, or whatever it is that they want to do. The result was a happier, more productive and more creative crew.

And they need also to be given some autonomy to decide the order of tasks, when the order is not time critical. For instance during space walks, they'll be told sometimes "Now you have two tasks to do, order doesn't matter, decide for yourself what order to do them." And inside the ISS they have a lot of autonomy deciding what to do in what order, as well as free time to just contemplate the Earth from the Cupola, take photos, do fun experiments of their own, play music, or whatever it is they want to do. 

And the result is not less work done in space, but more, and more creativity. The lesson they learnt is that humans in space actually work better when we don't have every last minute of our day timetabled to a strict regime.

There are differing ideas about how much of a "mutiny" this was. It seems a mild affair whatever the details. But it is clear that matters did come to a head in some way and that they had to change the way they dealt with astronauts in orbit, giving them more free time and more autonomy to decide the order in which they did tasks. And that this lead to a more productive expedition right away and has had benefits all the way through to the present day.

So this is a lesson that we are learning gradually. I think that this is a process that will continue in the future. That in future we will see more and more autonomy for the astronauts. 

Need for balance - space as a dangerous place needing discipline like life in a submarine

But at the same time, space is a very dangerous place. And your intuitions can easily lead you astray as well. 

There's a reason why they go through check lists and have to take many precautions for a space walk for instance. If the astronaut was permitted to just breeze through the checks and do whatever they like, we could have had many accidents.

Simple example - if they were allowed to just swing from handhold to handhold outside the ISS, without taking precautions of always having at least two tethers connecting them to the ISS at any time, we would probably have had at least a few accidents of astronauts lost in space, unable to return to the ISS or get rescued, or who had things go wrong with their spacesuits because of some mistake made donning them etc.

And it's a small space, and dependent on technology to a high degree. An astronaut who made a serious mistake could endanger the entire crew, easily. On Earth perhaps the closest we have to life on board the ISS is the likes of life inside a submarine that stays submerged for months on end. 

So astronauts have to be disciplined people, and willing to follow authority. And happy and comfortable doing so. 

The independent thinker who can't stand authority and who always has to make decisions for themselves would be a very unsafe astronaut at present. And probably into the future, also. They would not pass the screening process. 

That's not because of any prejudice against people who can't stand authority. It is just that space is not a suitable place for them. Similarly astronauts who suffer from claustrophobia would not be permitted in space (for obvious reasons, they are going to be living for months on end in a confined space not unlike a submarine again).

But within that need for discipline for things where it is absolutely essential at present - I think we'll see more and more autonomy as Kelly showed in a small way by offering to take on the responsibility for himself of deciding when to water his zinnia plants and how much water to give them.

It will be interesting to see how this pans out in future. 

For more background:

Letters to Earth: Astronaut Don Pettit about the first flower in space

Astronauts grow first zinnia flower in space - - CNN article

Astronauts Almost Killed Their First Flowers

How Mold on Space Station Flowers is Helping Get Us to Mars

No NASA, These Are Not The First Plants To Flower In Space - which has links to papers about the arabidopsis experiments

Growing Pains - about the early and at the time secret Soviet plant growing experiments including the arabidopsis ones.

Greenhouses and their humanizing synergies

And on the Skylab 4 "mutiny", see

The Skylab 4 mutiny, 1973

William Pogue, Astronaut Who Staged a Strike in Space, Dies at 84

And also discussed further down the page here: 8. ORGANIZATION AND MANAGEMENT - EXTERNAL RELATIONS from LIVING ALOFT: :Human Requirements for Extended Spaceflight

On growing plants in space for food and oxygen, see my

Could Astronauts Get All Their Oxygen From Algae Or Plants? And Their Food Also?

(This article comes from my answer to the quora question: Why is this flower's blooming in outer space special?)