Apple Inc. (AAPL -2.15%), Universal Display Corporation (OLED -1.06%), and Trex Company (TREX -0.18%) make the list of top growth stocks our Motley Fool contributors believe could fuel growth for investor's portfolios. These companies are leaders in their industries, and each has catalysts coming that could make their shares jump -- so let's learn more about them.

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I see six degrees of growth for Apple stock

Rich Smith (Apple): Let's play a stock market variant on the "Six Degrees of Kevin Bacon" -- and see if you can follow my reasoning here.

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As a stocks writer, when I hear "sharp investors," my first reaction is to look up flatscreen TV maker Sharp Electronics, and see how it's stock is doing. When I did that, though, I quickly saw that Sharp has been acquired by Taiwanese contract manufacturer Foxconn. Foxconn is privately owned, but it's also a name that I associate with iPhone parts manufacturing, and thereby, with its marquee customer.

All of which goes to explain why my first instinct is to recommend Apple as a "growth stock for sharp investors."

Apple stock, returned to growth mode in February, posting record quarterly sales of $78.4 billion, then kept growing in Q2 2017 when it reported earnings earlier this month. Despite falling gross margins, Apple used these growing sales to grow its net income a small fraction of 1% in H1 2017. Operating profits, and even free cash flow, both resumed growing in this year's first half.

The fact that Apple was able to manage any growth whatsoever, given how big it's grown already -- it's already selling $220.5 billion worth of iStuff annually, after all -- is pretty impressive. The fact is, even if Apple didn't grow at all, Apple stock is probably cheap enough ($802.9 billion market cap, $53.2 billion in annual free cash flow) to make the stock worth owning at its current valuation of just 15 times annual cash profits.

But Apple is growing again, and it could grow even later this year. I've got a hunch that consumers have been hoarding their money lately, in anticipation of the company's upcoming iPhone 8 release. And if you assume, as I do, that Apple is going to pack a whole lot of "wow" factor into its latest product, given that this will be the 10th anniversary edition of the iPhone, I expect we'll see considerably more than fraction-of-a-percent sales growth once the iPhone 8 comes on the market.

That prospect alone could make Apple stock a great growth stock for sharp investors.

Seeing more clearly

Todd Campbell (Universal Display Corporation): You might not be familiar with Universal Display Corporation by name, but you could be using its products every day without knowing it.

The company makes phosphorous emitters for the organic light emitting diode (OLED) displays used in consumer electronics, including smartphones and televisions, and lighting products.

Its focus is on high definition display solutions that reduce power consumption and because manufacturers are desperate to extend battery life, that's led to significant sales growth. In Q1, revenue was $55.6 million, up 87% year over year. Rising demand for phosphorous emitters propelled material sales up 92% to $46.6 million, while royalties and licensing revenue added another $7 million.

Profit is also improving thanks to leveraging sales growth against fixed costs. Net income was $10.4 million last quarter, up $8.5 million from the $1.9 million reported in the same quarter of 2016. That's an impressive increase, but it could only be scratching the surface of the growth that may be coming.

Consumer electronics companies are increasingly embracing OLED to display high definition content and that led management to bump its full-year sales outlook in April to at least $260 million. Previously, their forecast was for at least $230 million. If it can hit that target, it would represent 30% plus year-over-year top-line growth.

Samsung is one of the company's biggest customers, and the OLED display on the Samsung S8 is earning a lot of positive reviews. Apple is reportedly going to take the leap to OLED displays made by Samsung later this year, and if they do, it should force more consumer electronics companies to follow suit. Overall, rising OLED production capacity is good news for emitter demand, and that makes this an attractive growth stock to consider owning in portfolios.

The undisputed leader in a surprising growth industry

Jason Hall (Trex Company Inc): Few people would ever think of the decking industry as being a place to find a growth stock. However, one of my favorite high-growth companies is in the decking business. Trex, which makes an alternative to wood decking from recycled wood chips and plastic film, is doing more than just growing -- it's flat-out disrupting the decking industry by leading with a green product that also costs less to maintain and lasts much longer than wood.

Since 2011, Trex's annual sales have grown from 275 million to nearly $500 million. That's more than 10% per year in an industry that's growing at between 2% to 3%. Talk about taking market share.

Trex is doing it by developing great products that regularly sweep industry awards, investing in marketing its brand in traditional and social media (which has given it the best brand recognition in the industry), and developing deep relationships with national retailers, regional distributors, and home improvement contractors.

At last count, Trex made up more than 40% of annual sales of wood-alternative decking and has consistently grown its market share leadership position year after year. However, that doesn't mean the company is running out of room to grow. Even with a dominant share of existing alt-wood sales, Trex makes up less than 10% of total board feet of decking sold in North America each year.

Wood is still the big player, and that gives Trex a huge addressable market to continue taking share from. And with more environmentally conscious millennials buying homes in the years to come, Trex is a perfect "under the radar" growth stock to buy today.