Retirement is a big problem for many Americans. According to a recent poll from BlackRock, just 56% of people believe that they will be able to retire comfortably and have the life they want in their golden years.

One way you can improve your financial position in the long run is by generating dividend income. And you can accomplish that by investing in strong businesses that not only pay dividends today but that are also likely to increase them in the years ahead.

Three dividend stocks that are among the safest options for including in your retirement plan now are UnitedHealth Group (UNH 0.38%)Apple (AAPL -0.18%), and Walmart (WMT 0.92%). They may not offer high yields, but here's why they may be worth investing in anyway.

1. UnitedHealth Group

Health insurer UnitedHealth pays a dividend that yields 1.4%, which is broadly in line with the S&P 500 average. What investors will love about this stock is that its business is resilient and has strong growth prospects. Whether it's through acquisitions (it has closed on multiple deals within just the past year) or simply the population getting older and needing more ongoing care, UnitedHealth's financials should continue to improve, as they have over the past decade.

UNH Revenue (Annual) Chart.

UNH Revenue (Annual) data by YCharts.

The healthcare company's payout ratio is a modest 31% of earnings, leaving plenty of room for UnitedHealth to make rate hikes in the future. That means your dividend income should rise over the years. While the yield is modest right now, the dividend could end up representing more of your original investment the longer you hold onto the stock.

UnitedHealth recently increased its dividend by 14%. If it were to consistently raise its payouts each year by that rate, it would take six years for the payout to double. All in all, UnitedHealth is a great dividend growth stock to own and can be an ideal option for long-term investors.

2. Apple

Consumer tech titan Apple has a devoted legion of followers, which is why its high-priced devices, including iPhones, often enjoy strong demand. Even now, amid inflation and when consumers are supposedly tightening up their spending, the company still only reported a 4% drop in product sales last quarter, which ended on July 1. With profits rising and Apple often buying back shares, its per-share profit of $1.26 was still up by 5% year over year.

Apple has a robust business that continues to get more diverse and rely more on services -- a trend that's likely to persist for the foreseeable future. Although its dividend yield is low at less than 0.6%, this is another dividend growth play for long-term investors. The proof is in the charts below. Even as the company has more than doubled its dividend over the past decade, its payout ratio has actually been declining.

AAPL Dividend Chart.

AAPL Dividend data by YCharts.

There is a ton more room for the dividend to grow, and that's why this can make for an underrated income investment to own, particularly for long-term investors.

3. Walmart

Earlier this year, Walmart raised its dividend for the 50th straight year, making it a Dividend King. It's the only stock on this list to belong to that exclusive club. Investors shouldn't, however, expect huge rate increases from Walmart as in five years, its dividend has gone up by just five cents.

But like the other stocks on this list, investors get stability and consistency, which is arguably more important when you're talking about saving for retirement. Between the stock's potential to rise in value along with the dividend income you'll earn, Walmart, like Apple and UnitedHealth, can be a solid pillar to build your portfolio around.

The big-box retailer's strength is its size and versatility. Last week it reported earnings for the second quarter, which ended on July 31, and sales totaling $161.6 billion were up nearly 6% year over year. And operating income also rose by 7%. 

A strong grocery business makes the retailer more resilient than others in the industry. Walmart is another solid investment for risk-averse investors to consider loading up on for both its growth potential and its reliable dividend income.