When Hannan Braun felt stressed at work, he would treat himself to a houseplant.

"At one point, I think I had well over a hundred plants," said Braun, who lived in a studio apartment and was working on the front lines of the pandemic in Boston, "but it never looked cluttered or felt like I had too many."

For Braun, indoor plants have been a lifeline for dealing with the stress of medical training during the pandemic. Surrounding himself with lush greenery always calmed him down, he said, and helped him feel rejuvenated.

"Different properties of plants, such as how they look, smell and feel, impact us in so many ways," said Mengmeng Gu, an associate professor of horticultural sciences at Texas A&M University. "They can feel good to the touch, make a space more fragrant and please our eyes."

But how and why do plants have such positive effects on us? Here's a look at research over the past few decades that has shown how houseplants affect our psychological and physical health.

People and plants are naturally connected. Humans have an inherent connection to plants and other living things, according to what's known as the biophilia hypothesis, an idea popularized in 1984 by naturalist and writer E.O. Wilson. Since then, more than three decades of research spanning the globe have confirmed the hypothesis and shown that natural environments have a sizable effect on increasing positive emotions and decreasing negative ones.

"When people state the common belief that being in nature relaxes them, that it helps them recover from stress and tragedy, that it's a healing process to be in nature, we now know there's a solid basis for that," Wilson said in a 2015 interview with The Washington Post.

And when people started spending more time indoors, we brought in pieces of the natural world to continue feeling connected.

Plants can quickly improve mood. Our connection to plants is so strong that sometimes it takes only a few minutes of being in their presence to start feeling better. Studies have found that less than 20 minutes is enough to make us feel more at peace. In one experiment, participants who spent even five to 10 minutes in a room with a few houseplants felt happier and more satisfied than those in a room without plants. In another study, participants felt more peaceful and positive after spending 15 minutes in a room close to a tall plant (about five feet) compared with other objects.

However, Gu reminds us that "it is not only seeing a plant that improves our mood so quickly, but the smells can also make a huge difference," although studies on plants' effects on nonvisual senses are limited.

Plants bring relief in enclosed spaces. If you are stuck in an office or other small space for hours at a time, plants can bring about feelings of escape. In a study conducted during pandemic stay-at-home orders, participants who had indoor plants experienced significantly fewer symptoms of depression and anxiety than those who did not. Being surrounded by houseplants led to feelings of "being away" from social or physical demands.

Plants can reduce stress. Other studies have shown that interacting with plants suppresses the system in our bodies that gets activated when we sense distress signals. Young adults in one study who spent a few minutes repotting and transplanting an indoor plant reported feeling a lot less stressed at the end of the task compared with peers engaged in a computer-based activity. In addition, blood pressure measurements were much lower among people who handled plants, suggesting that plants have the potential to mellow the body's fight-or-flight response.

Plants can recharge us. "Plants also have a huge restorative capacity," said Melinda Knuth, an assistant professor of horticultural science at North Carolina State University. "Whether it's outdoors like in a yard or indoors with houseplants, nature can help us feel recharged and grounded."

When we focus on demanding activities for a long time, such as our jobs, it can lead to mental fatigue and negative emotions that can affect how well we can pay attention. Seeing a plant in this situation can provide a spark of interest, redirect our attention, and restore our depleted mental and physical resources, an idea known as attention restoration theory. Studies have found that the plant-induced "restoration" effect has a wide reach: renewing positive emotions and increasing productivity, creativity and attention capacity.

How do you choose what houseplants to buy? Research can provide some practical guidance:

Number of plants: Although there is no magic number, having five or more foliage plants can increase positive emotions. For example, in one study, participants in a room with bamboo palms, Chinese evergreens and heart-leaf philodendrons (five in total) felt more carefree and friendly compared with those in rooms without plants. Alternatively, one tall potted plant (about five feet) or three or more small floral displays (such as sweet pea, larkspurs or woodland sage) can elicit similarly positive responses.

Color: The greener, the better? In a study using English ivy, green-yellow and bright-green leaves increased feelings of cheerfulness and relaxation, whereas whitish-green leaves stimulated mostly negative emotions. As for flowering plants, a study found that purple, green, red, pink and white ones could lower people's blood pressure and heart rate. However, purple and green flowers were more effective in relaxing the body, reducing anxiety and improving mood. Another study found that red and yellow roses elicited a more calming response than white ones.

Real vs. artificial: In an indoor space, having any type of greens — including photographs of plants — is better than having none at all. However, real plants have a greater mood, attention and relaxation effect than faux plants. The same applies for real vs. faux flowers. In a study of high school students, participants looking at real pansies for three minutes felt more relaxed and comfortable than those looking at artificial ones. Gu's point about mood effects beyond visual cues may help explain these findings.

Placement: Although research on this is sparse, some studies suggest that having plants closer than 10 feet to a person has a positive mood effect. A study by Knuth of North Carolina State shows that most people put houseplants in living rooms, bedrooms and sometimes kitchens. With the expansion of working from home, placing plants in home offices or other work areas can be helpful.

It's important to remember the caveats of many of these studies: Some were carried out in highly controlled settings and primarily among college students. They reflect snapshots of time rather than long-term effects. And their real-world implications for a more varied group of people — for example, among older adults or those in low-resource environments — may be different. But it's hard to ignore the volume of research showing that houseplants have a significantly positive effect on mood and physical health.