Introduction
There is a unique kind of excitement that comes with planting tuberous begonias. These shade-loving stars are famous for their lush foliage and large, rose-like blooms that bring brilliant color to the quietest corners of the yard. Whether you are looking to brighten up a covered porch with hanging baskets or add a soft glow to a woodland border, begonias are a rewarding choice for any gardener. At Longfield Gardens, we enjoy helping you find the perfect varieties to turn your outdoor spaces into a personal retreat. Browse our Spring-Planted Bulbs for Shade.
Getting started with these beautiful plants is easier than it might look, especially once you understand a few simple rules of thumb. Many gardeners wonder exactly how deep to tuck these tubers into the soil to ensure they sprout strongly and bloom all summer long. This guide is designed to help you master the planting process, from identifying which side of the tuber is the top to finding the ideal planting depth for your specific garden setup. If you are comparing forms, see our Upright Double Begonias.
We will focus on the practical steps of planting, the importance of soil temperature, and the specific depth requirements for both indoor starting and outdoor garden beds. By the end of this article, you will have a clear plan for success with your tuberous begonias.
Thesis: To achieve the most vigorous growth and consistent blooms, tuberous begonias should be planted approximately 1 inch deep in well-drained soil, ensuring the concave side faces upward to allow sprouts easy access to the light.
Understanding the Begonia Tuber
Before we dig into the specifics of depth, it helps to understand what a begonia tuber actually is. Unlike a standard flower seed or a fibrous root system, a tuber is a thickened, underground part of the stem or root that stores energy. This energy serves as a fuel tank for the plant, giving it everything it needs to push out stems, leaves, and eventually, those iconic flowers. For a broader overview, see All About Begonias.
When you receive your tubers from us, you might notice they look a bit like small, brown, hairy stones or bowls. They are firm and may have a slightly rough texture. This rugged exterior protects the delicate growing points inside. While they might look dormant, they are ready to spring into action as soon as they encounter the right combination of moisture and warmth.
There are two main growth habits you will encounter. Upright begonias, like the Superba or Begonia Upright Double Roseform Pink, grow taller and are perfect for garden beds or large upright pots. Cascading or trailing begonias, such as the Hanging Basket types, are bred to spill over the edges of containers. Regardless of the variety, the planting depth remains largely the same, but understanding the growth habit helps you choose the right container or garden spot. For more container-friendly ideas, browse Summer Blooming Bulbs for Containers.
Identifying the Top and Bottom
One of the most important parts of getting the planting depth right is knowing which way the tuber should face. Because the depth is measured from the top of the tuber to the soil surface, an upside-down tuber will end up much deeper than intended, forcing the sprouts to work twice as hard to reach the surface.
Fortunately, tuberous begonias have a very distinct shape that makes orientation easy. One side of the tuber is usually indented or "dished" (concave), while the other side is rounded or "domed" (convex).
- The Concave Side (The Bowl): This is the top. The hollowed-out area is where the growing points, or "eyes," are located. You might see small, pinkish or reddish buds already starting to form in this indentation.
- The Convex Side (The Bottom): This is the rounded side. Roots will eventually emerge from the bottom and sides of the tuber to anchor the plant and draw in nutrients.
If you ever find a tuber that is particularly flat or hard to read, look for the remnants of last year's stem. This will always be on the top side. If you are still unsure, you can place the tuber on its side; the plant will naturally direct its sprouts upward and its roots downward, though planting it with the hollow side up is the preferred method for the fastest start.
How Deep to Plant Tuberous Begonia
The most frequent question we receive is exactly how much soil should cover the tuber. The answer depends slightly on whether you are starting them early indoors or planting them directly into their final outdoor home.
Planting Depth for Indoor Starting
Many gardeners choose to start their begonias indoors 8 to 12 weeks before the last frost to get a jump on the blooming season. When starting in shallow trays or small pots:
- Place the tuber with the hollow side up.
- Cover it with just enough high-quality potting mix so the top is barely hidden—usually about 1/2 inch of soil.
- Some gardeners prefer to leave the very rim of the "bowl" exposed until sprouts appear, then add a little more soil. Both methods work well, as long as the tuber stays moist but not soaked. See Tips for Growing Tuberous Begonias.
Planting Depth for Outdoor Beds and Containers
When it is time to move your begonias into the garden or if you are planting them directly outside after the weather has warmed:
- Aim for a depth of 1 inch.
- This means there should be 1 inch of soil sitting on top of the highest point of the tuber.
- In containers, you can stay closer to the 1/2-inch to 1-inch range. In open garden beds, a full inch of soil provides a bit more protection against temperature swings and helps stabilize the stems as they grow.
What to do next:
- Inspect your tubers for the concave "bowl" side.
- Prepare a light, well-draining potting mix.
- Position the tuber 1/2 inch to 1 inch below the soil surface.
- Ensure the soil is settled but not packed tightly over the top.
Why Planting Depth Matters
It might seem like a small detail, but planting at the correct depth is a quiet winner when it comes to plant health. Tuberous begonias are specialized growers, and their roots don't just come out of the bottom. They actually emerge from the sides and even the top edges of the tuber.
The Risks of Planting Too Deep
If a tuber is buried 3 or 4 inches deep, the sprouts may struggle to reach the surface. This uses up the tuber’s stored energy before the leaves can even start photosynthesizing. Deep planting can also lead to more moisture being trapped against the tuber for longer periods, which may cause the tuber to soften before it has a chance to establish a strong root system.
The Risks of Planting Too Shallowly
If the tuber is sitting right on the surface or has very little soil covering it, the roots that emerge from the top and sides may dry out. These roots are essential for taking up water and nutrients. Furthermore, as the begonia grows and produces large, heavy flowers, it needs the stability that a bit of soil coverage provides. A tuber planted too shallowly may result in a plant that tips over or stems that break easily at the base.
Starting Tubers Indoors for Earlier Blooms
Because tuberous begonias take their time to wake up—often needing 8 to 12 weeks to reach their full flowering potential—starting them indoors is a great way to maximize your summer color. This process is often called "pre-sprouting" or "starting."
We recommend starting this process in late winter or early spring. You don't need a professional greenhouse; a warm room with a bit of indirect light is perfect.
- Choose the right container: Use a shallow tray or individual 4-inch pots with plenty of drainage holes.
- Use fresh potting soil: A peat-based or coco-coir-based mix is ideal because it holds moisture while remaining light and airy.
- Nestle the tubers: Space them about 2 to 4 inches apart in a tray. Press them into the moist soil so they are about 1/2 inch deep.
- Provide warmth: Begonias love warmth. A room temperature of 70°F is perfect for waking them up.
- Water sparingly: At this stage, the tuber doesn't have roots yet, so it doesn't need much water. Keep the soil slightly damp to the touch, like a wrung-out sponge.
Once you see small red or green "pips" appearing in the center of the tuber, you know the plant is waking up. Soon, leaves will unfold, and the plant will begin to grow rapidly.
Timing Your Move Outdoors
Patience is a virtue when it comes to moving begonias outside. These are tropical plants at heart and are very sensitive to cold temperatures. Even a light frost can damage the tender new foliage, and cold soil can stall their growth. If you are not sure about your climate, check the Hardiness Zone Map.
A good rule of thumb is to wait until the danger of frost has completely passed and the nighttime temperatures consistently stay above 50°F (though 60°F is even better). In most regions, this is the same time you would plant out tomatoes or other warm-weather annuals.
When you move your begonias from the controlled environment of your home to the outdoors, it is helpful to "harden them off." This means giving them a few hours of outdoor air in a shaded, protected spot each day, gradually increasing their exposure over a week. This prevents the leaves from getting "sunburned" or wind-whipped. If you are ordering new tubers, see our Shipping Information.
Soil Preparation and Drainage
The quality of the soil is just as important as the depth at which you plant. If there is one thing tuberous begonias insist on, it is "drainage." Drainage simply refers to how fast water leaves the soil. Begonias do best when their roots stay moist but never sit in a puddle.
Improving Garden Soil
If you are planting in the ground, we suggest checking the area after a rainstorm. If water sits on the surface for hours, the soil may have too much clay. You can improve this by mixing in organic matter like compost or finely ground bark. This creates "pore space" in the soil, allowing water and air to move freely around the tuber.
Choosing the Right Potting Mix
For containers and hanging baskets, avoid using standard "garden soil" or "topsoil" from the yard. These are too heavy and will pack down, cutting off air to the roots. Instead, use a high-quality potting mix designed for containers. These mixes usually contain perlite (the small white "stones") or vermiculite, which help keep the soil light and fluffy.
Key Takeaway: Successful begonia planting combines the right depth (1 inch) with soil that allows excess water to drain away quickly. This balance keeps the tuber hydrated while preventing it from becoming waterlogged.
Site Selection: The Right Light
Tuberous begonias are the royalty of the shade garden, but "shade" can mean many things. To get the most flowers, you want to find a spot that offers filtered light or dappled shade.
- Morning Sun, Afternoon Shade: This is often the "sweet spot." The gentle light of the morning helps the plant grow, while the shade in the heat of the afternoon protects the leaves and petals from scorching.
- Dappled Sunlight: The light found under high-canopy trees is perfect. It provides a constant dance of light and shadow.
- Deep Shade: While begonias will grow in deep shade, they may produce fewer flowers and the stems can become "leggy" as they reach for the light.
If you are gardening in a very hot climate, your begonias will appreciate even more shade. In cooler, northern climates, they can often handle a bit more sun, provided they are kept well-watered.
Spacing and Container Tips
The space you give your begonias affects how they look and how well they grow. If they are too crowded, air cannot circulate around the leaves, which can lead to mildew. If they are too far apart, the display might look a bit sparse in the early season.
In Garden Beds
Space your tubers about 8 to 12 inches apart. This gives each plant enough room to reach its full width without competing for nutrients. If you are planting a large area, staggering the rows in a "zigzag" pattern creates a fuller, more natural look.
In Containers
Begonias are excellent candidates for pots.
- 12-inch pot: One to three tubers, depending on the variety.
- Hanging Baskets: For a standard 10 or 12-inch basket, two or three Begonia Hanging Golden Balcony will create a lush, overflowing effect.
- Window Boxes: Space them about 6 to 8 inches apart to ensure the box looks full from end to end.
When planting in containers, make sure there is at least an inch or two of space between the tuber and the edge of the pot. This ensures the roots have plenty of soil to explore in every direction.
Watering Correctly, Not Constantly
Watering is where many new gardeners feel a bit of pressure, but it doesn't have to be complicated. The goal is to keep the soil consistently moist, like a sponge that has been squeezed out.
Instead of watering on a strict schedule (like every Monday and Thursday), use your finger to check the soil. If the top inch feels dry, it is time to water. If it still feels cold and damp, wait a day.
When you do water, try to aim the stream at the base of the plant rather than over the top of the leaves. Keeping the foliage dry helps prevent spotting on the petals and keeps the plants looking their best. In the heat of midsummer, containers may need water every day, while begonias in the ground might only need it once or twice a week.
Essential Care After Planting
Once your begonias are established and growing, a little bit of maintenance goes a long way in extending the bloom season.
Fertilizing for Success
Begonias are "heavy feeders," meaning they use a lot of energy to produce those massive flowers. Once the plants have a few sets of leaves, you can begin using a balanced, water-soluble fertilizer. We recommend using it at a weaker strength (about half the recommended dose) every two weeks. This provides a steady stream of nutrients without overwhelming the plant.
Staking Upright Varieties
Some upright begonias, like the Begonia Upright Double Superba White, can produce flowers so large that the stems might bend under the weight, especially after a rain. Using a small bamboo stake and a loose tie can help support the main stem and keep the flowers upright where you can see them.
Deadheading
"Deadheading" is the simple act of removing flowers that have started to wilt. This keeps the plant looking tidy and encourages it to put its energy into creating new buds rather than trying to produce seeds. Simply snap off the spent flower at the base of its small stem.
Troubleshooting Early Growth
If your begonias aren't popping up as quickly as you expected, don't worry. Sometimes they just need a little extra time or a small adjustment.
- Slow to Sprout: The most common reason for slow growth is temperature. If the room or the soil is below 60°F, the tuber may stay in its "sleepy" state. Try moving the pot to a warmer spot.
- Pale Leaves: If new leaves look very pale or yellow, the plant might be getting too much water or not enough light. Check the soil moisture and perhaps move it to a slightly brighter location.
- Leggy Stems: If the stems are very long and thin with lots of space between the leaves, the plant is likely searching for light. Move it to a spot with better indirect sunlight.
Remember, gardening is a journey, and every season brings a chance to learn more about how plants respond to your specific environment.
Overwintering for Next Year
One of the best things about tuberous begonias is that they can be enjoyed year after year. When the weather begins to turn cold in the autumn and the leaves start to yellow, it is time to prepare them for their winter nap.
In most parts of the US, begonias will not survive the winter in the ground. Before the first hard frost, dig up the tubers (or bring your pots into a frost-free area). Let the foliage die back naturally, which allows the energy to return to the tuber. Once the stems are dry, they should pop off easily.
Clean off the excess soil and store the tubers in a cool, dry place (about 50°F) in a box of peat moss or paper bags. Next spring, you can start the process all over again. It is a rewarding cycle that makes these plants a wonderful investment for your garden.
Conclusion
Tuberous begonias are truly one of the most versatile and stunning plants you can add to your shade garden. By focusing on the simple basics—planting 1 inch deep, ensuring the hollow side faces up, and providing well-drained soil—you set the stage for a spectacular summer display. These plants remind us that gardening is about the joy of watching something grow and the simple pleasure of adding beauty to our surroundings.
At Longfield Gardens, we take pride in providing high-quality tubers that are ready to perform. Whether you are a seasoned gardener or just starting your first container, we are here to support you with quality plants and practical advice. We hope this guide helps you feel confident as you head out to plant your begonias this season. Shop with confidence under our 100% Quality Guarantee.
Final Takeaway: The secret to begonia success is a shallow start in warm, well-drained soil. Give them 1 inch of coverage and a bit of patience, and they will reward you with months of incredible color.
Happy planting! We look forward to hearing about your garden successes.
FAQ
Should I soak begonia tubers before planting them?
While some gardeners choose to soak tubers for an hour or two to "wake them up," it is generally not necessary. If the soil is kept consistently moist (but not wet) after planting, the tubers will naturally hydrate and begin to grow on their own.
Can I plant tuberous begonias in full sun?
In most climates, tuberous begonias will struggle in full, direct sun, especially during the hottest part of the day. The leaves can scorch and the flowers may wilt. They are much happier in filtered light, dappled shade, or a spot that only receives gentle morning sun.
What if I accidentally plant my begonia tuber upside down?
If you realize you’ve planted it upside down shortly after planting, you can gently dig it up and flip it over. If the plant has already started growing, it is often best to leave it alone. The sprouts will eventually find their way to the surface, though it may take a little longer.
Why are the buds falling off my begonia before they open?
Bud drop is usually caused by a stressor in the environment. The most common culprits are extreme temperatures (either too hot or a sudden cold snap) or inconsistent watering. Keeping the soil evenly moist and the plant protected from drying winds usually solves the problem.
