Introduction
There is a unique kind of excitement that comes with seeing the first pink sprouts emerge from a begonia tuber in the quiet of late winter. While much of the garden is still sleeping, these little "bulbs" are ready to begin their journey toward becoming the lush, rose-like showstoppers that brighten our shadiest corners. At Longfield Gardens, we know that giving your begonias a head start indoors is the best way to ensure a long season of brilliant color and healthy, vigorous plants in our Begonia Flower Bulbs for Sale collection.
This guide is designed for any gardener who wants to transition from buying pre-started nursery plants to the rewarding experience of growing their own from scratch. We will walk you through the timing, the planting process, and the simple care steps needed to move your begonias from the windowsill to the patio. For a companion walkthrough, see our Starting Tuberous Begonias Indoors guide. By starting your tubers early, you gain several extra weeks of blooms and much larger, fuller plants by midsummer.
Starting begonias indoors is a straightforward process that rewards patience with an incredible floral display.
Why Starting Begonias Indoors Matters
Tuberous begonias are tropical plants by nature, originating from warm climates in South America and southern Africa. Because they have a long lead time to reach their peak flowering stage—often 12 to 14 weeks—simply planting them in the ground once the weather warms up in June often results in a very short blooming season.
When you start your tubers indoors during the late winter or early spring, you are essentially "buying" yourself two or three months of growth. By the time the outdoor temperatures are safe for these frost-sensitive beauties, your plants will already have established root systems and several sets of leaves. For local timing help, check our When to Plant Begonias Outdoors article. This preparation allows them to hit the ground running, often beginning to bloom just as the summer heat settles in.
Beyond the benefit of more flowers, starting your own tubers is a cost-effective way to fill large containers or garden borders. You can choose from a wider variety of flower forms, such as ruffled, picotee, or pendulous trailing types, which are not always available as started plants at local garden centers.
When to Start Your Begonia Tubers
The most important factor in "how to start begonia bulbs indoors" is the calendar. Because these plants are highly sensitive to cold, you must time your indoor start so that the plants are ready to go outside only when the danger of frost has completely passed and the soil has warmed.
We recommend starting your tubers approximately 8 to 12 weeks before your area’s average last frost date. This window gives the tuber enough time to "wake up," develop a strong root system, and produce several inches of top growth. If you start too early, the plants may become "leggy" or outgrow their indoor space; if you start too late, you won't see flowers until late summer.
Understanding Your Planting Zone
Because the climate varies significantly across the United States, your specific USDA hardiness zone determines your starting schedule. For a quick reference, use the Hardiness Zone Map. Here is a general timeline to help you plan your indoor garden:
- Zones 3 and 4: Start indoors in early to mid-April. Plan to move them outdoors in early to mid-June.
- Zones 5 and 6: Start indoors in mid-March. Plan to move them outdoors in late May.
- Zone 7: Start indoors in late February. Plan to move them outdoors in late April.
- Zone 8: Start indoors in early February. Plan to move them outdoors in early April.
- Zones 9 and 10: You can often start these much earlier (January or February) or even plant them directly outdoors once the threat of frost is gone.
Keep in mind that these dates are averages. Local weather patterns can shift, so always keep an eye on your local 10-day forecast before making the final move outside. If you are timing an order, our Shipping Information page explains shipping windows by zone.
Supplies You Will Need
Before you get your hands in the dirt, gather a few basic supplies. Starting begonias doesn't require expensive equipment, but having the right materials on hand makes the process much smoother.
- Begonia Tubers: Choose firm, healthy tubers. They should feel heavy for their size and be free of soft spots or mold.
- Shallow Trays or Small Pots: You can start tubers in individual 3-inch or 4-inch plastic pots, or you can use a shallow nursery tray if you are starting a large quantity.
- Potting Mix: Use a high-quality, peat-based potting soil that is light and well-draining. Avoid heavy "garden soil" or "topsoil" from the yard, as these stay too wet and can cause the tubers to rot.
- Watering Can or Spray Bottle: A spray bottle is excellent for the early stages when you want to keep the soil moist but not saturated.
- Labels: Even if you think you’ll remember which color is which, it is helpful to label your pots or trays immediately.
Key Takeaway: Success starts with timing and the right soil. Aim for a start date about 10 weeks before your last frost and use a light potting mix to ensure proper drainage.
Anatomy of a Begonia Tuber: Which Side is Up?
One of the most common questions beginners ask is which way to plant the tuber. Unlike a tulip or a daffodil, which has a clear "pointy" top, a begonia tuber looks a bit like a dark, hairy potato or a small brown stone.
Take a close look at the tuber. One side will be slightly rounded or domed. The other side will have a depression or a "hollow" spot, making it look a bit like a shallow bowl. If you want a more detailed planting refresher, see How Do You Plant Begonia Tubers?.
- The Hollow Side (Concave): This is the top. This is where the "eyes" or growth buds are located. The stems and leaves will emerge from this indentation.
- The Rounded Side (Convex): This is the bottom. This is where the roots will eventually form.
If you look very closely at the hollow side, you might see tiny pink or white bumps. These are the eyes, similar to the eyes on a potato. If you absolutely cannot tell which side is which, you can plant the tuber on its side. The plant will naturally direct its stems upward and its roots downward.
Step 1: Waking Up the Tubers
When you first receive your begonia tubers, they are in a state of deep dormancy. They have been kept cool to prevent them from sprouting during shipping. To get them growing, you need to provide them with a "wake-up call" using warmth and light.
Before you even put them in soil, you can place the tubers on a tray in a warm room (around 70°F) that gets bright, indirect light. You don't need to water them at this stage. Simply letting them sit in a warm environment for a week or two will often trigger the eyes to start swelling and turn pink.
This step isn't strictly necessary—you can plant them directly into soil—but it allows you to verify that the tubers are healthy and ready to grow before you commit them to a pot.
Step 2: Planting for Success
Once you see those little pink sprouts, it is time to get them into their starting medium.
Preparing the Soil
Start by pre-moistening your potting mix. It should feel like a wrung-out sponge—damp to the touch but not dripping wet. If the soil is too dry, it can actually pull moisture out of the tuber. If it is too wet, the tuber may rot before it ever gets the chance to grow.
Placement and Depth
Fill your small pots or trays with about 2 to 3 inches of the moistened mix. Place the tuber on the surface with the hollow side facing up. For a closer look at planting depth, read How Deep Do I Plant Begonia Tubers?.
There are two schools of thought on depth. Some gardeners like to leave the top of the tuber exposed until it sprouts, while others prefer to cover it lightly. At our trial gardens, we have found that covering the tuber with about 1/2 inch to 1 inch of soil is very effective. Because begonias grow roots from the bottom, sides, and even the top of the tuber, being fully surrounded by soil encourages a much more robust root system.
Initial Watering
After planting, give the pots a light watering to settle the soil around the tuber. Be careful not to let water pool in the hollow center of the tuber, as this can lead to crown rot.
Step 3: Providing the Right Environment
Begonias are quite particular about their environment during the first few weeks of growth. They need a balance of warmth, light, and humidity to thrive.
Temperature
Begonias love warmth. Place your pots in a location where the temperature stays consistently between 70°F and 75°F. A spot near a radiator (but not directly on it) or on top of a refrigerator can provide the gentle bottom heat they crave. If your house is particularly chilly, a seedling heat mat can be a great investment.
Humidity
To mimic a tropical environment, you can loosely cover your pots with a clear plastic bag or a plastic dome. This traps moisture and keeps the humidity high around the emerging sprouts. If you do this, make sure to check the pots daily. If you see excessive condensation dripping, remove the cover for an hour or two to let some fresh air circulate.
Water Management
This is the stage where most beginners run into trouble. It is vital to keep the soil moist, but you must never let it become soggy. Overwatering a dormant or newly sprouting tuber is the fastest way to cause rot.
A good rule of thumb is to wait until the surface of the soil feels dry to the touch before adding a small amount of water. Always water around the edges of the pot rather than directly onto the center of the tuber.
Step 4: Light and Leaf Development
Once the sprouts reach about an inch in height and you see the first tiny leaves beginning to unfurl, their need for light increases dramatically.
Moving to the Light
At this point, remove any plastic covers and move your plants to a very bright location. A south-facing windowsill is often ideal, but be careful of direct, hot midday sun which can scorch tender new leaves. If you don't have a bright window, grow lights are an excellent alternative. Keep the lights about 3 to 4 inches above the tops of the plants, adjusting them upward as the begonias grow.
What to Expect
As the leaves develop, you will notice that the stems grow quite rapidly. Don't be alarmed if the plants look a little "stiff" or weird at first—this is normal. The first leaves are often smaller than the ones that will follow.
What to do next:
- Check soil moisture daily by touching the surface.
- Rotate pots every few days if they are on a windowsill to ensure even growth.
- Keep the room temperature above 60°F at all times.
- Watch for the appearance of the second and third sets of leaves.
Step 5: Transplanting and Pinching
As your begonias continue to grow, they will eventually outgrow their small starting pots. Usually, when the plant has three or four healthy leaves and is about 3 to 5 inches tall, it is ready for a "step-up."
Moving to Larger Pots
Gently lift the tuber and its root ball out of the starting pot. Be very careful, as the new stems are brittle and can snap easily. Move the plant into its permanent summer container or a larger transitional pot (such as a 6-inch or 8-inch pot). For more container ideas, see Can Begonias Be Planted in Pots?.
If you are planting in a large hanging basket or a 12-inch decorative urn, you can place two or three tubers together for a fuller look. Adding a bit of compost or a slow-release fertilizer to the potting mix at this stage will provide the nutrients the plant needs for the long blooming season ahead. For basket planting, try How to Plant Begonias in Hanging Baskets.
The Art of Pinching
If you want a bushy, full plant with dozens of flowers, you might consider "pinching." This simply means removing the very tip of the main stem once it is about 4 to 6 inches tall. This encourages the plant to branch out from the sides rather than just growing one tall, straight stem.
However, if you are growing varieties specifically for large, prize-winning blooms, some gardeners do the opposite: they pinch off the side shoots to focus all the plant's energy into one or two massive flowers. For most home gardeners, a bushier plant is usually more desirable.
Step 6: Hardening Off and Moving Outdoors
Moving your begonias from the protected environment of your home to the unpredictable outdoors is a process that should be done gradually. This is called "hardening off."
The Transition Period
Wait until the nighttime temperatures are consistently above 50°F and there is no danger of frost. Start by moving your pots to a shady, sheltered spot outside for just two or three hours during the warmest part of the day. Bring them back inside before the sun goes down.
Every day, increase the amount of time they spend outside and gradually expose them to a little more light (though remember, tuberous begonias generally prefer dappled shade or morning sun). This process usually takes about 7 to 10 days.
Final Placement
Once they are hardened off, you can move them to their final home. Begonias thrive in spots with filtered light, such as under a porch roof, beneath a leafy tree, or on a north- or east-facing patio. Avoid areas with heavy wind, as the succulent stems of begonias can catch the breeze and break.
Ongoing Summer Care
Once your begonias are settled outdoors, they are relatively low-maintenance, but they do appreciate consistent attention.
- Watering: Containers dry out much faster than garden beds. Check your begonias daily during the heat of summer. Water whenever the top inch of soil feels dry.
- Feeding: Begonias are heavy feeders. Use a balanced, liquid fertilizer every two weeks to keep the blooms coming. Look for a fertilizer with a slightly higher middle number (phosphorus) to promote flowering.
- Deadheading: While many modern begonias are "self-cleaning," removing faded flowers and yellowing leaves keeps the plant looking tidy and helps prevent fungal issues like powdery mildew.
- Toxicity Note: It is important to remember that begonia tubers and foliage can be toxic if ingested by pets or small children. Always keep your indoor starts and outdoor displays out of reach of curious nibblers.
Conclusion
Starting begonia bulbs indoors is one of the most rewarding ways to kick off the gardening season. It turns a cold, gray February into a time of anticipation and growth. By following these steps—matching your start date to your zone, planting with the hollow side up, and providing consistent warmth—you’ll be well on your way to a stunning summer display.
At Longfield Gardens, we believe that gardening should be a source of joy, not stress. While begonias have a few specific needs, they are incredibly resilient and generous with their blooms once they get going. Whether you are filling a single hanging basket or lining an entire shaded walkway, the effort you put in now will pay off in a spectacular show of color that lasts until the first frost of autumn.
Key Steps for Success:
- Start 8-12 weeks before the last frost.
- Plant hollow-side up in well-draining mix.
- Provide warmth (70°F+) and indirect light.
- Water carefully to avoid rot.
- Harden off gradually before moving outdoors.
We invite you to explore the many colors and forms available and start your own indoor begonia nursery this spring. For more growing tips, visit Tips for Growing Tuberous Begonias. With just a little bit of space on a windowsill and a few simple supplies, you can create a more beautiful yard and garden.
FAQ
Which side of the begonia tuber should face up when planting?
The hollow or indented side of the tuber should always face upward, as this is where the growth buds or "eyes" are located. The rounded, smooth side is the bottom where the roots will develop. If the tuber is very flat and you cannot tell the difference, you can plant it on its side, and it will naturally adjust its growth. For a general timing refresher, see When to Plant Begonias Outdoors.
Why are my indoor begonia starts looking tall and spindly?
Spindly or "leggy" growth is usually a sign that the plants are not receiving enough light. As the stems grow, they stretch toward the nearest light source. To fix this, move your plants to a brighter window or place them directly under a grow light, ensuring the light is only a few inches above the leaves.
When is it safe to move my begonias outside for the summer?
You should only move begonias outdoors once the danger of frost has completely passed and nighttime temperatures consistently stay above 50°F to 60°F. Because they are tropical plants, even a light frost or a cold snap can damage the foliage and set back their growth significantly.
How often should I water my begonia tubers while they are indoors?
In the early stages, water very sparingly—only when the soil surface feels dry to the touch. The goal is to keep the soil lightly moist, like a wrung-out sponge, but never saturated. Overwatering is the most common cause of tuber rot, especially before the plant has developed a full root system to drink up the moisture.
