Expert: Kenneth Joergensen - 2/25/2006

Question
Zone 7 (27203)
I want to know how to sprout begonia tubers. When I planted the tubers last year they never came up. All the articles I have found say not to pot until sprounts have formed. How do I get the sprouts to form?

Answer
This is best done indoors.

First find some small pots. I use 3.5 - 4.5 square nursery pots, but any type of small pot will work as long as it has drainage holes.

Use a good quality planting mix which is very well draining. If you go to garden centers, home depot, lowes or walmart right now, you can find bags of seed starting mixes. These are ideal for starting begonias.

Mix seed starting medium in a bucket with water until it is damp, but not soaking wet. I like to add a little water at a time until a handfull will stick together without falling apart. If you push your thumb into the ball it should crumble apart again. If the ball oozes water when squeezed then it is too wet and you should add some more of the dry seed starting mixture to your bucket. If the "ball" does not hold together, add more water to the mix. Slightly moist and darker brown is the goal.

Add enough of the pre-moisten mix into your planters so they are filled 3/4 full. Then take your tubers out and find the side that goes up. Tubers are usually concave, which means that there is a "cup" side (indent) and a rounded (concave) side. The cup side should have small red or white buds. If the buds do not show, then do not worry, but make sure you find the side that goes up.

Tubers are planted with the concave (rounded) side down and the "cup" upwards. Squeeze it gently into the soil mix with your hand while rotating the tuber. This will make a nice little hole for it. Do not cover the tuber yet. Leave the top of the tuber exposed.

Then water well once. The water should soak into the mix and thoroughly wet the mixture. Water should drain from the bottom of the drainage holes. After a while, empty the dish you have under the pots to avoid the pot sitting in water.

Place the tubers somewhere warm, but not cold and not hot. Room temperatures or slightly below is fine. I like to put them in an unused bedroom for the moment. Do not use bottom heat (such as seedling heatmats) unless you have a thermostat. The soil temperature should not exceed 60-70F. Regular room temperatures are fine. E.g. moderately warm temperatures.

Check every few days and water again as needed. The soil should not be allowed to dry out, but the tubers wont use much water at this time either (as they have no roots or top growth).

The warm temperatures and the moisture will make dormant tubers sprout. Therefore, if you have no tubers at time of planting, in 1-3 weeks you should see sprouts.

When you notice small pink buds showing above the soil level, you can cover them with more of the (pre-moisten) soil mix and water again really well. Do not burry them deeper than 1" (e.g. "just cover").

Keep watering on regular basis and small sprouts should show soon. At this time, you can start to use a diluted liquid fertilizer (such as miracle grow 15-30-15) but do make sure it is diluted. Such as 1/4 tsp per gallon of water max. You can increase this to 1/2 tsp per gallon of water when you transplant to larger pots. Transplanting is done once the rootball (in the smaller pot) is held together by the roots -- approximately when the sprouts are 4-6" tall.

Here is a link to a website. Just click the blue "slideshow" button to see all the pictures.

http://photobucket.com/albums/v345/kdjoergensen/gardening/begonias/startingtuber...

(copy/paste above link to your address or location bar in the browser. The link leads to my photo storage site)

a few tips:
- make sure the concave (rounded) side is planted down.
- if buds show already (on rounded "cup" side) cover with 1/2" of soil
- if no buds show (on rounded "cup" side) do not cover until they do.
- water regularily, never allowing the soil to dry out, but newly planted tubers do not require water very often. As they grow, they require more and more water
- always use a good quality seed starting mixture which is pre-moisten prior to planting. Water once really well at time of planting, and always use pots with drainage holes. Always empty saucer a few minutes after watering (or tubers rot if standing in water).

- Keep pots with tubers in moderately warm temperatures. room temperature is just fine. Do not place too warm or too cold. They do not need light until the sprout is well above soil level, so a spare bedroom can be used at this time.

- I like to cover my pots with plastic (must be suspended a few inches above the tuber) to increase humidity. This reduces need for watering frequently. This humidity cover (plastic wrap, or a plastic dome) is only kept on the tubers until the sprout show above soil level. If you use plastic wrap (or dome cover) then do not place too close to light source as it can raise temperatures inside dome.

- remember that begonias are shade loving plants and do not appreciate a window sill with hot afternoon sun. They also do not tolorate cold draft through a window.
A flourescent light bulb planced a few inches above the top of the plant is much preferred.

Enjoy the pictures, too.

Good luck.