Lee Valley & Veritas

 Gardening Newsletter
 
  Volume 8, Issue 11 - November 2013    
 
Forced to Perform
Forcing bulbs
Forcing bulbs is easy and will keep you in blooms until the next growing season.
 
I'm impatient and can't wait to see my forced bulbs grace windowsills and tables. It's incredibly easy to coax them into bloom by following these simple steps. The most important is to select top quality, robust, firm bulbs that have their skins (tunics) intact and that reveal no imperfections. Purchasing bulbs of the same size will provide near equal bloom heights. Buy extra bulbs, as some buds may fail to come through.

Forcing bulbs involves four stages:

1) The Rooting or Chilling Period
This is the most time-consuming and temperature exacting stage. Depending on the bulbs selected, it varies from 10 to 18 weeks. Squills (Scilla spp.), snowdrops (Galanthus spp.) and hyacinths can bloom in nine to 12 weeks, large trumpet daffodils (Narcissus spp.), dwarf irises (Iris reticulata and I. danfordiae) and grape hyacinth (Muscari spp.) can take 14 to 15 weeks, and some tulips can take up to 18 weeks. For those bulbs that require more time, you'll obviously want to start the chilling stage sooner. Staggering the time you start the chilling period for other bulbs will extend the winter bloom season. Remember that this is not an exact science and success varies with the robustness of the bulbs, light levels, humidity and temperature control. Consult bulb suppliers for their recommendations as to the best species and cultivars and their expertise regarding chilling and bloom times.

I use 20cm diameter pots for larger bulbs and 10cm diameter pots for smaller bulbs. Clay pots tend to dry out, so soak them overnight before planting and check the soil regularly during the chilling period. Non-porous plastic and rubberized pots need very little supplemental water. Plant at least four or five bulbs per pot and be sure to affix labels to the pots to indicate the type of bulbs and the date planted. This will help you to know what's coming up approximately when. Plant small bulbs very close together, almost touching. Larger bulbs (e.g. daffodils) should be planted 3cm to 5cm apart.

 

Tulips

Forced tulips provide beautiful color and a lovely indoor focal point.
 
Tulip bulbs often have a distinct flat side, which is the first leaf side. Place it towards the edge of the pot so that the emerging leaf will cascade nicely over the side. It's common to leave the top thirds of hyacinth bulbs above the soil line with their tips pointed toward the pot's rim. Many other bulbs can be planted with their tips at the soil line or just below it. Bulb aficionado Ferdinand Otawa suggests that larger daffodils benefit from being planted in deeper pots (15cm) to provide lots of rooting room for firm anchorage. The potting mix should be a soil-less, free-draining one with almost equal percentages of perlite, sand and shredded peat moss. Add in a sprinkling of bone meal. Before planting the bulbs, soak the pots and potting mix in water and let stand to drain overnight.

A fairly constant chilling temperature (approximately 2°C to 8°C) is critical while the bulbs are in this semi-dormant rooting stage. If the temperature dips below freezing, rooting will slow down; if it gets too hot, the reverse will happen. This critical stage can be achieved both indoors and out. One of the simplest outdoor techniques is to arrange pots in a circle on top of at least 20cm of leaves. Place 50cm to 60cm of leaves over the pots and cover with hardware cloth to keep the leaves in place and the mice out.

 
  Unopened hyacinth bloom
  The fragrance of hyacinth can fill the house during winter.
Another outdoor method involves digging a trench about 60cm deep and wide in well-drained soil with surface drainage directed away from the trench. Cover the base of the trench with 10cm of crushed gravel and fill the majority of the trench with a mixture of vermiculite, sand and shredded peat moss. Bury the pots in this mixture and secure the top of the trench with hardware cloth to keep out mice. For added insulation, cover the area with pruned evergreen branches. Place a plastic sheet overtop for rain protection. Some of the cons of this technique are that it involves quite a bit of work and water could seep in and freeze the pots. I also found it cumbersome to monitor the progress of the buried bulbs.
 
A variation on the trench method is to use the window wells along the foundation of your house. These have gravel bases for drainage, secure metal sides and removable plastic covers can be purchased to keep out the rain and snow. Insulation pellets in clear plastic bags or alternatively leaves (a bit messy at clean-up) provide extra insulation. Place a thermometer against the window so that you can monitor the temperature from inside.

Finally, if you are fortunate enough to have outdoor cold frames, these make ideal forcing facilities.

Forcing bulbs indoors can be as simple as repurposing a spare refrigerator dedicated to the bulbs. (To keep it ethylene free, do not store any fruit with the bulbs.) This method makes it easy to set and monitor temperatures, and you can simply open the door for ventilation.

Another indoor method, and one that I've been using almost exclusively for the last decade, is to place bulbs in large recycled cardboard boxes kept in my unheated garage. The cardboard boxes measure 70cm long by 45cm wide and deep and are lined with rigid 4cm thick outdoor insulation material on the sides, top and bottom. (The bottom insulation is critical.) Each box can accommodate up to six 20cm wide pots, and the boxes can be stacked to save space. I pour foam packing pellets around the pots for extra insulation. You can easily add or remove the pellets to ensure the bulbs are kept at the required chilling temperature. If the thermometers inside the boxes indicate that the temperature is too low, move the boxes to the warmer house-side of the garage. Lift the lids for extra ventilation and moisten the potting mix if needed.

2) Stem and Bud Elongation
During this transitional period, the bulbs acclimatize to the warmer temperatures (10°C to 14°C) and modest light levels of a basement or cold cellar for two to three weeks. Keep the soil slightly moist and monitor the developing stems for color (they should be turning a darker green) and the emergence of ample roots from the bottom of the pots.

 
  Cheerful daffodils
  Vibrant daffodils add cheer to any room.
3) Flowering
This stage requires approximately 20°C temperatures. If you place the pots on a sunny windowsill and water them regularly, the bulbs should be in full bloom within one to three weeks. Rotate the pots to ensure even growth.

To extend the bloom duration, move the pots to a cooler, darker spot in the evenings. Keep them away from heat sources (south-facing windows, heating vents) that would dry out the plant and shorten the bloom period. Don't be disappointed if the forced bulbs don't last as long or appear as brightly colored as they would outdoors. They have worked very hard to get to the blooming stage under artificial conditions.

 
4) Aftercare
This stage involves removing spent blooms and allowing time for the foliage to ripen (turn yellow) and send nutrients back to the bulb. When the frost is out of the ground, I replant the larger daffodil bulbs in the garden. Since the forced bulbs will probably need a year or two to bulk up or may not return at all, I prefer to naturalize them at the rear of my woodland garden with the ferns. I discard most of the tulips and smaller bulbs, as I have not had much success with replanting them outside.

Forcing plants is a worthwhile experiment for all gardeners and a wonderful science project for young students. Keep a diary of your successes and failures and the time and temperatures that work best for different bulbs. Above all, enjoy the cheery blooms all winter long!

Text by Frank Kershaw

Photos by Marnie Wright

Frank Kershaw is an award-winning horticulturist with more than 35 years of experience. He teaches garden design and horticultural courses at George Brown College in Toronto, Ontario, and at the Toronto Botanical Garden. Frank is also a presenter at the Lee Valley seminars at the Toronto stores.

Marnie Wright (gardenwright@muskoka.com) is a lifelong gardener, writer and passionate garden photographer. Her Rocksborough Garden, developed over 30 years, is located in Bracebridge, Ontario.

 
 
 
 
     
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