Planting bulbs in outdoor containers
Will daffodils bloom in springtime when planted in outdoor containers on a deck? If so, how often should they be watered? I would like to plant fall pansies in same container. Is there a pot size requirement (I.e. is there a size too small for planting bulbs for this situation?)
Howard County Maryland
Bulbs can overwinter in containers if the pot is large enough (both in volume overall and in depth, since bulbs often need a planting depth of around three times the bulb's diameter). There isn't researched information on pot size minimums based on bulb type, though in general, the larger the volume of soil, the more insulated the roots will be because larger masses don't change temperature as quickly as smaller ones. As a rough guess, a pot should be at least 10-12" in diameter to support both the pansies and the bulbs and give each sufficient root space, though of course it depends on how many plants are being used of each.
Ideally, the container should not be allowed to freeze solid, so a sheltered location like against a southern or western wall would be best if it also gets enough sun for the pansies to grow well. If the deck doesn't offer this option, than simply keeping it sheltered from wind will help moderate its temperature and evaporation. (Keeping the pansies from freezing will also prolong their bloom through the winter.) That said, faster temperature changes ("fast" is relative, so perhaps 24-48 hours) between mild and freezing conditions can be more damaging to hardy plants than the cold temperatures or length of a cold spell itself.
It may be tricky to make sure the pansies get enough water as the soil dries while avoiding over-watering the bulbs below since bulbs can be sensitive about staying too wet. Either way, be sure the container has good drainage (one or more holes in the bottom), and consider the use of "pot feet." These elevate the pot about a half-inch or an inch (if it's on pavement or another impermeable surface) to allow drained water to evaporate or drain away before it freezes and risks blocking the holes in the bottom of the pot. You can either purchase plastic or ceramic "feet" or use any sturdy materials around the house - scraps of brick or paving stones, scraps of wood you don't mind being degraded by the weather, etc.
How often the pots will need watering is mostly up to the pansies as they will need the moisture more so than the bulbs. Feel the soil about a finger's depth down (though this relative depth will depend on the pot size) and water well when the soil is fairly dry at this depth. If the pot isn't too heavy, you can usually get used to the feel of its weight when adequately damp versus needing water so you don't have to probe it each time, particularly when the surface may be lightly frozen and difficult to check.
Thank you, Miri, for your comprehensive and very helpful response.