By Larry Hodgson
In March and early April, it’s time to wake up most of the plants that you put into dormancy in the basement or garage for the winter, including tender bulbs like cannas, callas, tuberous begonias, caladiums, dahlias, gladioli, and alocasias, and also subtropical plants like pelargoniums and mandevillas.
What to Do
Bulbs in Pots
If you kept your bulbs in their summer pot and simply brought them indoors in that state, storing them in a cold but frost-free spot, you can save a few steps. Just place the pots in a sunny and relatively warm location (normal room temperatures are fine) and start watering!
Water gently at first, as the plant has no roots at this point. So, barely moisten the soil. But as growth resumes, increase the amount of water and the frequency of watering. Once the plant is in full growth, and that may only take a month, you should be watering it according to the golden rule of watering, that is: water deeply enough to moisten the entire root ball, then allow the soil to dry before watering again.
It’s also when the plant starts to put on new growth that you can start fertilizing it again. So, a month or so after you start rewatering them.
If the bulb was stored without soil, pot it up in a temporary container. The idea is to start it early indoors to give you an earlier bloom or harvest when you plant it in the garden.
The dimensions of the temporary pot are of little importance as long as the bulb fits easily into it. Leave at least 1 in (2.5 cm) of space all around it and 2 in (5 cm) or more of space underneath where it can root into the potting mix. A 4 in (10 cm) pot would be fine for a single bulb of modest size, for example, while some large bulbs will need, obviously, a larger pot. An 8 in (20 cm) pot would be suitable for 3 to 6 bulbs, depending on their size. Lots of gardeners start them in trays, in which case you’d probably have enough room for 10 bulbs or more.
Plant the bulb in a slightly moist planting mix. Position it so that any sprouts (probably visible by now) are directed upwards. Begonia tubers, though, prefer to be barely covered at first—no more than ½ inch (1 cm) of soil or even not at all.
Cover other bulbs with about 1 to 2 inches (2.5 to 5 cm) of mix. (When you plant them outdoors in late May or June, you can adjust the depth further, according to the table above.)
As with bulbs still growing in their previous year’s pot, little watering will be necessary at first, because the bulb has no roots. As they begin to grow, their roots will dry the soil out more and more rapidly. You’ll have to increase the frequency of watering in consequence, according to the golden rule of watering above.
Subtropical Plants Forced Into Dormancy
Many gardeners force subtropical plants into dormancy as well. This has long been the case with the zonal pelargonium (Pelargonium x hortorum), formerly known as zonal geranium. It has no dormancy in the wild and will willingly grow and bloom all winter on a sunny windowsill at room temperature. However, this pelargonium is very drought resistant and can be forced into winter dormancy by ceasing all watering and keeping it cold (about 45°F/7°C) and in darkness.
You can do this quite simply with a potted pelargonium: just stick in a cold, dark corner and stop watering it! However, many gardeners like to save space by taking it out of its pot and knocking off the soil, then storing the plant in various ways, such as in a paper bag or cardboard box. My father used to hang his upside down in the cold room!
By the end of winter, most of the leaves will be dead or dying and several stems will be in a similar state, but there ought to still be some life left in the plant. So, as spring dawns, prune off the damaged parts, then bring it into warmth and bright light. Repot it now if it is not in a pot, then begin watering gently, as explained above, until growth resumes. Increase your watering as the plant fills in. And you can start fertilizing when growth restarts as well.
Several other subtropical plants, such as mandevilla (Mandevilla spp., formerly Dipladenia), bougainvillea (Bougainvillea spp.), agapanthus or lily of the Nile (Agapanthus spp.) and Chinese hibiscus (Hibiscus rosa-sinensis) also are often put into dormancy in much the same manner, although usually kept in their pots. If so, the recommended treatment for pelargoniums applies.
When Should You Start Waking Up Your Plants?
When you should be starting your bulbs and other dormant plants for the coming summer is going to depend largely on your local climate. The idea is to have them growing robustly, but not yet blooming, at planting out time.
Some of the plants mentioned above need a long period of growth indoors in order to be ready to bloom fairly early in the summer, so should be started indoors in March or early April. This is the case of alocasias, caladiums, cannas, tuberous begonias, and pelargoniums. But others are faster growing: only pot them up 4 to 6 weeks before you expect to be able to plant them in the garden; otherwise they tend become too tall indoors and produce weak stems likely to fall over. That’s the case of dahlias and gladiolus. And if you start callas (Zantedeschia) too early, they’ll often come into bloom indoors on the window ledge, probably not what you wanted. Just give them a 3 to 4 week head start!
So, go right ahead. Pull yourself out of your own winter lethargy and invest a little effort in preparing your dormant plants for summer. It may still be wintery outside where you live (or maybe not), but as soon as spring is near, with the days getting longer and longer, it’s time to wake up any plants that are dormant and start them on the route towards a summer of beauty and growth.
Text based on an article originally published in this blog on March 2, 2016.