Perhaps the summer sun in your area is too intense for some of the bulbs that you want to grow, or the winters are too cold for tender bulbs. Perhaps summer rains prevent you from growing summer-dormant bulbs that need dryness, or winter rains interfere with growing bulbs that need dry winters. You can overcome these problems by providing shelters. Even a simple shelter can expand the range of bulbs you can raise. Some bulbs won’t need to remain in the shelters the year around, but just during the seasons when they need protection.
Wherever the summer sun is too hot for bulbs, the simple, old-fashioned lath house is the perfect answer. Even in mild coastal climates, tuberous begonias look better if they’re grown under lath. So do other bulbs whose natural habitats are shady. A lath house is an enclosure roofed with lath. The laths are usually spaced about their own width, so the bands of sun and shade are the same width. They are set closer where the sun is particularly hot, and farther apart in milder climates. Laths should run north and south, so that bands of sun don’t linger too long in one spot.
Alpine Houses and Bulb Frames
An alpine house is an unheated greenhouse used to grow bulbs and alpine plants. A bulb frame is a high cold frame dedicated to bulbs. A typical alpine house accommodates lots of bulbs, including tall-growing ones such as dierama and large-scale gladiolus; a typical bulb frame accommodates fewer and smaller bulbs (which most bulbs are). A bulb frame usually represents a more modest expense and effort than an alpine house. Otherwise, these two shelters are functionally identical, or nearly so.
A bulb frame is essentially a raised bed of bulbs with a glass cover; the bulbs may grow in containers rather than the ground. Glass panels adjust for ventilation, and lift off easily. An alpine house is similar. It may be thought of as a walk-in bulb frame or an unheated greenhouse with particularly good ventilation. Like a frame it may contain bulbs in pots or in raised beds. Or, it may be meticulously landscaped—a display rock garden under glass.
Either kind of shelter makes growing Mediterranean-climate bulbs possible in regions with rainy summers. It also allows northern gardeners to raise such winter-growing bulbs as freesias, ixias, romuleas, geissorhizas, calochortuses, hesperanthas, brodiaeas, many of the moraeas, and Bulbinella floribunda.
An unheated shelter is also a safe place to cultivate spring- or summer-growing bulbs that are not hardy in the open garden. In some regions where semi-hardy bulbs survive winter in the open garden but aren’t robust, they perform beautifully in an unheated shelter. Even some of the hardiest bulbs are susceptible to damage from late frosts and thus benefit from the protection of an alpine house or a bulb frame. Bulbs being forced, as well as seedlings and newly planted offsets, do better in a shelter.
Perhaps setting up even a bulb frame is a larger project than you want to take on. If you need a separate shelter for a small group of bulbs, perhaps to bake them during their dormancy, try cloches (literally “bells”): miniature lift-off tents. You can improvise them, using fiberglass or plastic sheeting over a wood or wire framework. Leave holes near the top to ventilate them when the sun is out.
Heated greenhouses greatly expand the gardener’s scope. They protect their contents from the natural elements and provide whatever heat and humidity the plants need. Greenhouses are useful for growing bulbs in most climates, and indispensable for growing many kinds of bulbs in cold-winter regions.
A cool greenhouse, whose thermostat is set at just above freezing, is the perfect environment for bulbs that do their growing during a cool, rainy winter and early spring. Some examples are given above, in the discussion of alpine houses and bulb frames. But a cool, well-ventilated greenhouse can do everything an alpine house or a bulb frame can, and it can do it in the coldest regions.
A warm greenhouse is the ideal environment for tender and semi-hardy bulbs, including many of the bulbs commonly grown as houseplants. Some of the bulbs that thrive in a warm greenhouse (though not all require identical conditions) are clivia, achimenes, gloxinia, tigridia, eucomis, Scarborough lily, crinum, sprekelia, eucharis, gloriosa, and worsleya. Greenhouses are also useful for forcing bulbs.