Wednesday, October 23, 2013

Guidelines for Winter Protection of Lily Bulbs

Cold Winters - Midwest & Northeast
Although moderate climates only require enough mulch to reduce winter weed germination, colder climates need a bit more attention. In colder areas of Wisconsin, Minnesota and other Northern Tier States, gardens can experience very cold temperatures with frost levels greater than 5 feet and without enough snow cover to prevent the loss of rose bushes, trees, shrubs and other “above ground” plants. Protecting lily bulbs under such trying conditions is easy if the ground is properly mulched.  In the severe winter climates you MUST place an insulating layer of straw, ground leaves or other material over all Trumpet, Oriental, OT Hybrids or species where gardens are subject to deep freezes (generally colder than -10o F.), this is especially true if a good snow cover is lacking or not expected. Asiatics do not usually need extra protection, but all other lily bulbs in the Midwest, Northeast or Mountain states should be covered for protection.

In early Spring, remove the mulch in layers as the ground thaws and warms, to evaporate rainfall (or snow melt) as quickly as possible.
Too much sudden moisture can cause your bulbs and sprouts to rot. If lilies are planted at the top of a ridge or small hill, frost will not settle on the plants, so those areas may get by with less mulch  Avoid soggy, wet areas as well as low spots where frost will be heavier in winter.  A good layer of mulch may consist of 8 to 12 inches of straw or hay, tree boughs or other fluffy material. Avoid compacted, whole leaves or wet grass clippings because these have the potential to hold too much moisture around your underground bulbs and the sprouts will rot in spring if you do not remove the mulch fast enough. Shredded leaves are better because they allow air movement and are easier to remove in spring.

Wet Winters - Southern & Coastal States
In mild climates, be very CAREFUL about mulching newly planted bulbs if you were affected by an unusual amount of rainfall last winter or spring, that resulted in your garden becoming overly saturated.  Lily Bulbs by their very nature are designed to store water for times of drought; plant on a slope or in a slightly raised bed to avoid flooding—they do not swim well and will suffocate if the soil stays constantly soggy over winter.  Moisture replenished from rainfall on a daily basis is OK just as long as it is draining away quickly and not sitting stagnant.  During cold and wet springs, a thick mulch over lily bulbs may cause the sprouts to decompose while still underground or as they begin to emerge. (See Fungus: Stump Rot (Phytophthora) on our website under “Problem solving - Animals to Weeds”.) In moderate/mild Southern and Coastal areas, use only enough mulch (one or two inches) to suppress winter weed germination, especially if this winter is expected to be wetter than normal.
During very moist winters, if the soil surface is not allowed to dry between rainfalls and the garden remains saturated, you will risk losing even established bulbs. 
Use a porous mulch (e.g. straw or large nuggets of bark) will allow your soil to breathe and dry faster. Go easy on compost; only an inch or two on top of the soil is recommended per year.  Your goal is not deep freeze protection as in the Midwest or Northern states, but rather a tidier garden.

Gardeners in maritime areas of the Pacific Northwest, that are subject to large amounts of heavy rainfall over winter and into June, need to be especially careful about using bark, shredded leaves or other decorative materials in winter.  Add a new layer to your garden in spring after wet snow and/or heavy rainfall has diminished and just before the new weed seeds begin to germinate, which will keep the garden looking good all next summer.  It is important to allow the soil surface to dry properly between storms. For good looks, freshen your mulched garden in June, after the wet and cold weather has settled and the days have become drier.

NEW - Alaska & Gardens at High Elevation
(Not printed in the Planting Guide.) Planting in the garden is preferable (See Midwest & Northeast Instructions above), but customers in the far north sometimes pot up lily bulbs for winter and keep them in a colder area until spring.  These instructions are important should the soil suddenly freeze too hard and deep before our bulbs can be harvested and mailed in mid October.  Strategies include placing lily bulbs planted in pots with moist—not saturated soil—in the crawl space under their house or wrapping them in pink fiberglass insulation in an attached, cold garage or protected shed over winter.  A heavy cover of insulating snow also helps to moderate the temperature so that the soil surrounding the pots slowly freezes around them.  Quick freezes or freeze-thaw conditions all winter will leave lily bulbs looking like a frozen, thawed onion—limp and not viable.
Do not store pots in a greenhouse or root cellar that stays warm over winter OR is allowed to experience quick freeze-thaw conditions.
Lily bulbs need 8 to 10 weeks of cold, preferable below 35 degrees F. to reset themselves for bloom the next summer.  If they do not have enough cold treatment, they will either emerge weak and spindly or simply grow leaves and not bloom.  Should pots be stored in an greenhouse that stays constantly over 40 degrees day and night, the bulbs will also not bloom well either.  Modern hybrids are derived from wild lilies in temperature climates, where the soil slowly freezes deep over a period of time or the soil regularly only freezes a few inches below the surface each winter.  If stored in an unheated greenhouse, where the temperatures drop below freezing at night, but warm up to above freezing on sunny days, the freeze-thaw will compromise the integrity of your lily bulbs and they will most likely rot before spring.

Your goal is to allow soil surrounding the lily bulbs to slowly freeze.  We do the same in our winter coolers.  Wet peat surrounding the bulbs is frozen solid, which in turn slightly freezes the lilies in place and mimics a natural winter.  They stay in this dormant frozen state with proper hydration and the cooler air temperature is just a degree above freezing, until the cases are removed and prepared for shipping across the USA in April.  This is not something you cannot do at home, because the lilies need to be in a large block of soil/bulbs (about 45-50 pounds) to be protected and the air temperature is dropped only a few degrees each week.  Home freezers are designed to freeze solidly to zero or below within a few hours, which would turn the lily bulb to mush upon thaw.

Planted in a well-drained garden, soil freezes at a slower rate and acts as a buffer.  If you are lucky to have a good snow cover as well, your sleeping bulbs are quite happy.  If in those odd years of either no snowfall or snow that comes and goes over winter,  a thick mulch (foot or so deep) of shredded leaves, cut evergreen branches, bark or other insulating material is in order.  Avoid whole, large leaves (like Bigleaf Maple) because they will tend to mat down and become slimy.  Remove the layers as the snow melts to allow soil to slowly warm and wake up your lilies.

Also see:  Hardiness for bulb types and a brief Planting Guide for bulb depth on this Blog.

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