How to Chill Bulbs: What Bulbs Need a Winter Chill?
Several spring-flowering bulbs from cool regions must be chilled in warmer regions or forced to flower early indoors. Gardening expert Madison Moulton breaks down which bulbs need a winter chill and which don’t, with a step-by-step guide on how to do it.
In their native environments, many bulbs are exposed to lower temperatures in fall and winter before they pop up again in spring. This temperature drop signals a rest period, causing the plant to direct nutrients back into the bulb and develop a stronger root system until temperatures rise again.
This cycle of cooler and warmer temperatures triggers both flowering and dormancy. If temperatures don’t drop low enough for them to rest in fall and winter, they will struggle to flower, even if all other conditions are perfect.
Those in cooler zones that match the requirements of these cool-region bulbs won’t have a problem chilling them in the ground or a pot. However, if you live in a warmer zone (around USDA Zone 8 and above), you must provide the winter chill to trigger growth.
You can also use this natural cycle to your advantage to force bulbs to flower indoors at a certain time. Cooling in the fridge and then exposing them to warmer temperatures will allow you to have happy flowering bulbs indoors long before they appear outdoors.
Which Types Need To Be Pre-Chilled?
Spring-flowering types are made in fall, right when temperatures dip. Most spring-flowering bulbs require pre-chilling if you live in a warmer zone or want to force them to flower.
This cooling emulates fall soil temperatures in cooler climates, allowing you to get them in the ground when the soil cools sufficiently in winter. You can also chill for longer before planting them in pots to force them to flower indoors.
These are just a few of the bulbs that need to be chilled to flower successfully:
If you’re unsure about your specific types, consider their native environments and typical planting times. Those that go into the ground in fall and come from cooler regions need it, while those from warmer regions don’t.
Which Types Don’t Require Chilling?
If you don’t want to go through the effort of chilling, there are also a few that don’t require any pre-chilling.
These are typically types that bloom later in the season or come from warmer climates, such as:
When To Chill
Different bulbs require different chilling periods to flower successfully. Work off the minimum amount of time here; they can chill happily for a few weeks longer but usually won’t perform if chilled a few weeks less.
For example, tulips need chilling for about 10 weeks minimum before planting. Crocus and snowdrops that really appreciate cooler temperatures need closer to 15 weeks to flower successfully. Check the recommended chilling time for your specific bulb to avoid missing the window.
You’ll be chilling them for a period between 6 and 14 weeks. This usually starts toward the end of October, but you can adjust the timing based on temperatures in your region and when you plan to plant.
How To Chill
Chilling is not difficult or technical and becomes part of your regular seasonal chores once you get the hang of it. Follow these tips to get it right.
Pick Healthy Bulbs
Successful bulb growth starts with choosing the right ones or, more specifically, knowing what to look for in a healthy bulb. This will greatly increase your chances of success, whether chilling to plant outdoors or force indoors.
Healthy bulbs should be firm to the touch and free of any signs of damage or disease. Avoid those with mushy spots or mold, as this issue will only spread in storage. Also, choose larger over smaller ones if you have the option, as these typically translate to better flowers.
Purchase from a reliable source or check them thoroughly if you’re using those you’ve lifted from your garden. The better the start, the easier the growing process will be.
Store bulbs correctly to avoid disease issues and keep them properly cooled without overheating or freezing. Preparation is key to success over the next few weeks while you’re pre-chilling.
Start by cleaning off any dirt or debris, along with old roots or areas of damage. Place them in a paper or mesh bag, ready for storage. Any diseases or pests can spread quickly in storage, so make sure you’re satisfied with the health of your bulbs before placing them in a bag.
You can also chill them in soil in a small container (usually easier if you plan to place them in a cool room rather than your refrigerator). Layer them in a potting mix and water lightly to encourage root growth. This is ideal if you’re keeping your bulbs in a container as when forcing indoors, limiting any need to transplant later.
Keep In Storage
You can store them in a few places, depending on how many you have and the space you have available.
The easiest chill method is placing them in the refrigerator, as long as you’re careful about what you store them with. Unheated rooms in your home, such as storage rooms or cellars, also make great chill zones if you can successfully manage temperatures.
Mark Your Calendar
Accuracy in timing is essential when pre-chilling, especially if you’re dealing with a few different bulb varieties simultaneously. Once you place your bulbs in a fridge or cold room, mark your calendar to note the date they began chilling and when the chilling period is set to end, ready for planting.
Although this step seems like unnecessary extra effort, it may be one of the most important steps you complete. Caught up in the excitement of the process at the start, it’s easy to forget when you started chilling, potentially leading you to take them out too early.
For pre-chilling to effectively ‘chill’, you must manage temperatures closely while in storage. Typically, a consistent temperature of 35-45F will best simulate the fall and winter conditions they are accustomed to, forcing them into rest.
Monitor the temperature closely to prevent them from freezing or becoming too warm, which may end your blooming season before it even starts.
Ethylene is the gas certain fruits and vegetables produce when ripening, such as apples and pears. You may have heard the hack of ripening tomatoes at the end of the season by storing them with a ripe apple or banana, which is caused by ethylene.
Unfortunately, ethylene can harm chilling bulbs, damaging the internal structures and impacting flowering. If you’re using a refrigerator as your chilling environment, ensure it doesn’t contain produce that emits ethylene.
Alternatively, use a dedicated area for chilling or a different chilling location altogether to safeguard against potential ethylene exposure.
Once the chill period is over, you can get to planting. For indoor forcing, plant in a well-draining potting mix with the pointed end facing upwards, ensuring they are at the correct depth and spacing. You can space those in pots a little closer than you would in-ground or layer them (known as lasagne planting).
For outdoor planting, prepare your beds beforehand by incorporating organic matter like compost. Water thoroughly after planting and keep up with care to enjoy a flush of blooms in the upcoming weeks.
Unfortunately, most spring bulbs that are forced or pre-chilled are unlikely to flower successfully again. It’s not impossible, but treating them as annuals and purchasing new ones the following year is best.
Whether you’re a warm-zone gardener who wants the excitement of cool-season spring bulbs or forcing them to flower indoors to brighten your winter, chilling is the way to get it right.