Helping Plants Keep Their Cool

The majority of home gardeners probably haven't tried their hand at bulb and seed chilling, but it's a great way to gain more control over when seeds will germinate and bulbs will flower; and cold storage can help preserve seed viability. These tips from Dermot Molloy, Horticulturalist (Nursery) are a great jumping off point! 

What's the deal with winter dormancy?

There are two types of plant dormancy which occur during the winter months - endo-dormancy, in which plants will not grow due to internal mechanisms rather than environmental conditions, and eco-dormancy, which occurs when plants are ready to grow but the environmental conditions aren't suitable. Eco-dormancy usually occurs because temperatures are too cold and the plant in question hasn't experienced enough chill hours required to germinate. Chill hours are the units of time that plants need to spend at cold temperatures which don't drop below freezing - temperatures between 5°C and 10°C are ideal. As the plant enters dormancy, it tracks these chilling hours to gauge the passage of winter so it knows when to germinate. 

Time to chill out

The number of hours required for chilling depends on the plant. Some require less than 500 hours, and others might need upwards of 1,500 hours. Depending on how warm of a winter it is, plants can accumulate their chill hours earlier than normal, and as a result, the plants are no longer in endo-dormancy after chilling is completed. They are now in eco-dormancy, solely due to cool or cold weather. Warmer temperatures will be the trigger that causes the plants to begin growth. Once they start growing, they lose the ability to readjust to colder temperatures. Examples of plants grown in the home garden which require chilling time in order to germinate include peonies and some apple varieties, as well as Tulips, Hyacinths and hardier bulb varieties.

Overcoming Winter Dormancy: Bulb Chilling 

If you want your bulbs to flower quickly after planting and stand tall and strong, chilling is the answer! Refrigerating bulbs in your veggie crisper for between 5 - 8 weeks so that they receive enough chill hours is a great way to have your garden put on a spectacular display! 

Breaking Dormancy: Seed Stratification

Seed stratification is the process of breaking seed dormancy in order to promote germination. For seed stratification to be successful, you need to replicate the exact conditions that they require when breaking dormancy in nature. Some seeds require a warm and moist treatment, some require a cool and wet treatment, and others require a combination of both warm and cool treatments followed by a warm treatment. Knowing specifically what seeds require to break dormancy is critical before beginning the process of stratification. To get started, soak the seeds for 12 to 24 hours and put them in a sealable container with equal amounts of sand and peat or vermiculite. Store the seeds for 1 – 3 months in the fridge, and then move them to a warm place or sow them. Seed stratification is regularly used by the Royal Botanic Garden's Nursery team as a method of germinating the seeds of rare and threatened species which have been collected on expeditions or provided by DELWP or other organisations. It's often successful (for example in the case of the endangered Discaria nitida) and is a good option for the trial-and-error process of testing germination methods for rare species with very little to go off in terms of literature and research. 

Freezing seeds

Freezing seeds is a fantastic way to extend their lifespan. The secret to success when freezing seeds is to store them in an airtight container and ensuring that they are thoroughly dried before being frozen, as the freezing process can cause moist seeds to crack or split. Storing them in an airtight container prevents them from absorbing any humidity and taking on any damaging moisture. Storing seeds in the freezer will provide them with more consistent temperatures than refrigerator storage. For every 1% increase in humidity, a seed can lose half its storage life. Whether you are freezing seeds for a couple of weeks to use for succession plantings, or for use in a year or more from now, these are some critical steps to ensuring a good result:

  • Make sure the seeds are clean and dry before freezing. You can use silica gel to help thoroughly dry seeds.

  • When placing seeds in an airtight container for cold storage, you should label and date the container to avoid confusion around when it’s time to plant.

  • When it is time to use them, take the seeds out of the freezer and allow them to thaw at room temperature for at least 24 hours before planting them.

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