Alyssum is a popular plant to grow for its pretty little flowers and spreading nature.
Whether you are purchasing a young alyssum plant or growing it from seed, an important consideration is when you should plant it outside.
Before we get started, it should be clarified that we are focusing on alyssum, which are annuals, with this information. Perennials (plants that come back every year) don’t necessarily need to be planted in the spring, so the answer to when to plant outside will vary for them.
Whether you are planting in your garden, a flower bed, or doing some container gardening, choosing when the best time to plant your alyssum outside is a decision that should be made carefully.
If you plant alyssum plants too early, they have a higher risk of being damaged or killed by cold snaps. If you plant them too late, you may be missing out on their full potential and in some cases, the summer heat can hinder their early growth.
One way to get the most out of that one growing season is to be smart about when you choose to plant your alyssum. Timing can be a crucial factor when it comes to getting your alyssum started off right.
We put together this guide to discuss some important factors to consider when choosing when to plant your alyssum.
The best time to plant alyssum outside
There are a number of factors to consider when deciding when to plant, but you can always use your growing zone’s average last frost date as a general rule of thumb.
For Pennsylvania, planting outside around Mother’s Day (the second Sunday in May) is the best time to plant alyssum. This will lessen the chances of the alyssum freezing with an overnight frost.
Planting around Mother’s Day also gives you a full growing season, allowing the plant to mature and grow to full size for a full summer of beauty.
If you are starting your alyssum from seed, you can determine when they’ll germinate and sprout so that you can better time when to plant the plant outside.
If you have a greenhouse or some other protection from early spring cold nights, alyssum can be planted earlier than Mother’s Day.
4 Things to Consider When Choosing When to Plant alyssum
If you are really getting eager to plant your alyssum, there are a number of factors that you can consider to decide if you can safely plant your alyssum earlier than Mother’s Day, including:
- What growing zonal area you are planting in
- Whether the alyssum are hardened off
- The size of the plants
- Where you are planting the alyssum (or what you are planting them in)
We’ll break down each of these factors below.
1. Considering your growing zone
For most varieties of alyssum, frost will kill or cause the plants to struggle significantly, so it’s important to determine what growing zone you are in.
The United States is broken up into 11 different growing zones, each with general growing conditions that can help you as you plan your garden.
What growing zonal area are you in?
Not sure what growing area you are in?
Here’s an example: If you are in Ephrata, PA, your growing zone is 6. In growing zone 6, the last frost happens on average sometime between April 20th-May 20th. For this reason, we normally recommend that you wait until after Mother’s Day to plant your alyssum outside.
To find the growing zone for your specific zip code, here’s a helpful tool to determine your growing zone.
When to plant your alyssum by zonal area
Below is a guide for when the average last frost date is for popular growing zones in the United States.
This list can be used as a reference for when the average last frosts are, but please keep in mind that these are only estimates, and a later than normal frost is always a possibility.
Average last frost date for popular United State growing zones
- Zone 3: May 20th-June 20th
- Zone 4: May 15th-June 15th
- Zone 5: April 15th-May 15th
- Zone 6: April 20th-May 20th
- Zone 7: April 15th-May 15th
- Zone 8: March 15th-April 15th
- Zone 9: February 15th-March 15th
- Zone 10: January 15th-February 15th
- Zone 11: Rarely-January 15th
2. Are your alyssum hardened off?
Typically, a alyssum that is tough and hardened off will grow better once planted in another environment.
Growing centers and greenhouses don’t always harden off their plants before selling, so it can be something to ask them when shopping for your plants.
3. Size of the plant
The size of plants affects how established and ready to plant outside it really are. Obviously, seedlings won’t last very long during typical spring weather.
Many greenhouse and garden centers sell plants in varying sizes of pots and packs, so you’ll want to consider the size of the plants when determining when to plant your flowers. Some of the more popular sizes of packs and pots that you’ll find alyssum for sale in:
- 3 packs
- 4 packs
- 3 ½” pot
- 4 ½” pot
- 6” pot
Due to the variation in pot and pack sizes, the size of the plugs will vary significantly. Along with this, the plants’ strength, maturity, and hardiness are tied to how developed their root structure is.
Simply put, the larger the plug and the more developed the root structure, the better the plant will handle the shock of being taken into a new environment.
However, keep in mind, that no matter how well-developed the root structure is, cold temperatures can always kill plants that are not hardy enough to handle the weather.
4. Where are you planting your alyssum?
You can also consider where you are planting the alyssum. Whether it be a container, in your landscape, or in a hanging basket, each has its own requirements and advantages when started at a particular time.
When to start growing alyssum in hanging baskets?
For hanging baskets, the earlier the better. Hanging baskets look best when they are filled and overflowing with mature plants, so the earlier you can get them started, the more beautiful they will get and the longer they’ll be beautiful, provided, they are taken care of.
A benefit of hanging baskets is that they are more mobile than plants in a pot and especially in landscape. So, if a late frost happens, they can be taken inside overnight to protect blooms and plants.
When to start planting alyssum in the landscape?
Planting alyssum in the landscape is where you want to be especially careful and the most conservative. Many plants have been wiped out by an unexpected late frost when planted in the landscape.
In some cases, buckets or clothes can be placed over plants when a late frost hits — it’s generally recommended to be better safe than sorry with landscape alyssum.
When to plant alyssum in pots?
Unlike landscape alyssum, planting alyssum in pots does give them some mobility, so they can always be taken inside overnight if necessary.
An important thing to consider is how full you are filling your pots. If you are planting your pots later in the season, you can fill the planters much fuller and have an almost instant great look. If you are planting alyssum in pots earlier in the season, you’ll want to make sure that you are not overfilling the pots with plants, as the plants will grow considerably with the proper care and water.
Consider what type of protection it has
You’ll want to keep in mind the amount of protection your plant will have when planting. As mentioned earlier, you can always manually protect alyssum by covering them with a bucket or a blanket or taking them inside.
Buildings, fences, etc. can also protect plants from harsher elements like wind that might make a cold snap more severe. Some gardeners also opt to plant their flowers in a mini greenhouse.
When is it too late to plant alyssum?
Is it ever too late to plant your alyssum? The answer is no. While there are certain times that are optimal, if you are busy during the spring, you can certainly start and still grow some magnificent alyssum beginning in the summer.
Keep in mind that some greenhouses may be sold out of plants you are looking to start too late, but there’s normally always something to grow, even if you are starting mid-summer!
What to do if you plant too early
If you plant too early and cold temperatures wipe out your plants, you can always come back and purchase flowers to try again. Many gardeners are hit by surprise frosts (and for farmers, it can lead to losing an entire crop), so it’s normal to be hit with a surprise cold snap from time to time.
Conclusion
To wrap things up, there are a number of factors that will influence the correct answer to “when is the best time to plant alyssum?”
Consider the following when deciding if it’s safe to plant your alyssum outside:
- What growing zonal area you are planting in alyssum in
- Whether the alyssum are hardened off
- The size of the plants
- Where you are planting the alyssum (or what you are planting them in)
Seasons and weather patterns vary, so there’s no exact date. But using these questions will guide you in knowing when the safest and best time to plant your alyssum is.