One of my personal favorite flowering plants is Eryngium. Also known as sea holly, these unique flowering plants are in the same family as carrots (Apiaceae).
Here in the US, sea holly is not all that popular, and it can be difficult to find seeds or plants for sale. So, if you want to have these gorgeous spiky blue flowers in your garden, you’ll have to grow it yourself.
In this article, I’ll share the entire process of growing eryngium from seed to bloom. You’ll learn everything you need to know, including germination, soil requirements, watering needs, and more for your sea holly plants. Let’s get started!
Sea Holly Plant Overview
There are many types of eryngiums, from vibrant blues to purples and even white cultivars. However, the most popular varieties produce blue blooms on sturdy stems.
Quick Sea Holly Facts
- Winter hardy to USDA zone 3 ⛄
- Sandy, low-nutrient soil is ideal 🌱
- Tolerant of salt-air (shoreline climates) 🌊
- Full sun is ideal 🌞
- Deer resistant 🦌
- Flowers attract a wide variety of pollinators 🐝
- Unpleasant flower smell (some liken it to the smell of dog feces) 💩
Is Sea Holly Perennial?
Sea holly is a cold hardy perennial (USDA hardiness zones 3-9). The foliage will die back in winter, and will regrow in spring, flowering each season. In an ideal location, sea holly plants can live for many years with very little care.
Sea Holly Growth Stages
Like many perennials, eryngium will not flower in its first year of growth. During the first year, sea holly will grow some foliage, but it will be focusing its energy on forming a large taproot (much like a carrot or parsnip root).
So, this means that the first year of growth is not very spectacular. Mostly, you should expect to see low, broad leafy growth. Strong foliar growth indicates good root development, and will usually lead to a more vigorous plant the following year.
After the first year, sea holly regrows in early spring. First, a leafy base forms, followed by spikes that will soon form flower stems.
In early summer, flowers will begin to blossom, and your sea holly will be a pollinator factory! Our sea holly plants are always buzzing with dozens of different types of insects as the flowers bloom.
As summer progresses, sea holly flowers and stems become dyed a gorgeous deep blue or purple color (depending on the species and variety). Later summer is a great time to harvest the flowers for arrangements, as the somewhat unpleasant aroma will have subsided.
As winter approaches, the foliage of your plants will once again die back to hibernate. The flower stalks will remain in place, and early winter is a great time to harvest sea holly seeds. Otherwise, birds will likely come in to feed on the mature seeds.
Germinating Eryngium Seeds
Since sea holly is a hardy perennial, the seeds benefit from cold stratification. This means the first step should be to expose them to cold/damp conditions for at least 4 weeks before planting.
- Freezer stratification. Place seeds in a baggie with a damp paper towel. Seal it, and place it in the freezer for 4-6 weeks. Then, move the bag to the refrigerator for another 1-2 weeks. After this, your seeds are ready to plant.
- Winter sowing. Eryngium makes a great candidate for winter-sowing. Basically, you plant seeds in mid-winter, outdoors. The natural freeze-thaw cycles stratify the seeds, and they sprout naturally as the weather warms up. Learn more about winter sowing here.
After your seeds have been cold stratified, they can be planted in containers for germination. They do not require heating to germinate, normal room temperature is adequate.
Note: It is best to use a deep container to plant sea holly seeds because they will form a taproot. Using deeper nursery pots makes the transplanting process less traumatic.
Plant sea holly seeds about 1/8-1/4″ deep in sandy, well-draining soil. Alternatively, place the seeds on the surface of your soil, and cover them with 1/8″ of pure sand.
Keep the seeds constantly damp until they germinate, usually within 1-2 weeks of planting.
Allow the plants to grow and establish in their small pots before transplanting outdoors. I only recommend transplanting once, as the plants need to establish their taproot in a permanent location for the best results.
Transplanting Sea Holly Seedlings
Once your sea holly seedlings have 4-5 true leaves, they should be transplanted. This process is delicate and should be planned out in advance.
- Choose the right location. Sea holly plants prefer soil on the sandy side, with very good drainage. They also thrive in full sun, or at least 6 hours of direct sun each day.
- Avoid disturbing the roots. Sea holly forms a large, central taproot that goes deep into the soil. Touch the root ball as little as possible while you transplant, as it could cause damage to the plant’s main root.
- Plant at soil level. Do not plant your sea holly plants deeper than they were in their original container. Instead plant them at soil level.
- Water them in. After transplanting, water at the base of the plant to saturate the soil around the roots.
The most important consideration to make when transplanting sea holly is the location. Sea holly is extremely resilient, and actually prefers poor soil.
So, find a spot with full sun and good-draining, sandy soil, and transplant it in. Don’t worry about adding soil amendments and fertilizer, as it is better to allow the plants to acclimate to your native soil.
When to transplant? Eryngiums can be planted outdoors almost any time of the year (even in winter if the soil is workable). However, for seedlings, the best time to transplant is spring or summer. This allows time for the plants to establish a strong root system before going dormant in winter.
Try to be as swift and gentle as possible when removing the plant from its pot. The roots may be delicate to disturbance. Never wait too long to transplant your eryngium into the ground. If the plants outgrow their container, the taproot may reach the bottom and begin to bend or deform.
Once transplanted, don’t worry too much about watering. Unless your soil it bone dry, your eryngium should be able to access the water it needs to get established.
How to plant bare-root sea holly
One of the easiest methods for growing sea holly is to buy bare-roots. These are very affordable (we bought a 3-pack for $5), and are easy to plant.
- Start by soaking the tap roots (they look like small , pale carrots) in water for about 30 minutes. This will help rehydrate the plants and prepare them for transplanting.
- Then, dig out a hole deep enough for the entire taproot. You can also use the handle of a shovel or trowel to open up a hole for the plants.
- Place the taproot into the hole (skinny end down), burying it up to the top. The soil-line should be just about at the top (crown) of the root.
- There may not be any foliage on the upper part of the plant yet, and that is okay. The root contains the energy and life needed to grow the plant.
- Water in your plants, and place a marker where they were planted so you don’t confuse them for weeds when they sprout! From here, the growing requirements are the same as seed-grown plants.
Again, don’t expect these plants to flower in the first year, even when planted in early spring. The plants will spend energy developing a large root system in the first growing season.
Soil, Watering, and Fertilizing Eryngium
As your sea holly plants get acclimated to the soil and outdoor climate, you may be tempted to “over-care” for them. Watering and fertilizing may sound like a good idea, but it isn’t.
Don’t over-water or over-fertilize your eryngium plants! Sea holly is the king of “thriving on neglect.” Unless the leaves are drooping or turning pale yellow, there is no need to supplement with water or nutrients.
If your soil is overly wet, the roots of sea holly can rot. Once this happens, it is very difficult for the plant to recover. So, only water when the soil is completely dry 3-4″ below the surface.
If you planted in a well-drained soil, then your plants should be fairly difficult to over-water. However, it’s best to keep it too dry than too wet!
Too much nutrition in the soil will cause tall, spindly plants with fewer blooms. Eryngium seems to find a way to get all the nutrition it needs from very infertile soil – so give it what it wants!
I have found that a light application of compost is enough to feed these plants all season long. You can also mulch the plants with a natural material like woodchips or shredded leaves, which add a small amount of nutrient back to the soil over time.
Year 1 vs Year 2 Growth
As we’ve established, sea holly does not flower in the first year. Set realistic expectations, and consider growing some annual flowers nearby to fill in the space while your sea holly gets rooted.
However, in year two, you can expect the plants to send up tall shoots. These will branch off, forming flower stems. Depending on the exact variety, you may get dozens or hundreds of flowers!
Once winter arrives, your plant’s foliage will die back. You can cut the flower stalks down to the ground and mulch the plant if you live in a very cold climate (zone 5 or less). This isn’t necessary for us in zone 6A.
FAQs about sea holly
Where to buy sea holly seeds? There are not many US seed suppliers that carry sea holly. Select cultivars are available on Redemption Seeds, Johnny’s Seeds, and Swallowtail Garden. In Europe, there are many more options, like Jelitto’s or Thompson-Morgan.
What are the types of sea holly? Many species of Eryngium exist, from the floriferous E. planum, to the vibrant blue E. alpinum, to the impressive and showy E. amethystinum and E. bourgatii. I hope you can find a good selection of sea holly varieties to grow in your region!
Can you divide sea holly? Unfortunately, sea holly cannot be divided like many other perennials. This is because the root system has a single taproot in the center, anchoring the plant in place. It is also not recommended to dig up sea holly plants, as they may not recover from the move.
What do sea holly flowers smell like? One of the drawbacks of growing sea holly is the unpleasant smell. Many compare the aroma of the blooms to that of dog or cat poop. I’ll admit, it doesn’t smell very good, but it does subside after the flowers finish blooming. Plus, the pollinators don’t seem to mind!
I hope you are excited to grow one of the most unique and interesting flowers on earth! Eryngium is a personal favorite of mine, and I hope I can inspire others in the US to give it a try. Maybe increased demand will bring more varieties stateside!