Growing Cup and Saucer Vines

The annual vine known as the Cup and Saucer vine is beloved by many gardeners. Their large, cup-shaped purple or white blossoms and their distinctive flaring corollas look very much like teacups. Many of us never really think about that, like. Like so many familiar garden flowers, the Cup and Saucer Vine or Cobaea scandens belong to a much bigger clan – it has many close relatives within the genus CObaea, all native to South and Central America. However, most if not all are still relatively unknown or even discovered.
I decided to do a deep dive on the genus cobaea (or at least as deep as I could grow a few of the more hard-to-find species). My real goal here is to solve many people’s problems when trying to grow Cup and Saucer Vines. If you’ve even had trouble with germinating or getting plants to flower, then maybe you’ll find some bits of information here useful.

You know me – I like to ask the obvious question first. For example: Why does nearly every garden writer repeat the directive: ‘Sow cup and saucer vine seeds vertically. Go ahead, google “how to sow cup and saucer vines” and see. I suppose there might be a good reason, but honestly (and botanically), I can’t imagine why, as no botanist nor peer-reviewed journal of Botany seems to be able to back it up. Seeds don’t land vertically on the soil in nature.

Oh, and while you’re snooping around for advice, you may also notice that there is a wide range of suggested depths that one should plant the seed ranging from surface sown (because some writer once wrote that the seed needs light to germinate) or 1/2 inch deep in total darkness. Which method is correct?

Now, before I address all of that, a few facts to get out of the way.

Cobaea scandens (the common Cut and Saucer vine) isn’t actually an annual at all; it’s considered a tender perennial (but tropical). Just a fun fact, really, as this shouldn’t change how most of us grow it.

This is one of those tropicals that just must be grown as an annual in cold climates, but it should serve as a warning to those in warm-winter climates where the plant is a well-known invasive. As with any vine, plant with caution – stand back, they run.

Cobaea scandens isn’t even new, though some might think that it is. You might call it an heirloom because it was once more common than now, though a new gardening generation is rediscovering it. Like the ones I am showing here, other species are rare, if not impossible to find – yet. I am just sharing some to show folks that there is diversity in the tribe, and maybe some plant breeder will do some work with the genus.

Cup and Saucer Vines are quick-growing when started from seed, and while some may read that they require the short days of late summer or Autumn to bloom, this isn’t true. Cobaea is considered to be ‘day-neutral plants – and unlike their neighbors that also grow in central America (morning glories, zinnia these are plants that don’t respond to a shorter photoperiod. Cobaea must be mature enough to bloom. As such, must produce a certain number of leaves during a period of summer with high ultraviolet light (as in Central America). Thus it is believed that light intensity and maturity stimulated plants to bloom.

Some gardeners believe that they can trick the plants into blooming earlier in the summer by starting earlier (and perhaps they could if the vines are allowed to grow long enough under strong, full-spectrum lights) but who like the room? For a 20-foot vine indoors?). Generally, cobaea will bloom naturally near late summer, anywhere between mid-August through September until frost. In fact, seed sown in February and seed sown in late May bloom around the same week in September for me in Massachusetts.

On Pinching
Another myth exists around pinching plants trying to make the plants branch or remain a manageable size. Pinching isn’t necessary at any stage as it’s a seriously vigorous vine even if left alone. It will branch multiple times left to its own devices. Some gardeners feel that they can train cobaea to a short 6-foot teepee by doing this, but plants will most likely suffer from such restricted treatment, resulting in yellowing foliage and few blooms. It’s simply not a trainable plant. Cobaea needs height and room, and plenty of both if it is to grow well.

I think the pinching advice began when people started sowing seeds too early indoors, soon discovering that plant quickly became unruly. Pinching is fine, but again, only if necessary. I’d say that if you need to pinch your plants before setting outdoors than you probably sowed your seed far too early.

I would advise pinching plants once you set them into the soil outside where they grow, though. This will help stimulate lower branches which might be handy if you are trying to cover a fence or a low, long structure. Know that cobaea’s natural habit to want to grow up and over something big (like a tree or over a brushy cliff). It wants to produce 12-15 foot long stems before it forms flower buds. I rarely pinch our plants as they grow on 1″ diameter black mesh that is stapled to our porches which sit on a 4-foot foundation. Our vines must first grow up 7-8 feet before they are expected to branch, which they have no problem doing in rapid order all summer.

Can I Grow Cobaea in Pots?
Safe to say that Cup and Saucer Vine is not a good candidate for most containers. I Can grow well in a large tub set at the corner of a sturdy arbor, but it’s far too vigorous for a small pot or even for a window box. Believe me, we’ve tried, and not only did our lower story windows become covered by mid-summer, so too did our second-story windows.

I have heard of some gardeners attempting to grow Cup and Saucer fines on a teepee – on bamboo canes, but I can’t imagine them having much success. I may have to try it (I grow plenty of annual vines on teepees, and while most become too crazy by late summer, I can get some blooms). Still, with cobaea, I would imagine that there would be few flowers and that the volume of foliage and leaves would be too much for the entire structure.

Rare Species
I obtained some seeds of newly discovered and rare species of Cobaea a couple of years ago. My favorite species to grow so far was Cobaea campanulata which had beautiful lime green flowers smaller than C. scandens but had longer stems. The best feature with this species, though, was the seed pods that looked like torpedos and were very ornamental as they hung straight down with their weight, like Christmas ornaments. The foliage, though, was far too rampant – it truly nearly took over our house on two and a half floors in just one summer. It shut in windows and even encased a screen door on the deck, locking us in.

I also grew a newly discovered species C. pringlei, but it only produced a handful of blooms that looked like white, waxy versions of C. scandens.
I had a seed of C. paneroi, another new species, but it only produces a couple of flowers, yet plenty of foliage that seemed to take over our house (and it’s a big house!).

The genus cobaea is much larger than you might imagine, with 18 species at least, but the most common if not the only species you will find is C. scandens – the traditional Cup and Saucer Vine. It does come in an all-white variety (alba) and the more traditional purple one. Know that all C. scandens begin green, then white, and then turn purple as well, as the flower ages.

How to grow Cup and Saucer Vines
Start with knowing where you are going to plant your cup and saucer vines. As the most vigorous of any annual vine sold as seed in packets, never underestimate their size. Morning glories are a close second, but if given a race, a cobaea will always win and take over even the most rambunctious morning glory. Believe me; we’ve raced them on our gourd tunnel.

Remember that they are a tendril-producing vine (like grapes) and not a vine that twines (wraps around) a column, cobaea grow more like sweet peas, producing thin, twisty tendrils that only wrap around fragile materials such as twigs or netting. As such, you may need to tie stems to thicker pieces of wood if, for example, you are trying to train them up a post to a trellis. Once they reach the top of their structure (like a trellis),, they will grow madly entwining in and out of each other.

New gardeners often discover this fact when they try to grow cobaea in a pot thinking – let’s say with three bamboo canes in it. Unless your canes are 8 feet tall or more, you are more likely to end up with a hot mess. They really need a wire or plastic mesh wrapped over a very tall structure or against a fence or wall. We staple black plastic 1″ diameter mesh onto our 16′ high porches, which looks terrible in early spring, but they become completely covered with cup and saucer vines by August and then flowers in September.

Can I grow Cobaea indoors?
In a greenhouse, yes. In your house? No. You might be able to winter over a plant you have dug up in a cold cellar window, but cobaea isn’t a plant for the indoors.

Wintering Over Vines
In the 19th, vines grown on some of the bigger East coast estates were often cut back near a hard freeze in late October, and their crowns dug up, potted, and brought into a cool greenhouse for the winter. Savvy gardeners also sowed seed in early summer directly into the soil of a conservatory where vines would bloom effortlessly until the New Year (such as at the Isabella Stewart Gardener Museum in Boston).

Raising Cobaea from Seed
Cobaea is generally seed-raised, but there are many bits of misinformation out there about seed, starting when it comes to Cup and Saucer Vines. Much is it is myth or lore, and the truth is told, Cobaea seed will germinate just fine if it is fresh (collected and stored properly from the previous year) and if the seed is sown just under the surface of the soil, kept warm near 70 and lightly moist. No-fuss really. That means no soaking, no wrapping seed in a polybag of medium and refrigerating it, no chipping the seed, and no special way to set it into the soil. Those are ‘tips’ that are unnecessary steps.

What about sowing seed vertically ,again? Why do I keep seeing this?

I don’t know where it started, but I guess this is just one of those tales passed along from gardening books to gardening writers over time. It appears nearly everywhere, though, so it may have started in the late 19th century as I found a source in an old 1878 seed catalog suggesting it. If anything, early texts suggest that the seed can rot, but then greenhouses were kept much cooler than germination chambers are today (usually under warm lights indoors).

The bigger question I asked other botanist friends was, does seed orientation matter? It does for very few plants (like coconuts, for example, or lychee fruit, but any benefit from setting flat, papery cobaea seeds on their sides only gave me furrowed eyebrows and a clear “It doesn’t matter.”.

Some home gardeners like to pre-soak or pre-germinate many large seeds like peas, beans, and morning glories. While it will help a seed coat absorb water and start the germination process a day or two sooner, it’s always considered risky as you can and will damage the tine root hairs on the radicle, and aside from the task being fun or seeming like a science project, few if plant scientist would recommend it.

What about soil depth for Germination? OR light?
I’m often asked this: “Don’t Cobaea seeds need light to germinate?” No, they don’t. In fact, some commercial growers recommend sowing seed three times the depth of the seed, while others suggest 1/16″ deep. Others advise that the seed needs to be half-buried in the soil. Bottom like is to sow seeds 1/8-1/4 inch deep, and you’ll be fine.

What about those who say to pre-germinate seed in paper towels?

Pre-germination is never considered good horticultural practice. The damp paper towel method might allow seeds to germinate a day or two quicker because of the consistent moisture, but these same seeds would have germinated in the soil as well. Great for teaching young children about seed germination, though. But practice it with beans. If you have poor Germination, it’s most likely because of temperature or, more likely, seed quality. Your seed may be old and too dry.

Do Cobaea seeds need temperatures warm to germinate well?
Yes. At least when germinating and while growing in early to mid-summer.
Plant seedlings outdoors once the soil is hot (60° F) (when you plant tomatoes) or even later—no need to hurry.

Do I have to soak or file the seeds first before sowing? Or soak seed?
No. Never chip seed either. This is risky and not necessary with Cobaea.

Why aren’t my seeds germinating then?
It could be soil temperature (try to keep them warm to near 70° F), or you most likely have poor seed. Seed quality is a common issue with Cobaea as so much of our ornamental seeds come from China or India. The best way to know that you have fresh seed is to buy seed from a big seed seller (Burpee, Harris, Parks, etc.). Cobaea seed that is greater than 2 years old will have poor Germination. If your seed is brittle or dry, it may be old. Even if I save seed and keep it in a jar in my closet, it becomes too dry to stay viable.

Seed not germinating? Don’t feel bad.
That said, Cobaea is just notoriously fussy to germinate as it is, so don’t feel bad if you are getting low germination. The best way to ensure a good number of plants is to order seed from reputable seed sources (real seed catalogs), not eBay or Etsy or private seed sellers – and try a few sources. As an invasive plant in many parts of the world, opportunistic seed sellers might be selling old seeds they found in a park. You never know.
To quote my germination guidelines book: ” germination patterns are varied and complex with cobaea. It is of much value to know the exact pattern.”
As an invasive vine, I wouldn’t say Cobaea are not difficult, but seed viability can be irregular, especially if not fresh.

What about pinching seedlings?
Pinch if you need to, but unlike sweet peas, pinching will only cause early branching (ok if you want to cover a smaller object), but it won’t cause flowers any earlier. If anything, vines may grow more vigorously if you are pinching in a pot, and you risk causing too much growth about the size of your container.

Are Cup and Saucer Vines pollinated by Bats?
Well, Yes, in South America they are but not in North America. Don’t worry; you aren’t going to attract any fruit bats. Flies are known to pollinate them here (they’re stinky), or you can do it with a paintbrush or feather.

What about flower color? Why are my purple Cup and Saucer Vines blooming with white or green flowers?
Flowers on Cobaea start greenish-white when immature but mature to a deep purple flush, depending on sunlight. There is an all-white flowered selection sold simply as Cobaea scandens ‘alba’. It’s a good choice if you want all-white blooms.

Is there anyway to get my Cup and Saucer vines to bloom earlier?

I’m not sure what the answer is, but shoot down a few more myths about this – once again, Cup and Saucer vines are not short-day plants, yet they still like to bloom naturally in autumn. It’s complex, but the vines need to produce enough foliage before they bloom, and this foliage requires high light quality with UV light, as found in India or Mexico. Remember, these are tropical perennials that we are trying to grow in a northern climate, most likely. They need to reach a certain age before they bloom, and in the north, seed-raised plants get going in late summer – it’s just how it is.

Is Cobaea winter hardy?
Not below zone 9 -10, but…they are rather cold tolerant in autumn. After a hard freeze, I often have our porch covered with vines growing and blooming in October, even way past Halloween. Sure, some freezing happens but only on the leaf tips. We’ve had vines survive and bloom with snow on them. Still, Cobaea is not winter hardy. I’m guessing below 26° F might do them in completely, at least when the roots freeze.

Can I save seeds from my Cup and Saucer Vines?
Sure, but the challenge is getting pods that have enough time to fully mature. I can rarely get dry, papery seed from my pods as my cobaea here in Massachuetts rarelyo bloom until September 1.

A mature pod will be dry and papery, as the seeds are wind-dispersed. It’s probably best to buy fresh seed every year, as the few times I have been able to save dry seed from other species of Cobaea, the seed dries out too much in the winter – I suspect that it needs to be cryovac’d or saved in a climate-controlled device with the proper level of moisture.

When should I start Cobaea indoors?
Late winter or Spring is fine. I sow my seed on May 1-15. You can try earlier, but it isn’t essential as the plants’ won’t start to speed up growth until late June once the sun is brighter. As I’ve said before, I’ve sown Cobaea in April, May, and late June, and they all bloom at the same time no matter what. No need to hurry.

If you live in a tropical climate or near the equator, cobaea can be planted at most anytime.
Old New England conservatories often listed Cobaea as a winter plant, but it seems mostly as a foliage vine (particularly a variegated selection) or as a roof-shading selection. I know that in India, seeds of Cobaea are planted in July and August for winter blooms and in September for flowers in April, but I am not sure at what latitude they are speaking about. Obviously, they can grow in moderate greenhouses in the winter, but my greenhouse is too cold at 40° F.

In the end, cobaea or Cup and Saucer vines make for a lovely green vine even if they don’t bloom. Yet it’s their flowers that are so fantastical and worth the extra work upfront to get a good seed, and healthy plants started on a hefty trellis or netting.

HOW TO GROW CUP AND SAUCER VINES

Start with fresh seed

Sow in late April through May

Sow just slightly under the surface of the soil

Keep soil at 70° F, indoors under lights

Plant outdoors, at least half day of sun, near a strong trellis or netting after frost has past

Keep watered and fertilize weekly with a balances feed

Expect flowers by late summer

I encourage you to try growing Cup and Saucer vines if you haven’t tried yet. They are easy to grow once you get the seed germinated, and aside from some patience and finding the right structure for them to grow on, they provide an extra boost of fresh color in late autumn and some pretty cut flowers to pick.