Are you craving something a bit sweeter than your standard white potato? Enter the humble sweet potato.
While the sweet potato (Ipomoea batatas) isn’t directly related to the white potato (Solanum tuberosum), it’s become a trendy alternative for dishes like French fries and classic mashed potatoes. But viewing this yummy vegetable as nothing more than an understudy would be a shame.
Sweet potatoes are straightforward to grow, especially in warm areas. In this article, I’ll walk you through the sweet potato growth stages and provide some tips and tricks for a successful harvest.
Conditions for Growing Sweet Potato
Sweet potatoes belong to the Morning Glory family and come from tropical areas throughout the Americas. They can be grown in most climates, though they are only winter hardy in USDA Zone 8 and warmer. Fortunately, you only need a single growing season to produce a sweet potato harvest.
All sweet potatoes are planted in the spring after all risk of frost is gone. The soil temperature must be at least 60°F before the sweet potatoes grow. For this reason, many gardeners plant sweet potatoes in raised beds where the soil can warm up more quickly.
Your sweet potatoes will need full sun to thrive. Once established, about an inch of water per week is required to support healthy growth and root development. Be careful not to let the soil get too damp, especially during the final weeks of the growing season.
Two basic kinds of sweet potato plants are available: vining varieties and bush varieties. Vining sweet potatoes will climb a trellis or similar structure if trained but naturally prefer to crawl along the ground. Bush varieties are ideal for space-conscious gardeners.
Though sweet potatoes enjoy fertile soil, over-fertilizing can trigger excessive top growth. I recommend amending the soil with a material like aged compost at the start of the growing season and letting your sweet potato crop fend for itself.
Sweet Potato Growth Rate
Sweet potatoes are fast-growing. Some vining varieties can grow several inches per week during the peak season. The most extensive types can quickly grow up to 10 feet tall or extend in a year.
While top growth is easy to track, remember that it doesn’t always coincide with what’s happening below the soil’s surface. Here are some techniques to encourage vigorous root growth:
- Loosen the soil before planting.
- Plant sweet potatoes at least 6 inches deep
- Water consistently, especially during dry periods
- Avoid over-fertilizing (excess nitrogen will trigger unwanted leafy growth).
Also, note that bigger isn’t always better for sweet potatoes. The roots will grow indefinitely unless harvested. Past a specific size, however, the harvest quality starts to drop, and the seeds become prone to cracking and other deformities.
Growth Stages of Sweet Potato
It takes an average of 100 to 150 days for sweet potatoes to mature and prepare for harvest. However, warm weather is far more critical for maturation than the sheer passage of time.
The most accurate way to determine the growth rate of a sweet potato crop is to calculate the Growing Degree Days. Growing Degree Days (GDD) is a unit of measurement correlated with the accumulation of daily temperatures throughout the planting season. Much math goes into calculating GDD, but PennState Extension offers an excellent introduction.
Early sweet potatoes typically need at least 1,200 GDD during the growing season to reach a good size.
1. Slip Sprouting
Sweet potatoes aren’t traditionally started from seeds but from young sprouts called slips. You can purchase slips or produce your own using an existing sweet potato (even one bought from a grocery store).
There are a few ways to start slips from a mature sweet potato. Most notably, you can either submerge the tapered end in a water container — use skewers to support the rest of the sweet potato — or bury it halfway on its side in soil.
New roots will eventually grow from the bottom of the sweet potato. If you opt for the water method, you can watch this growth as it happens. Green shoots will then emerge from the top of the sweet potato not long after.
This process can take 40 days or longer, regardless of the method used. However, the soil produces faster results than the water method.
(Sweet potatoes do grow from seed. Sources are commonly used to develop new cultivars and hybrids. For the home gardener or even large-scale farmer, however, slips are far more accessible and economical!)
2. Root Formation
Once the sprouts (a.k.a. slips) are at least 5 inches tall, gently remove them from the mother sweet potato and place the bottom ends in a fresh cup of water. The slips will then develop their root systems.
Remember that the slips are growing, so they’ll need access to sunlight or an indoor grow lamp. While growth rates vary, healthy slips should begin building new roots within a few days. Monitor the roots until they’re about 4 inches long when the slips are ready to transplant into your garden.
3. Vegetative Growth
Yes, what we care about is out of sight, located several inches beneath the soil’s surface. However, a good sweet potato harvest still relies on healthy vegetative growth.
As I mentioned above, sweet potatoes are broadly classified as bush or vining varieties. Both types have a sprawling, vine-like habit, but the former tends to be far more compact.
Bush varieties are excellent options for home gardeners looking to make the most of limited space. These sweet potato vines typically grow no more than 3 feet long.
Vining varieties are more traditional but can eat up a lot of space. A trellis or similar support can help keep the vegetative growth tidy. Vining sweet potatoes can grow up to 20 feet long in warm climates.
Most culinary sweet potatoes have large, bright green foliage. The leaves are usually heart-shaped or similar to those of a maple tree. Ornamental sweet potatoes (a vine plant with purple flowers) boast far more aesthetic variety than their edible counterparts.
New vegetative growth will continue to form until either harvest time or the first fall frost — about 100 to 150 days after planting. While sweet potato vines can grow several inches per week, watch out for excessive growth that could indicate too much nitrogen in the soil.
4. Tuber Development
Sweet potato ‘tubers’ are, botanically speaking, not really tubers. They are simply specialized store roots that amass energy for the rest of the plant.
Each slip contains several nodes that can produce adventitious roots. According to Mississippi State University, these adventitious roots can start growing within just one day of planting and have the potential to turn into harvestable sweet potatoes if the growing conditions are right.
Although roots can begin forming almost immediately, it usually takes at least 30 days for any visible growth to occur. By this point, storage roots will be noticeably enlarged. They’ll continue growing throughout the summer months.
Adventitious roots can also turn into fibrous or pencil roots. These roots will not turn into ‘tuber-like storage roots but support the sweet potato plant as a whole.
Perhaps the most important thing to take away from this is that the quality and number of storage roots are determined very early after planting. So take good care of your sweet potato slips and young transplants to ensure a good harvest at the end of the year!
5. Flowering
Sweet potatoes will sometimes flower in late summer. These blooms closely resemble those of morning glories belonging to the same family. Watch for tubular, light purple flowers along the lengths of the vines.
Remember that — in stark contrast to vegetables like tomatoes and squash — sweet potatoes don’t need to flower to produce the edible part of the plant. It’s more or less normal for sweet potato plants to not flower (or at least not flower much) during the growing season. This won’t affect the harvest!
6. Pollination & Seed Production
Sweet potato flowers are pollinated in the morning by various flying insects like butterflies and bees. Each flower usually only lasts for about a day, and will simply fade away if not pollinated. (Again, successful pollination isn’t required for tuber production. It only matters if you’re trying to collect new seeds.)
Pollinated sweet potato flowers produce small, insignificant seed pods that typically measure less than a millimeter in length. Curious gardeners can save these seeds to plant the following year. You never know what new variety you could accidentally create!
Note that many varieties of sweet potato on the market — edible and ornamental — have sterile flowers. Though the flowers will bloom, pollination isn’t possible. Naturally, these sterile varieties also won’t produce seeds.
When to Harvest Sweet Potatoes
Often the first sign that sweet potatoes are ready to harvest is that the ends of the vines begin to turn yellow and die back. This usually happens in late summer or early fall, or about 100 to 150 days after planting.
Don’t be afraid to gently dig around your plants to investigate the roots. This is the most reliable way to tell if your sweet potatoes are a good size for harvesting! Different plant varieties mature to different sizes, so keep this in mind as you work.
As you wait for your sweet potatoes to develop, be cognizant of the nightly temperatures (especially if you live in a cooler climate). You want to harvest all of your sweet potatoes before your garden experiences a hard frost.
Sweet potatoes must be cured after harvesting for maximum sweetness. The Alabama Cooperative Extension System has a detailed guide for curing sweet potatoes before storage.
If you have enjoyed reading this article, then click here for helpful information and advice on Butternut Squash Growing Stages.
For more growing tips, here’s a link to Wheat Growth Stages that may be of interest.
FAQ Sweet Potato Plant Stages
How many sweet potatoes will one plant produce?
Most home gardeners can reliably harvest 3 to 5 sweet potatoes from each plant in their vegetable bed. Sweet potatoes grown in warmer climates are generally more productive and may produce six or more tubers per season.
How do you know when sweet potatoes are ready to dig up?
One of the clearest signs that sweet potatoes are ready to harvest is when the ends of the vines start to turn yellow. In cooler climates with short growing seasons, you may need to dig up sweet potatoes before the leaves turn yellow to beat the year’s first frost.
Citations
- PennState Extension Growing Degree Days (GDD)
- Mississippi State University Sweet potato root types
- Alabama Cooperative Extension System How to cure sweet potatoes
Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.
