Sweet Potato Whitefly

I'm always amazed that every year some new pest or disease finds my farm. So I'm constantly learning new things about how to grow food here in the tropics. Today while going back to harvest from a remote sweet potato patch, I immediately noticed something was wrong. First of all, the patch didn't look lush and happy. The growing tips were small, curled, and bunched. Below is a sample of what the tips looked like.....,,

Normally the growing tips have smooth, flat leaves fanning out, like the variety showed below......

On closer inspection, when I touched the vines, a plethora of small whitish flies jumped or flew this way and that. There were dozens of them flying from one vine tip. So I flipped some leaves over for a closer look. The bottoms were covered in small, light colored insects of varying size. Below, I've circles in yellow some of them for you to see. 

One flying adult happened to land on an adjacent potato patch and I managed to take a photo of it (circled below).........

Back at the house, I pulled up the Internet for a search. Conclusion : sweet potato whitefly. 

Control of this pest didn't sound optimistic. But then, I'm not a big user of chemical pesticides, so I immediately planned to fall back to my usual solution --- remove and destroy the infested plants. Then don't replant a susceptible crop in that area for a while. 

My plan was to harvest this patch anyway, but I did so very carefully so as to not spread the problem to other areas of the farm. None of the plant material went to the compost bins. Instead it went to the dump in order not to perpetuate the problem on the farm. It was carefully bagged, the bags sealed, and placed into the bed of the pickup truck for disposal. I next dumped several bags of fresh compost over the growing bed and immediately tilled it in. I'll then plant a non-suspectible crop. Don't know yet what I'll plant, but I want to get something growing that the whitefly doesn't like so as to starve out any survivors. Oh by the way, I was also careful not to spread the bugs back to other sections of the farm by immediately changing my clothes and showering. While the wind could easily be the culprit for introducing this pest to the farm, there's no need for me to help spread it around by being sloppy.

I also harvested every sweet potato that I found while removing the infested vines, or when rototillering. These went into a bucket, then taken to the chicken cookstove for making into mom's famous slop & glop. Boiling water was used to rinse out the bucket to kill any possible white fly eggs that tagged along. At least the chickens and piglets will benefit from this crop failure.   

My Sweet Potato Observations

In addition to what I've already answered or posted about sweet potatoes, here's some of my observations noted while experimenting with this crop...........

Things I've observed while growing sweet potatoes on my farm :

...From October 1st to March 1st the plants grow slower than the rest of the year. And the tubers tend to be smaller and less in number. This is no problem for me. If a home gardener notices this difference, it's nice to know that it's the time of year doing it, not their soil fertility or an inferior variety. 

...While I can grow sweet potatoes year around, I see the most vigorous vine growth from mid-spring to mid-summer. The plants look absolutely gorgeous.

...My best tuber production occurs during early autumn. 

...While I'm at 2400 foot elevation, I can still grow sweet potatoes. I was told I was too high. A fellow gardener at around 4000' successfully grows sweets too. Perhaps the elevation recommendation refers to commercial production. 

...Sweet potatoes don't like soggy or wet soils. Mine do better when a little on the dry side compared to other veggies, but I don't let them get overly dry. They will however survive a drought. So I don't worry about watering the vines that I grow for livestock fodder or as a living mulch. I only watch the soil moisture on my tuber producing plantings. On years that I get 50-60 inches of rain where it is spread out during the year, I do not have to water the sweet potatoes at all. In drier years or during long dry stretches, I will water as needed to keep the soil from drying out. 

... I don't use manure my sweets. Instead I use my "vegetarian style" compost. I found that by pumping the nitrogen to the plants, I ended up with some very weird looking tubers, lots of vine, and less tubers than anticipated. My standard vegetarian compost mix includes all sorts of weeds, grass clippings, waste fruits, kitchen garbage, coffee grounds, plus a tad of coral sand, lava sand, burnt crushed bone, biochar, wood ash. 

...I plant cuttings 12" apart in rows about 3' to 4' apart. The rows is just so I can get in there to apply mulch for a few months while the vines are growing. If I'm just intending to grow the plants for livestock fodder, I'll plant even 12" in all directions, making a solid mat of growing vines. I will initially apply mulch when I plant the cuttings, but that's it. The vines grow too densely to be able to reapply mulch. 

...The vines will often send out additional roots from the leaf nodes. These roots can form tubers. Thus one of the reasons some tubers are found far away from the mother plant. In order to encourage tuber production closer to the mother plant, I discourage this type of rooting. I repeatedly apply mulch, moving the vines to do that, in the process uprooting or breaking off any node roots. This helps keep the tuber production closer to the mother plant. It doesn't stop it altogether, but it helps. 

...I don't have much trouble with pests. But they are out there, just not on the farm just yet. When I first started out I ignorantly planted weevil infested tubers. Luckily I noticed the problem the first year. I destroyed everything and didn't grow sweet potatoes again for 4-5 years. Since that time I haven't had a problem. But as a precaution I'm quick to mulch my cuttings, and keep the plants well mulched as a barrier to the weevil. The other destructive pest I have occasionally is the stem borer. I see a little damage from time to time, but not much. So far I haven't had to take action. And one last pest I've seen is the Sphinx moth. It eats the leaves. But they don't eat much, and I don't have scads of them, so I just let them be. Live and let live. For some reason I like those big moths. 

...Diseases. I'm told that there are a number of serious diseases out there but I haven't had a problem so far. My preventative measures include in never bringing questionable sweet potato material onto the farm. I'm hoping to stay disease free. 

...While curing harvested roots is the norm on the mainland, I don't do that here. The temperature and humidity in my house maintains the tubers just fine. And besides, I'm harvesting new tubers on a regular basis. No need for long term storage. When I harvest fresh tubers, I feed the old ones to my livestock. 

Recipe from "S".......

For lunch I have created a very passable sweet potato dumpling, without salt and without other seasoning (a mistake).... dropped into gently simmering water for about 15-18 minutes.
      One third cup flour, dollop of olive oil, dollop of canned cow, one elderly egg, and enough mashed sweet potato to make a thick, sticky dough.  Made four.  Need some sort of spices in the dough and should be cooked in stock. They were perfect dumplings, just a tad on the very, very bland side....

 

Potatoes vs Sweet Potatoes

Recently "D" sent me some questions about potatoes, and among them was a question of "white vs purple vs orange". I mistakenly thought she was talking about sweet potatoes because I'm not aware of any orange "Irish potatoes". In a follow up email, I became aware that "D" might have had questions about Irish potatoes. My mistake. I was totally off base babbling along about sweet potatoes. You see, sweet potatoes and potatoes are two totally different vegetables. I'm sorry "D" if I confused you! But I will go and answer your questions as if you meant Irish potatoes. 

Above, Irish potatoes. These grow bush-like, approximately 18" to 30" in height. 

Above, sweet potato vines. They creep along the ground surface, seldom attaining much height. 

Below, if you pull up a stem, you can easily see that it's a low growing vine. 

Working with many new gardeners, I find that it's not uncommon for people to confuse the two, at least until they're grown them both. The differences then are really obvious. The two plants don't look anything alike. Their growth habit is different. The tubers even look different. Their flavor is different that even the non-sweet varieties of the sweet potato won't be mistaken for an Irish potato. 

Above, potato leaves. 

Below, sweet potato leaves

So after one more rambling post about sweet potatoes, I'll move in the potatoes and hopefully lift the cloud of confusion I've caused around "D". But first, some comparisons.......

Sweet potato 

...vining growth habit

...tubers develop as an enlargement along the root, thus there is a root coming into the tuber and a root leaving the other end of the tuber

...leaves are edible

...tuber skins may be white, tan, orange, pink, red, purple 

...flesh may be white, creamy, light yellow, pale orange, bright orange, reddish orange, light violet ranging to deep dark purple. Streaks and blotches of contrasting colors is not uncommon. 

...tubers generally mature around 5 months 

...flavor is general sweet to some degree

Potato

...bush growth habit

...tubers develop at the end of a rootlike stem or along the underground stem

...leaves are not edible 

...tuber skins may be white, tan, brown, pink, red, dark purple. May be blotched. 

...flesh may be white, pale gold, medium gold, pink, light reddish, bluish, purple. Streaking of pink, red, or blue may occur on white or whitish flesh. 

...tubers generally mature around 3 months

...flavor is generally somewhat nutty, but not sweet

Above, a very young potato. It's attached to the plant via one rootlike stem. 

Below, a young sweet potato. Usually a thick stem-like root coming to the tuber on one end and a thinner one going out the other end. 

Sweet Potatoes : questions 4 thru 6

Question #3- "Where do you plant-ground or grow boxes?"

Both. It depends upon the variety. All varieties can be ground in the ground, but some lend themselves well to container gardening too. What's the difference? Some varieties will develop tubers right under the mother plant. These can be grown in containers. Other varieties don't do that. Plant the wrong variety in a container and you're won't get any tubers.  I have some sweets growing in my pallet grow boxes. A gardener on the top of Oceanview has success growing them in smaller containers (half barrels and old coolers). 

From what I've read, ancient Hawaiians grew at least some varieties that produced their tubers right under the mother plant. They may have had other more far ranging types, but I've only read accounts of them making stone rimmed beds for sweet potato growing, and also making a puka (scooped out hole) in the ground and preparing it for sweet potatoes. Since they had dozens, possibly hundreds, of cultivars, I wouldn't be surprised that they had all the variations that we see today in modern cultivars. 

Question #4- "How far from the main plant are the potatoes located?"

That depends upon the variety. Some produce all their tubers right under the mother plant. On the other extreme, there are types that never produce tubers under the mother plant. Most tubers are within a few feet of the mother plant, but I tried one variety once where I would find tubers up to 15-20 feet away! The neatest growing variety I had once was a bush type with 3-5 foot long vines and which produced lots of tubers only right under the mother plant. It was an ok sweet potato for eating but it wasn't outstanding. But it was easy to grow and very productive. Sadly I lost the variety when my neighbor's cows destroyed the garden. They ate every bit of vine and tuber! I've never been able to find that variety again. 

Question #5- "How do you know when they are ready?"

That depends upon the variety. Some will be ready by 3 1/2 months from the time they were planted (for small tubers), others not until close to a year. For most varieties, check at about 5 months and see what has developed. If tubers aren't big enough, wait another 1-2 months and check again. If I know a variety's habit, then I can simply mark a date on the calendar for harvesting. But of course that date will vary a little due to the weather and time of year. They are slower to form good tubers during the winter and during drought. 

Tubers can be harvested at any size. I don't need to wait for tubers to get big. In fact, I prefer the smaller sized ones myself. I'll sell the larger ones. Some varieties tend to produce fibers in the tubers if they grow too slowly or get too mature. Thus another good reason for harvesting young tubers. 

"D" says that she's growing a purple variety and wanted to know when to harvest them. Not knowing which variety it is, I can only suggest that she do a little gentle digging and take a look-see. If it is an Okinawan type, I wouldn't expect harvestable tubers until 9-12 months. It's a very slow variety. But there are numerous purple types, some of which are ready at 5 months. 

She also asked if there was visual clue for when the tubers are ready. While there might be for some particular varieties, I haven't noticed them yet. Some varieties will flower, most do not. Some grow long vines which slow down in growth, others don't. Some start producing smaller leaves, some don't. I haven't seen anything consistent as an indicator. If there is one, I'd like to know about it myself. 

Sweet Potatoes : questions 1 thru 3

(Note: I didn't have time to gather some photos for today, but if you're interested in pictures, I will upload some to this post tomorrow. Just check back sometime later. Thanks.)

"D" sent me a list of questions about sweet potatoes. Luckily, she also lives in Hawaii, which makes answering a whole lot simpler for me. While I'm no expert on growing this crop, I do indeed grow a lot of sweets. Basically I use the greens for livestock feed, but I also harvest the tubers. Plus over half the sweets I grow serve as a cover crop or living mulch. 

Above, a living mulch to shade the soil around the taro. This one us a bush variety so it won't aggressively spread. 

Above, sweets used as an aggressive ground cover. This one produces 15' to 20' vines, grows quickly, produces lots of side shoots, making it ideal for a quick ground cover. 

Question #1- "Any specific time to plant?" 

I've planted cuttings in every month with success. They always root. Their growth may be slower during early winter, or when things are dry and windy, but they don't die. They just wait for better conditions. My latest cuttings were planted in January and they're actually looking pretty good. Not at all stunted. If I had to name a month or two that are the least desirable for planting, I'd say November and December. Growth is slow those two months even if all other factors are even. But I don't hold off planting cuttings if I have them in Nov/Dec. Nope, they go into the ground anyway. Now for the best months to plant? For me I'd say Mar-May. 

Question #2- " Where you get starts?"

99% of the time I use tip cuttings from other plants. When I harvest or thin my sweets, I'll choose the healthiest tips to make cuttings, about 12"-14" long. I'm looking to plant at 3 or so leaf nodes into the soil while having 2 leaf nodes (plus the small tip) out of the soil. Usually I make the cutting and place it into fresh rainwater for 3-5 days until I see roots starting. Then I plant the cutting. You don't really have to wait for roots to develop. The cutting can immediately go right into the ground as long as you give it a good drink of water. 

Occasionally I'll make slips. Making slips means planting the tuber and havesting the shoots that come up. Those shoots are what is meant by slips. I'll snap them off the tuber when they are at least 6" long and plant then. I'll do this when I'm given a new tuber variety to try. On the mainland where it is cold, gardeners don't have vines to take cuttings from. Thus they plant the tubers in a warm area and create slips for transplanting out in their gardeners. 

I remember as a kid suspending a sweet potato in a glass of water, using toothpicks as props. Then over the next couple of weeks watching the sprouts grow. I guess we kids lost interest after a couple of weeks, because I never recall planting any of those slips. 

Question #3- "white versus purple versus orange"

All.  I like to experiment, so I grow all sorts. Sweet potatoes are highly variable. The skin can be white, creamy, gold, orange, red, pink, purple, even blotchy. The flesh can be white, cream, orange, purple...ranging from pale to intense, and adding streaks and blotches. Not only the color is variable, but all sorts of other traits too. I have tubers that range from smooth to lumpy, round to snakelike, big to small. Some tubers are oh so pretty, others are downright ugly. Some varieties are good for baking, others are better for mashing, and other yet better for frying. Some are dry fleshed when cooked, other very moist. And anyone who has grown several varieties of sweets will tell you that some varieties are incredibly sweet, others mid-way, and yet others not so sweet at all. Some varieties produce lots of tubers, while others produce very few. Some have blocky roundish leaves, others handshaped leaves, others thin lacy leaves. Some plants are thick and robust, while others have thin somewhat wispy vines. Plus there are bushy varieties, and aggressive running types with incredibly long vines, and dozens of intermediate length vines. Some sweet potatoes mature early, others very late, plus lots of varieties maturing somewhere in between the two extremes. Boy, there sure is variety in sweet potatoes! 

So back to the question, white vs purple vs orange. It comes down to personal preference. With me, it depends upon what I'm doing with the tater. Hubby prefers the non-sweet ones. I like them all. 

Heat Units & Sweet Potatoes

One of my friends ("S") grew sweet potatoes where it is much cooler than my location. As a result, her sweets took far longer to produce tubers. Plus when she harvested them, they were smaller.

(Harvested at 9 months)

Initially I didn't know why, but I've been researching it. I what I found was that sweet potatoes have a heat unit requirement. They need a certain number of warm days of suitable warmth to grow tubers. 

Sweet potatoes are adapted to the warmth. And they prefer sandy, well drained soil. Plus give them a summer of hot days and warm nights and they'll produce really well. Thus I find my sweets produce better at my seed farm than at my homestead location, primarily because it's much warmer there. I've also noticed another trend. Although I grow them year around here, they produce more and bigger tubers when they are ready to harvest in the fall. Must have something to do with the shortening days. 

From various university websites, I've learned that most sweet potatoes require 1200 heat units. Some varieties require more. A definition of a heat unit for sweets is.... "the daily 24 hour average temperature over 55°". So if my high for yesterday was 80° and the low 60°, then that day's heat units were 15 (80+60=140; 140/2=70; 70-55=15). But I often get days where the high is only 70°, low of 55°. Heat unit is 7. Thus the hotter my days are, the more heat units. (I don't get days over 90°. When it gets into the high 90s it actually slows tuber development according to the websites.) Based upon heat units, I can now see why my sweet potatoes grow way slower during the cooler half of the year. And this is most likely the reason my friend's sweet potatoes takes much longer to develop. 

I was aware that peas and corn have heat unit requirements. Now I've learned that sweets do too. Something for me to keep in mind. 

Sweet Potatoes - Where Are They?

"E" wrote to say he was really surprised to discover that his sweet potato tubers weren't right under the plant like it is with Irish potatoes. For most sweet potato varieties, he's right. First of all, sweet potatoes and Irish potatoes are two totally different crops. They aren't related. So I wouldn't compare them. 

There are indeed some sweet potato varieties that will grow most of their tubers right under the mother plant. But I'm finding that most varieties do not. In fact, I have one variety that produces tubers several feet away from the mother plant, making it challenging to harvest them. Truthfully, I don't even try. I just use the greenery as livestock fodder. Occasionally I'll happen upon a tuber or two. 

But I've found some varieties can be quite variable. In the photos below, I pulled out some young plants in order to open a space to plant a moringa seedling. These 3 plants nicely demonstrate the variation I find I this variety. (photos taken on the tailgate of my pickup truck.)

Plant #1.... No start of tuber development yet. With this variety it is not uncommon to find a percentage of plants with zero tubers. This particular plant is just a baby, so it would have had plenty of time still to produce tubers, if it were so inclined. 

Plant #2..... Very young tuber starting close to the base of the plant. 

Plant #3..... Tuber starting away from the base of the plant. 

As I said, this is normal for this variety. I find tubers all over the place. So I can't plant this one in rows in the normal garden. Instead I grow it in wide beds, acknowledging that some will end up well outside the bed area. 

Sweet Potato Container Failure

This was sent to me by "S", who tried growing sweet potatoes in a half barrel container........

"Eight months in a major barrel and this is all I got..... all very deep and right under the plant, by the way.  But lots of unproductive roots thru out.  These cook up bright translucent yellow and are quite sweet.  Waste of time for a container situation, far as I could see.  Could have grown almost three generations of Peruvians for ten times the yield in that same amount of time, and space.  Plants make effective and attractive ground cover, however. "

I don't have any experience using containers for sweets, so I don't know if this would be an expected result.....undersized tubers, long growth period. I'm sure there are climatic factors going on here, but also variety factors too. 

Growers often say how the non-commercial varieties of sweets are fickle. Some years they produce, some years nary a tuber. Some parts of a field will be great, other spots in the same field...nada. That's one of the reasons commercial varieties were developed. There was a need for consistent production. A field that didn't produce after all the time, effort, and expense put into it was a major hurt to a farmer trying to make a living. 

So why grow non-commercial sweets? They often taste way, way better. And there's lots of variations that fit the various nooks people deal with. Some withstand heat and dry, some do good with moist and humid conditions. Some can take a chill, others no. Some taste too sweet for some folks, and others are barely sweet at all. Some produce in rocky or sandy soil, others do fine in compost. Some produce tubers under the mother plant, others spread tubers all around. Some make only a couple giant tubers, others abundant small ones. Some are only good bakers while others are only good fryers. So there's sweets to fit just about every gardeners' situation and desire. But they can be challenging at times. 

I've not even come close to figuring out sweet potatoes. It's always a miracle that I find tubers when I dig them up. 

How I Start Sweet Potatoes

Living in an area without freezing winters, it's easy to start new sweet potato plants year around. We don't have to grow slips from tubers. It's faster and easier just to make cuttings from my established plants. 

Today I finished two new grow boxes and want to use them for growing sweet potatoes. In truth, I want to propagate the sweet potatoes, making new plants and rooted cuttings for resale. Whatever tubers they produce will be a bonus. I have some varieties that produce their tubers right under the plant, so they are excellent candidates for the grow boxes. 

Step 1- I took cuttings from the plant. I cut a 12 inch piece off the tip of a robust vine. Then I removed all the leaves except for the 2-3 on the very end. This is what they ended up looking like......

I then scooped out a hole in the soil about 3-4" deep and placed the stem into it, like this.......

The stem lays sideways in the hole rather than straight up and down. 

I pour a cup or two of water into the hole to well moisten the soil, then fill the hole in with dirt. Only the very end tip of the cutting is above the soil, like this........

Once the entire box is planted, I'll give it a good watering, then a light covering with grass clippings so that the soil surface is shaded from the sun. 

I've planted the cuttings quite closely in the box, perhaps 12" apart. My goal isn't tuber production so the close planting won't matter. I'm looking for plants to take future cuttings from. In 6 months I will be opening these boxes, harvesting whatever tubers developed, using the compost and soil elsewhere, then reloading the boxes with fresh biomass. The cycle repeats itself. 

If you've read previous posts about my pallet grow boxes, you'll recall that only the top 6 inches is garden soil. The majority of the box is filled with assorted chopped up biomass. Not a true compost pile in the sense that it will heat up. Heat is something I don't want because it would kill the young sweet potato cuttings. So I don't include much in the way of manures, but I do inoculate the layers with some garden soil that has active micro organisms so that decomposition can take place over the coming months.