SWEET POTATO GROWING INFORMATION
There is no crop that has more misconceptions of how to grow it in this country. I can relate to many of these misconceptions, as I too had them at one point in time. I can remember being very frustrated with the place that I ordered slips from when I lived in Idaho. The gentleman refused to send them until mid June, even though the place where I lived at that time rarely had frost in May. I had read about sweet potatoes since I had started to read and had read all about how they are a southern crop and they need such a long season. I never even attempted to grow them when I lived in the high mountain valley of southern Idaho. It just seemed, from what I read, it would be a waste of time. I waited until I got to the Banana Belt of the Lewis Clark Valley of northern Idaho during my college years before I even tried. The gentleman at the place where I first ordered those plants really knew his stuff and I now, some 30 years later, realize all of the stress he went though trying to get the right information out to his customers about this crop. He kept telling me that you want it hot and stable before you plant them. Even though the frost free time was past, I still did not have the heat they need in May where I lived. He was right, of course. They will do so much better if planted in stable, warm soil - not just the first day after the last expected frost of the Spring. I do hope those of you first timers (and a few who may still be learning) do take the time to read the following information. I have faithfully grown sweet potatoes ever since I moved to Iowa in 1984 and have taken a great deal of time to try to clear up some of the misinformation that is out there about this crop. I can thank Lowell Martin for getting me started once I was here in Iowa. Lowell was in his 80’s when I first met him in 1985 and he had a piece of “worthless” river bottom sand much like our “worthless” sand hill. Lowell made the desert bloom, so to speak, with his wonderful melons and sweet potatoes. He taught me a lot that first year and one could not argue with his success. The first thing he straightened me out on was that yes, even here in Iowa, you can plant them late. It was after the Fourth of July and I was helping him set out his second crop of slips. He told me these will make the best for sprouting and the most uniform. He said they won’t get as big as the earlier ones, but they will keep better. And, because they grow so fast, they will have less surface fungi and just all around be healthier. Lowell was right. I have experimented year after year with anything from making monthly plantings from early May to early July. Consistently the best, most uniform roots will almost always come from the early July plantings. Several years the early May plantings have been a bust with lots of foliage and few roots while the July plantings have sustained us with the best crop. It really almost always seems to boil down to the heat they receive. The following are some tips that may help you get a crop where your local county extension service thinks you are crazy for trying.
SWEET POTATO CULTURE
This information is sent on a red sheet with your order. We include it here for further reference.
Several years back we switched from starting our sweet potato slips in the greenhouse in dishpan tubs to field starting all of them. There were several reasons for the switch: first, we had outgrown this method, both in the number of varieties and numbers of slips we needed to meet the demand. It was also apparent that the quality of the slips was not as good and they did not survive or thrive as well as the field started slips. We also encounter many insect problems with the greenhouse culture and it just was neither environmentally sound nor wise to continue that process. There are some drawbacks to field starting when you live in Iowa. As with any agricultural venture, you are always at the mercy of the weather. Sweet potatoes are extremely sensitive to cold - both as roots and as young plants. At temperatures below 55 deg. F, fungi grow on the sugars in the root and they quickly rot. It is fruitless to start the bed in cold soil. We have determined over the past 20 years that the best time to start the field beds here in Iowa is usually the last weekend in April or the first part of May. This coincides with the usually last frost of the season. The soil is still too cold for the slips, but we make the beds by digging a wide trench several inches deep. Place the roots in it and cover with peat moss. Wet down then and cover the beds with clear plastic and wait several weeks. If we have warm weather they will start sending up slips in about 20 days. Cool, cloudy weather means added time and fewer slips as the parent roots will sometimes rot.
While many will think that it is getting too late to plant sweet potatoes after the first of June, year after year of research done here has indicated that slips set out when the weather is very warm will outgrow and out-produce ones set out even as much as a month earlier. A slip set out in cold soil will many times become stunted and not produce as large a yield. I typically do not get my slips planted until June 25 thru July 5. With the exception of just a few varieties, they are all ready to harvest by the middle to the end of September. Some tips to increase yields are as follows: Sweet potatoes like to have hot “feet” the opposite of Irish potatoes which like to have cold “feet”. The best way to increase yields is to plant the slips on black plastic. If you can find mulching plastic, that is fine. If not, you can go to a hardware store and get a 10’ x 25’ piece and cut strips 2’ to 3’ wide. If you have light, sandy soil then lay the plastic on the tilled ground and bury the sides of the plastic with dirt. Poke holes 1 foot apart with a trowel. Place a slip in the hole slightly deeper than where the developing roots currently are located when the slips arrive. Make sure that you leave the leaves on the slip above ground level. Then water it and wait for growth. If you have clay or heavy soil, it sometimes works better to make a mound the length of your row and lay your plastic over that and then bury the sides of the plastic with dirt. Then proceed as above with planting the slips. This elevated mound helps to heat up the soil when the soil is heavy. Never place your slips in the refrigerator if you are not ready to plant them when they arrive in the mail. Put them in a container with wet soil or wet peat moss. Don’t store them in jars of water as they will get mushy and rot. You can keep them in the container of wet soil (or wet peat moss) for up to 10 days before setting them out if necessary (but this is not recommended). The longer you do this the more damage you do to your yield and the greater the chance you have of not getting true edible size roots but a bunch of stringy worthless roots.
Your plants are pulled as slips, wrapped in a wet paper towel with a plastic label inserted. They are then wrapped and rubber banded in waxed paper. Once your whole order is collected, the plants are then put in a plastic bag, padded in newspaper and shipped in a Priority Mail box. Your plants should arrive within 3 days and, if they appear wilted, take them out of the waxed paper and place them in a container of very wet soil or peat moss and wait a day or two. Most slips have been growing more roots in transit. If the weather is nice and warm, immediately transplant and keep them very wet in the garden for a week or so to let them establish themselves. Once this happens, they will take off and will not need to be watered as much. As the plants grow, you will need to take care that the plants don’t set down roots away from the main plant. Check for this by gently lifting the vines every once in a while to keep them from setting down roots along the vine. If they do this, and they will in moist soil, your yield can be decreased to next to zero. The only place you want them to root is at the spot where you planted the original plant. When harvest time occurs, make sure that they are not subjected to cold soil. Even if you don’t have frost, soil temperatures under 55 deg. F will cause surface fungi to take hold and your crop will either rot in the ground or in storage. I usually dig mine the end of September here. However, the past few years we have had summer weather then and I have waited until early October. After you dig them, be careful to lay them out in a warm, dry area or place them near a furnace vent for a few days to cure. Ideally, you should cure them at 80 to 90 deg. F for 10 days to 2 weeks. Once curing has completed, it is ideal to store them at a temperature of about 65 degrees F. Roots stored where it is too cool will quickly rot. Once you place them in their storage container (plastic tub or cardboard box), do not handle them any more than necessary. The more you handle them, the more likely they will rot.
HOW WE START SWEET POTATO SLIPS
Many of you who have ordered sweet potatoes from mainline nursery catalogs in the past cannot understand why, when you have ordered from a company in the northern United States, you are able to get your sweet potato slips more quickly than when you order from us. The reason for this is that with other companies, they are “drop shipped” from growers in the south. This practice is also the reason that some companies are not able to send sweet potato slips to California as we are able to do. Ours is an entirely different operation where the slips are started here on our farm in Iowa from roots that are grown the previous year here on our farm in Iowa. We cannot start the beds any earlier than the first part of May. It is simply too cold.
We start the slips by placing the roots in a slightly dug out area. Then cover with peat moss and wet it down. Number nine wire hoops are placed over the area and the whole area is covered in clear plastic (forming a grow tunnel). We lay a soaker hose down the 2-3 foot wide bed and bury the edges of the 50-75 foot long tunnels on both sides with dirt. In the garden it looks like long, clear worms. The beds are not disturbed for the first 2 weeks. Then the plastic is lifted and any weeds removed and a check is made for slip development. Generally 3 weeks after bedding down the first slips are ready to be shipped out. (If we have not had any heat or sunshine during that time, it takes longer for the potatoes to start sprouting.) When those slips are pulled, then more develop and are ready to go out about every 5 days. PLEASE NOTE: The slips for each individual order are all pulled within a few minutes of each other and the order is mailed out that same day. Technically (and then again it depends upon the variety) every 5 days we have a fresh supply from any given potato. This allows us to make slip pickings about 5-6 times during our normal shipping season of May 25 to June 25.
Please Note: Many people have been asking about what a “slip” is. A slip is a single plant (with small roots) that is sprouted on the sweet potato root and then slipped off so that you may plant it in the garden to grow a sweet potato plant. We ship out only slips (plants), but do not ship out sweet potato roots.
SWEET POTATO TIPS FOR SUCCESS
First: Forget what climate zone you are in. That has nothing to do with growing annuals, that is for over wintering plants. Parts of coastal Alaska are in zone 7b. Here in Iowa we are in zone 4b. If we set out plants in the garden on the same day in both places it is almost certain the folks in Alaska will not have anywhere near the success we will have here.
Second: Do not set the plants out when it is very cool. They hate cold and just sit there shivering and waiting for heat. Do whatever you can to make it warm and toasty for them and they will reward you.
Third: You do not need lots of roots on the slips when you plant them to be able to insure success. The more roots the more stress when transplanting and the more they will be stunted. The key when setting out the slips is to have very few roots and keep them as wet as possible in the garden for the first 7 to 10 days. Then back off on the water and they will go crazy. The worst possible thing you can do upon receipt of the slips is to pot them up, wait a few weeks and then transplant to the garden. You will have lost a good 2 weeks of positive growth and will have given them two chances to slow down. The more you slow them down the less yield you have. The more times you transplant them the more non-uniform roots and the less roots you will get. You will frequently get one ball shaped twisted root from plants that have been repotted in pots and then transplanted into the garden.
Fourth: This is the most important thing when it comes to sweet potatoes. It is the heat units that determines success, not the number of days nor plant zone, but heat units. I have been an avid weather observer for over 40 years and have files of weather data to go with files of planting data. A few years ago , thanks to the help of one of our workers, I was able to put the two sets of data together and arrive at some conclusions that I had already suspected, but had never had the time to confirm. It takes about 1200 heat units for our early varieties to reach a decent crop of usable size roots. I use the term usable size as I think for many a sweet potato the size of a nice fat bratwurst is about the best size for keeping and for baking. Bigger than that is okay, but they do not sprout as well nor keep as well because they suffer from bruising much easier. The question you must then ask yourself is: “How is 1200 heat units determined?” I offer the following examples. To get heat units you take the day’s high temperature (maximum) and the day’s low temperature (minimum) and add them together. Then divide by 2 and subtract 55 from that. That gives you the heat units.
Example 1. Daytime high (maximum) 75 deg. F, night time low (minimum) 45 deg. F. Add those together and divide by 2 you get 120/2 or 60. Subtract 55 and you get 5 heat units. If that is your typical summer, then you will need 240 frost free days to get a crop. It doesn’t take a genius to realize that if you have summertime days like that, you are probably not going to have 240 frost free days because that is 8 months.
Example 2. Daytime high of 90 deg. F, night time low of 70 deg. F. That gives you a heat unit for the day of 25 which is just about perfect for maximum growth. Heat units per day greater than 25 seem to have more of a negative impact because of the massive amount of water lost through transpiration. If you can keep 25 heat units a day, then you only need about 48 days to get some useable roots. This is pushing it a bit as there are some limits to daily plant growth. The best growth I have ever seen here is planting around July 18 and having a decent crop by our first frost of October 2, which is about 76 days. By no means do we have temperatures that are perfect for growth each day here in Iowa, but hopefully this shows some data that can help you determine if you can grow a crop.
Fifth: For those who have read the above information and feel that it is now hopeless for them to try to grow sweet potatoes, I offer the following challenge. You can “alter” your heat units in a cold climate. The only way I was able to grow much the three years I spent in the Panhandle of Northern Idaho, where nighttime temperatures got down to the 30’s most nights, was to trick or alter the environment. I used lodge pole pine saplings and made an A frame structure and covered it with plastic. I then enclosed both ends, only opening the ends on the hottest of days and faithfully closing the ends every night. There, the average daytime high was around 80 deg. F in the summer and around 40 deg. F at night, in other words about 5 heat units a day for most days. By using the A frame plastic enclosures, I could get it up to 95 deg. F in there in the daytime and keep it at around 55 deg. F at night without any supplemental heat source. Therefore, I could get 20 heat units a day instead of 5. I never had the courage to try sweet potatoes there, but by using this method I was successful with melons, tomatoes, and cucumbers that otherwise were either impossible or close to a miracle. You can speed it up even more with planting on black plastic, something I never had access to at that time . Living here in Iowa there are no lodge pole pines so a similar structure could be made out of stiff number 9 wire or, if you are talented, plastic electrical conduit.
Once you get a crop, you will then need to preserve it properly. The most common mistake we encounter with folks is storing the sweet potatoes in too cool a place and not properly curing them prior to storage. The best way to cure them is to dig and do not wash. Then carefully lay them out in a warm area (80-90 deg. F) for a week or so to cure. Next, carefully place them in the storage container. Do not just toss them into the container as that will cause bruising and they won’t keep as well. We use plastic storage tubs and place the tubs in a location that is above 60 degrees F for storage. If you store them in a place cooler than 55, the fungi go wild and thrive on the sugary roots. Also, please remember to not handle the roots a lot as bruising causes the fungi to take over and will ruin your harvest.
We have achieved organic certification for our sweet potato plants. Our sweet potato plants are started in the field and are grown on soil that receives no chemical treatments. The plants also are not sprayed for insects or disease and are grown organically.
SWEET POTATO PRESERVATION PROJECT
Growing up in the mountain valleys of Idaho, sweet potatoes were something my family only got at Thanksgiving and occasionally Christmas. I can remember, as a child, counting the days until we were close to Thanksgiving and that tasty treat. Once I moved to Iowa and had a chance to start locating some of the rarer varieties and also had good conditions for growing them, sweet potatoes were no longer just on the dinner table at Thanksgiving. We enjoy them year around though Thanksgiving wouldn’t be Thanksgiving without them. Once I started rounding up the rarer and old time varieties, I soon discovered a few things. Most of the sweet potatoes sold in the local grocery store are really not all that tasty. The greatest discovery of all is that there are a large number of varieties, not on the main commercial lists. These are early enough that many folks can grow them in places where people thought they could not be grown. We have had reports back from customers from Maine to Washington and all across the north that have had great luck growing sweet potatoes. We have set up this sweet potato preservation project as both a preservation effort and as an educational tool. Our first and primary goal is to make sure as many varieties of sweet potatoes get preserved as is possible. Our second goal is to spread the word to as many folks as we can to try growing some of the earlier sorts and find out what a great garden item they are. We will continue to search out any new (old) ones we can and propagate them and get them out to the mainstream public as quickly as possible. We have learned that many rodents of all sorts love the plants and roots and we do lose varieties in the field at times. We maintain a backup pot of each variety in our greenhouse. We have no plans to ever reduce or eliminate varieties just because they don’t sell. Neither do we intend to focus on just a few and mass produce thousands of plants of only those varieties. Our sweet potato collections are not driven by sales only, but by making sure each variety is maintained. Therefore, when you order assortments you are getting many of the types that have not been requested as individual varieties. Please don’t think of these as being less worthy. They, for some reason, just don’t have a catchy name or an interesting history. Regardless of what varieties you choose, we are hopeful you will have a great harvest and can have Thanksgiving everyday in your house.
SWEET POTATO PESTS
Sweet potatoes are a common food item for a great number of animals. They all seem to know how great and delicious sweet potatoes are. We are attempting here to give you some ideas on how to deal with each of the problem animals that harm our crops.
Deer- They love the plants at all stages and will paw at the ground to get the roots in the Fall. We have had few problems with these animals as long as we visit the planting area frequently and our scent is in the area. I try to take my dogs with me and go among the rows every few days. The plants can also be covered with row cover or plastic net for the growing season. We have had great success spreading bad egg residue along the rows.
Rabbits- These are a real problem for us here as the rabbits have a taste for the foliage and will many times eat off the plants as soon as we plant them and walk away from the area. I have literally lost plants to rabbits while I walked back to the greenhouse to get more to set out from rabbits descending on the rows the second they are planted. Our best method of success in dealing with them is to plant the slips on black plastic to hold back weeds. Then use number 9 wire cut into pieces to make wire hoops over the rows and cover with row cover for 3-4 weeks until the vines fill in under the row cover. Even after the plants are large when the cover is removed rabbits can cause substantial yield looses from munching continuously on the foliage. We have had great success spreading bad egg residue along the rows.
Field Mice- Usually the white bellied deer mouse that likes to build nests under the black plastic and will nibble on the tops of roots that reach the surface of the ground. While a nuisance, these little rodents are more interested in the seeds from neighboring crops and use the heavy foliage and black plastic for cover to keep predators away.
Rats- These can be an issue if they move from areas around livestock into field areas. They love the roots and will vigorously dig and eat the crop. You have to be proactive and watch for the signs of fresh dug soil and their tell tale droppings. Cats or dogs are the best method to remove them from the area.
Pocket Gophers- I have seen them travel for several hundred feet underground digging tunnels all the way to get to the sweet potato growing areas. They will search until they find them and then go up and down the row devouring any roots they find. They will not leave the area until all the roots are gone. Many times you may not realize they are there as their mounds will be hidden under the foliage and the plants may not die as they only eat the larger roots. You go to dig in the Fall and the ground will be loose and easy to dig, almost as if it has been sifted. Generally there will be no useable roots left. We have tried all kinds of approaches to control over the years and with our sandy soil they are a challenge. Poisons and smoke bombs are minimally effective if at all, flooding with water is temporary and works for a day or two at most until they rebuild. To date our most effective control method has been the battery operated sonic controls you place in the ground at various intervals. While expensive, they have kept them out of our starting beds for the past few years.
Voles- These are a short tailed plump mouse like rodent and are currently our number one enemy and hardest to control. They move in early from the grassy areas near the gardens and live under the black plastic mulch and feed as fast as the roots form. We have learned they are hardest on the plants nearest the garden edges where they come out of their natural grassy areas into the garden. They are famous for eating the roots from the top down leaving the outer shell in the soil where they have feasted. They tend to lay low until the season is nearing an end then move rapidly in the rows. Frequently you will find well built grassy nests and mothers with up to 6 to 8 young nursing on her as she moves down the row eating the roots. Our dogs smell them and dig and get some, our barn cats venture out and are our best control. Care must be used when encountering them as though they appear harmless, they do and will bite and fight aggressively when disturbed.
SWEET POTATO DESCRIPTIONS
We try to limit descriptions about flavor as flavor varies depending upon the individual and also upon the soil type and the climate where the variety is grown. My sweet potato tastes and Linda’s are not the same. I like dry and firm sweet potatoes with a slightly sweet taste. Linda likes smooth, moist and very sweet flavor. Your best bet is to keep trying different varieties until you find the one that fits your taste buds.
We are not using days to maturity for the sweet potatoes. Sweet potatoes need hot weather. Days to maturity is very subjective. 90 days in my native Idaho mountain valley and 90 days here in Iowa are not anywhere close to the same. Heat units or growing degree days is the only way to accurately determine maturity. Our plants here are usually planted the last week of June and are dug the last week of September (or about 90 days).
Early - at 90 days here in Iowa these have reached full size.
Early/Midseason- at 90 days the majority of roots are full sized and mature.
Mid-season - at 90 days here in Iowa these still have roots that need a few more weeks to bulk up.
Late - at 90 days here in Iowa these only have about 25% of the roots mature.
Very Late - really nothing much at 90 days. These need around 140 days.
Plant Growth Type Criteria
This is our criteria that we use to classify the varieties’ growth habits. This is from data gathered at our farm taking measurements from the location where the plant is growing to the distance the vines cover on one side of the plant.
Very vigorous - vines go to 12 feet or more.
Vigorous - vines usually go from 8 to 12 feet.
Vining - vines go from 6 to 8 feet.
Semi-bush - vines go from 4 to 6 feet.
Bush - vines are less than 4 feet.
For short season areas, success will come easier if you plant on black plastic that has been laid down several weeks prior to setting out the slips Remember they like it hot. Those who live where summer night temperatures are usually less than 60 deg. F, should probably stick to early varieties. Clear plastic tunnels with both ends open also are great in cooler areas.