Handling seed potatoes

Whether you save your own seed potatoes or buy them, producing a vigorous crop depends on making sure your seed potatoes are as healthy as possible when they hit the dirt. But what does healthy mean for a potato? A couple of weeks ago, we talked about the importance of having low disease levels in your seed potatoes – a goal you can achieve by sourcing certified seed potatoes.  Today, Erin will share what she’s learned about storing and preparing seed potatoes so they are ready to grow, and grow fast, as soon as they’re planted.

Sprouty seed potatoes:

I was sorting through my crude potato storage set-up in the basement, groping around for a handful of potatoes to bake for dinner –comfort food for a cold early March evening. Some potatoes I pulled from the bin revealed compact little green sprouts, others exhibited spindly white stems reaching upward, while a few remained solid, ‘eyes closed’ still in dormancy. I started to wonder what causes potatoes to sprout and break dormancy? Do I need to re-work my storage set-up? What might I do to influence health and vigor of potatoes? What is the proper technique for encouraging potatoes to sprout? Here are a few tips I learned from Ruth and on-line, to ensure a full life when it comes to storing and caretaking your ‘aging spuds’.

Tips for ensuring proper storage for potato seed pieces:

Potatoes start aging as soon as they form on the parent plant – even before they are harvested! One of our roles as farmers is to slow down the spiral to the compost heap and keep our produce fresh, tasty, and nutritionally dense until it reaches our forks – or in the case of seed potatoes, until they return to the soil. We can do this mechanically by regulating temperature, humidity and air circulation. Potatoes, by nature of physiology, tend to live a longer life than most other vegetables, post harvest. In storage, tubers are metabolically active, using the carbohydrates packed into them to produce energy, thus consuming oxygen and releasing carbon dioxide, water and heat. Aging is, in part, a measure of how fast the tubers are going through their resources. Keeping temperatures low, humidity high (90-95%), and air circulating will help to slow down the aging process.

For potatoes that are destined for the plate, storage at 40-45 F will keep them fresh and nutrient-packed. Seed potatoes need a deeper sleep, and are generally stored at 34-38 F. However, if you just received your seed spuds in the mail, it’s ideal to store them at 50 – 55 F with high humidity for a week or two. In these conditions, potatoes will go through an active healing process, creating a layer of suberized skin – a kind of scar tissue – to repair cuts and bruises that might have occurred during shipping. Of course, it’s always good to take a look at your seed potatoes when they arrive. Especially if they were handled roughly in shipping, this is the time that rot problems might emerge. Then, if there are still a couple of months till planting, it’s best to cool them down to 34-38 F and let them get back to sleep.

physiological age

 

As tubers age, they come closer to breaking dormancy and sprouting – signalling that they are ready to produce the next generation of plants.Young tubers will have a single sprout, or maybe two, at the bud end. More sprouts will form as the seed reaches middle age, and if you have bushy sprouts forming, you have old seed. If your seed is forming tiny tubers on its sprouts, sadly it has given up on ever seeing soil and is unlikely to produce vines at all. Tuber physiological age is in part dictated by variety – some varieties simply have a longer dormant period than others. Field conditions experienced by the parent plants also impact the aging process – more stress means ‘older’ tubers. I find it fascinating that last year’s wet June and droughty July might have played a role in early sprouting Red Maria tubers in the root cellar.

Middle-aged and old tubers are ready to grow – so they will emerge more quickly than young seed. More sprouts ‘eyeing’ a break in dormancy equates to more stems, but each with less vigor – which then leads to smaller tubers, but more of them. Younger seed tubers? You’ll see slower emergence, few but vigorous stems, and fewer but larger taters. If your customers enjoy smaller spuds at the market and in their CSA boxes, planting ‘aged potatoes’ works to your advantage (though you can also manipulate size by harvest timing and spacing tubers closer together).  The key is to optimize your storage conditions and learn, through field and storage observation and good record keeping, what to look for in a good seed piece.

As farmers (and caretakers of the tuber aging process) our role is to know when to intervene. Inevitably, your tubers need to grow up as you don’t want your seed potatoes to linger too long in dormancy and risk losing your planting window. You can speed up the aging process with a little preparation prior to planting. If you’re not sure how old your seed tubers are, just take a handful out of storage and let them sit at 60-70 F. After about a week, take a look and see if you have any sprouts – and how many. If there are no sprouts, your tubers are still dormant and will need some waking up before planting.

You’ve got to be Chitting Me

I never knew that it would take me 36 years of living on this planet before I finally discovered how to chit. Potatoes that is. One way to encourage varieties which are slow to break dormancy is to greensprout, or chit potatoes. Greensprouting seed potatoes will hasten emergence and overall development, leading to an earlier harvest. It’s a simple process, so don’t be like me and wait 36 years before chitting. Rather consider starting this season and follow these basic steps:

1. If your seed potatoes are still dormant, warm them to ~70 F for a week to help break dormancy.

2. Let the tubers cool down to 50-55 F, and keep them in low light for 2-4 weeks.

All of the tubers need light exposure. Depending on how many tubers you want to greensprout, you may be able to simply set them in a shallow tray, or you may need to build or buy many trays and stack them so that light can penetrate. In Europe, where greensprouting is a common practice, commercial setups with hanging bags can be purchased. For her trials of greensprouting, Ruth has taken mesh bags and sewn channels into them, filled them with potatoes and hung them from racks in a lighted basement.

Large scale Pre-sprouted seed potato storage system
Large scale pre-sprouted seed potato storage system demonstrated at Joppe. Pre-sprouting seed potatoes is a wide-spread practice throughout Europe.

After a week or two, you will start to see stubby green sprouts. These are strong, hardy sprouts that will not break off easily. And they will burst into life when you plant!

image of greensprouted potatoes
Example of greensprouted. or chitted potatoes

 

Pre-cutting Potatoes

Pre-cutting is another way to age young seed potatoes. Not all seed potatoes should be pre-cut – only seed of young or middle physiological age. Pre-cutting can improve emergence, and, since planting is a busy time, it can also improve labor flow on the farm. The steps are:

1. Warm seed to 50-55 F, and cut into 1.5-2 oz seedpieces.

2. Allow to heal for 2-7 days with high humidity.

3. Cool to 45-50 F and hold for 1-6 weeks.

4. Warm to 50-55 F before planting.

If your seed doesn’t need any waking up, it’s still good to warm it to 50-55 F at least a week before planting, and to cut no less than 2 days before planting. Ideally the soil and the seedpiece will be at about the same temperature when you plant.

In closing:

Storage, seed piece size and physiological age of the seed piece are just a few factors that determine the final yield. We’ll delve deeper into other factors in preparation for this season’s planting that influence yield such as soil conditions, accuracy of the planter, and diseases in future blog posts.

In the interim, we’d love to hear from you. What challenges have you encountered in storage? Are there discoveries you’d like to share from your experiences aging potatoes? Are there advantages and disadvantages to chitting that you’d like to add? To chit or not to chit – is that indeed the question? I’ll let you know in another 36 years.

Learn more:

From the Archive (January 2014): How old is your potato?

Resources on storing seed potatoes:

CIP Bulletin –Physiological Development of Potato Seed Tubers

University of Maine Cooperative Extension – Potatoes

Chitting.com

 

Erin