It’s the time of year again when we begin to think about next years potato crop. We may have already decided where we will grow them, what variety we will plant, and how much potato fertilizer we will apply. One aspect which can make a massive difference to the speed our crop establishes and the yield and quality of potatoes we produce is seed planting depth.
The importance of seed potato planting depth
As we become more experienced growing potatoes we start to notice how things we thought were of small importance have a big effect on our crop. The depth we plant our seed potatoes is one of these things we become more focused on the longer we have been growing potatoes.
After growing different potato varieties we soon find out that each variety behaves a little differently as it grows. Some varieties will grow slightly downward in the drill, some stay in the middle and some will form their potatoes slightly towards the surface of the drill.
How deep should I plant my potatoes
Setting the correct depth to plant your potatoes usually follows 2 strategies –
- Plant at 6 inches deep and add more soil (moulding up) after the potatoes emerge through the soil. The main reason for this strategy is to allow the potatoes to establish leaves sooner.
- Plant your potatoes at the required depth with no moulding up needed later. The majority of people prefer to plant at the required depth at planting as the soil has a tendency to become less workable later on in the year making moulding up difficult. Unless you are planting very late in the year the potatoes will have plenty of growing season.
As a general rule, most seed potatoes should be planted 8″ (200mm) below the surface of the soil. Different potato varieties will develop tubers slightly higher or lower than the seed potato you plant. Experience of how your variety behaves is helpful when setting the depth.
Consider drill width
Potato varieties that are high-yielding will require more space in the drill to form so if you have drills that are quite narrow- you will need to be careful that the potatoes do not begin to show at the sides of the drill. There is a balance between the yield of the crop, the width of the drill, and the depth the potatoes are planted.
If you kept a narrow width drill and decided to plant your crop deeper you could run into a few other issues. When you plant your seed potatoes too deep – say 12″ to 14″ (300-350mm) you could negatively affect the growth of your plant in a couple of ways.
Although you could avoid the issue of potatoes showing at the sides of drills by planting into a wide bed. The issues I have listed below apply to planting seed potatoes too deep in any situation.
Excessively deep planting issues
- The deeper you plant your seed the colder the soil is – this slows the growth of the plant – making it take much longer before it reaches the surface. You generally want to get your potato buds to the surface as fast as possible because when the plant forms leaves and has access to sunlight that it really takes off and grows well. Planting very deep can delay this emergence by 3 to 4 weeks, this will make it lose valuable growing season and you may end up with small potatoes which havent quite had long enough growth.
- Sometimes the deeper you plant the more wet the soil is. This can expose the seed to be sitting in excessively wet soil for prolonged periods and could eventually cause the seed to rot and die. If you are planting into drills or beds the seed potato should not be placed below the level of the bottom of the alley of the drill or at the edge of the bed. Each ally of the drill acts like a mini drainage trench. growing potatoes in drills is much better than beds for this reason. Unless your soil is very free draining I would advise caution with growing potatoes in beds. Caution is needed when tilling soil for beds because if the soil is too finely tilled it prevents good drainage in beds which can lead to a mush of wet soil which has difficulty drying leading to more seed rot issues.
- Deep planting usually means the seed is not surrounded with finely tilled soil but is instead surrounded with more compacted soil. Compacted soil will harm the free development of the tubers and may have negative effects on the availablity of soil nutrients.
Just as there are important points to pay attention to if planting your potatoes too deep, there are also points to be made against planting too shallow
Planting potatoes too shallow
- Planting seed potatoes too close to the surface can increase the risk of your potatoes showing through the soil. If left uncovered these potatoes will become “sunburned” – this will have the effect of turning their skins green if exposed to sunlight for a short period and black if left for a long period of time. either way the potato will be unuseable and could make you feel unwell if eaten due to the formation and presence of the toxin solanine.
- Another important point to consider if your plant your potatoes too close to the surface is the effect of drying out the soil. As a potato plant grows it needs quite a good supply of water – especially when it has developed into a fully grown plant above ground. At this time of full plant growth the next stage is the formation of flowers and then it will begin to form potatoes under the soil. At this stage it is vital that the plant has access to enough water for it to develop properly. If you have planted your seed too close to the surface it will grow pretty much ok until it get to this high water demand stage. But this is when problems can arise. The top part of the drill has the least volume of soil this means the shallow planted potato plant will quickly dry out the soil on the top of the drill and because the roots are not deep into the base of the drill it will not have access to the moister soil. The plant can become stressed due to the lack of water and the tuber formation in the soil has a much greater risk of developing scab. The supply of water at tuber formation is one of the most important strategies against scab along with keeping soil ph lower.
Accurate planting depth by hand
It may be easier to define and set your planting depth if you are using a mechanized potato planting machine than by manually planting seed potato by hand with a shovel but let me give you a few tips.
- After you have cleared you dug over your soil or tilled it quite finely to a depth of around 6″ (150mm) set the first seed potato on top of the tilled soil where you want to begin your row.
- Set each seed potato at it required spacing (around 14″ apart) until you reach where you want the end of the drill to be.
- Now shovel the soil from both sides of the potato – keeeping about 15″ from the potato to the centre of your shovel until you have 8″ of soil over that one seed potato. I keep a depth stick with a marker at 8″ on it in the soil until i reach the mark.
- Now move along about 4 feet and do the same until you reach the end of the row.
- These mounds will alow you to get an idea of how much soil 8″ looks like on top of the seed potato. You can fill in between the mounds in the confidence you have enough soil over them.
- As you get used to this process you will eventually no longer need the mounds or the depth stick as often.
As you can see there are important points against planting too shallow and planting too deep. I would advise you to start with planting your seed potatoes 8″ (200mm) deep and make note of how each variety responds- you can then make a slight adjustment the next time you plant that variety. Growing an excellent potato crop has many factors to consider but each year you have the opportunity to improve on the last. Good luck.