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Growing Pumpkins in Containers

I was looking for something big to grow and thought pumpkins might be an interesting challenge. For my first try at it I decided to grow sugar pumpkins which are popular smaller pumpkins great for pies and stuff. And I choose jarrahdale, a bluish white squash from Australia, because they seemed different. I used self watering containers because I knew they'd grow to be thirsty giants. I tried hard with those pumpkins but it wasn't enough. No pumpkins for me. Although my pumpkin growing efforts didn't yield anything 'fruitful', I did learn much about growing pumpkins in containers.

Sugar pumpkin growing fast

The sugar pumpkin shortly after being transplanted outside. I had to work fast to get a trellis up.

Pumpkin leaf damage

Nutrient deficiencies and mildew making a mess of my pumpkin plants.

I stared my pumpkins in the kitchen window from certified organic seed at around the end of April and planted them out the first week of June when the temps started to warm up a bit. I've heard sugar pumpkins referred to as "small" sized pumpkins and as far as pumpkins go, sure, they are on the small side. Same with the jarrahdale. But when it comes to container gardening there is no such thing as a small pumpkin. In previous years when I've started melons and cucumbers from seed in the spring I tended to start them earlier and was always faced with having to put long spindly vines outside before temperatures warmed up enough for them. For these pumpkins I held off a bit and put them out when temps were just right.

Container pumpkin trellis

The trellis for my sugar pumpkin. Will it be enough?

Both plants grew fast once transplanted. Especially the sugar pumpkin which I placed in the largest container in a nice sunny corner of the balcony. The leaves on these plants were huge with thick meaty vines and giant flowers. In the heat of the summer these things were sucking back gallons of water every day. All that leaf surface area equates to big time transpiration. The small watermelons I've grown are nothing by comparison. Thankfully I had my plants in self watering containers. The sugar pumpkin was planted in a converted storage tote container and the jarrahdale in a 5 gallon self-watering bucket. This was far too small for that plant. And even growing the sugar pumpkin in a container twice that size was pushing it. Pumpkins have massive root systems and I was constraining mine to 5 and 10 gallon containers. Obviously I had set myself up for some trouble.

I had a big problem with nutrient deficiencies like I've never seen before. I started out with a good amount of compost in the containers and some organic fertilizer. As the plants grew they each came to a point where the growth slowed and some of the leaves started showing signs of stress. Perhaps there wasn't enough nutrient available in the soil yet. I added more dry fertilizer and added some inorganic water soluble ferts to the water also and I think that got things going again. I was using everything I had. Eventually the plants started growing again and continued to flower.

I had constant problems with mildew. Powdery mildew started spreading on the leaf surfaces which wasn't so bad but there was also lots of downy grey mildew on the undersides of the leaves. It started at the bottom leaves and quickly spread to the vines and other leaves. At first it was just my sugar pumpkin but later my jarrahdale suffered also. Downy mildew is nasty. This stuff kills patches of leaf tissue and spreads, eventually killing entire leaves. Fortunately I had some neem oil and I mixed a mild solution of neem and dish detergent and sprayed it on the top and bottom of the leaves. This instantly beat back the mildew but it didn't eliminate it completely and eventually I had to respray. My spray bottle is always loaded and ready to do battle.

Sugar Pumpkin Male Flower

Sugar pumpkin male flower. Big flowers, big pollen.

For my sugar pumpkin in the larger container I constructed a new trellis to support the vines. It was made from 1 inch wood furring strips and a sheet of plywood. I made a platform for the container and attached the frame for the trellis to the sides of this platform. The frame was a little over 6 feet tall and 3 feet wide. To the frame I secured some bamboo sticks horizontally. My plan was to weave the vines back and forth over the structure as they grew. I had done it before with watermelons so I figured it should be the same for pumpkins. I should have made the trellis bigger but I just didn't have enough space on the balcony. The pumpkin vines were very thick and stubborn and training them to grow where I wanted on my trellis was difficult.

I made a smaller trellis entirely out of bamboo for the jarrahdale pumpkin just as I've done in the past for several other plants. The jarrahdale was a bit behind the sugar in terms of development and was getting less sunlight. That and the smaller container meant there wasn't much chance of it producing anything so I didn't want to put much effort into a support structure for it. I considered it to be my backup plant in case something happened to the sugar.

New sugar pumpkin

A new sugar pumpkin. They don't take long to get to this size.

Eventually both plants flowered. First my sugar pumpkin and eventually the jarrahdale. The sugar grew faster, most likely because it was in a very sunny spot where as the jarrahdale received some shade from a container in front of it. It was fairly easy to pollinate the female flowers by hand when they opened. There were lots of male flowers around and the flowers were big. The grains of pollen seemed massive compared to what I've seen with watermelons or cucumbers. After some hand pollination I managed to get a couple of sugar pumpkins to start growing nicely. On the jarrahdale I had a couple take as well but they didn't get very big before withering and falling from the vine.

The sugar never really grew all that big. At one point it put out an unusual number of male flowers. Perhaps that was a sign that it was getting desperate and stressed. Once the sugar had a couple of pumpkins on it I had another nutrient crash and the mildew was getting out of hand. I realized I just couldn't keep this plant going so I decided to chop it down and focus on the jarrahdale that was still quite healthy looking. With the sugar gone, I moved the jarrahdale over to the trellis and it started to really take off. But soon it too was showing signs of stress. On top of that it was starting to get a little late in the season. The container it was in just wasn't big enough for the size of the plant and although I could get pumpkins to start forming on the plant I couldn't get them to stay for very long. I eventually gave up on this one too and took it down also.

Jarrahdale filling the trellis

I got nice growth from the jarrahdale but no pumpkins. It was much happier once I moved it over to the big trellis. But still no pumpkins :(

Some lessons learned and things to try next time around:

  1. Keep my trellis out from the wall. One of my failures with the trellis was that I put it up against a wall and I couldn't get behind the plant easily to spray the leaves to keep the mildew in check. It also would have helped to have a sprayer that could reach the bottom leaves without requiring me to kneel down next to the plant.
  2. Make the trellis much bigger. Bigger spacing between openings. More width and height. I gave some of the pumpkin seedlings I started to my parents which they planted in their garden. I was surprised at how long those vines grew. Especially the jarrahdale. My trellis clearly wasn't big enough.
  3. Use a bigger container. The storage tote was ok but I would like to make something a bit bigger if that's possible. The 5 gallon bucket was way too small.
  4. Put some mulch over the soil to retain heat and moisture.
  5. Find a smaller pumpkin variety. I like sugar pumpkins and think I'll go with them again but if I can find a smaller variety that is edible and good in a pie, I'll do that instead
  6. More compost and fertilizer. I need to plan out the fertilizer more carefully. A couple of times now I've been caught near the end of the season without fertilizer and usually by that time I find it hard to get motivated to go out and buy more. I might consider switching to an inorganic fertilizer just for this one plant, just to ensure I can get enough nutrients into it fast if things start to go wrong. Instead of one big batch of fert at the start of the season, a regular application of a water soluble fert throughout the season might be better in this case.
  7. Lower the pH of the potting soil. I've read that pumpkins will grow in anything from 5.5 to 7.5 but I have yet to find any real consensus on a more narrow range. But even within that range, 7.0 (which is where my potting soil typically tests) is on the high side and I might do better around 6.5. I should be able to provide that by using more organic matter in the mix.

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