The best fertilizer rhubarb growers should reach for is a low-phosphorus blend like 24-0-15 or 30-0-10. These mixes give your plants plenty of nitrogen and potassium without the extra phosphorus they don't need. Skip the balanced all-purpose products. Your rhubarb wants a formula that matches its specific hunger for nitrogen and potash above all else.
Rhubarb is a rhubarb heavy feeder that pulls large amounts of nutrients out of your soil each year. Those giant leaves and thick stalks need a lot of fuel to grow. The plant craves nitrogen for leaf growth and potassium for strong cell walls and disease resistance. Ohio State data shows rhubarb prefers about 1.25 pounds of nitrogen and 4.5 pounds of potash per 1,000 square feet (93 square meters). That's far more potassium than most garden crops need. If you don't feed your rhubarb enough, the stalks will come in thin and your yields will drop off fast. You'll notice the difference within one growing season if you skip a feeding. Keep your plants well fed and they'll reward you with armloads of thick stalks each spring.
You should go with a low phosphorus fertilizer rhubarb growers can find at most garden stores. The experts at the University of Minnesota back this up. Most garden soils have plenty of phosphorus from years of compost. Adding more just creates runoff that hurts local streams. A 24-0-15 lawn fertilizer that has no herbicides gives you the right ratio. You can also grab a 30-0-10 blend if your store carries it. Both work great for rhubarb because they load up on what the plant needs and skip what it doesn't.
I switched from a balanced 10-10-10 to a low-phosphorus blend three years ago. The results showed up the very first spring. My stalks came in thicker and the red color looked deeper than before. I used the same amount of product at the same time of year. The only thing I changed was the formula. That one swap convinced me to never go back to a balanced mix again. I tell every new rhubarb grower I meet to make the same switch. It costs the same and takes the same effort. You just pick a different bag off the shelf at your garden center.
Timing matters just as much as picking the right product. Apply your fertilizer in early spring when new shoots start poking through the soil. Spread it in a ring around the drip line about 6 inches from the crown so you don't burn tender new buds. Water it in right away so the nutrients move down to the root zone. A second feeding in late spring after your first harvest gives the plant a boost during peak growth. Don't fertilize after July because you want the plant to slow down and store energy for winter. Late feeding pushes new growth that won't harden off before the cold hits.
Top your fertilizer with a thick layer of compost for the best results. Compost feeds the soil microbes that help break down nutrients for your plant. It also holds moisture close to the root zone so your rhubarb doesn't dry out between rains. Spread 2 to 3 inches of compost around each crown every spring right after you fertilize. This combo keeps your rhubarb producing fat stalks for years. You won't see any slowdown as long as you stay on schedule with both the fertilizer and compost each spring. Your crowns will keep getting bigger and better every single year. The best fertilizer rhubarb plants can get is one that gives them exactly what they need without any waste.
Read the full article: Growing Rhubarb: Expert Advice for Success
