Planting spring-flowering bulbs is an essential part of my fall gardening to-do list. Every spring, I anxiously await their staggered flowering times. Knowing when to fertilize bulbs ensures that I enjoy that variety of blooms in my ever-growing collection of naturalized and perennial bulbs year after year.
Flower bulbs essentially already have all the energy and nutrients they need to sprout, bloom, and flourish. But you do want to make sure you provide optimal growing conditions for that bulb to start growing in the first place—loose, well-draining soil is a must. Bulbs prefer a neutral soil pH. A soil test will reveal the pH if you have concerns. You also want to make sure those bulbs do not sit in wet soil or they will rot.
Once you’ve chosen where to plant your spring bulbs, amending the soil with an organic fertilizer, like composted manure, will help to prepare the site. And most of the time, that’s all you’ll need to do before you plant and patiently wait for spring.
However, after you’ve set up your flower bulbs for success in their environment, you may want to think ahead for future flowers. Timing is key if you’re going to help those bulbs bloom again for you.
When to fertilize bulbs so they bloom again next year
For bulbs that you want to rebloom the following year, such as fritillaries, puschkinia, snowdrops, hyacinth, and perennial tulips, plan to add your fertilizer in the springtime. It’s worth noting that if you are going to plant spring-flowering bulbs as annuals, such as tulip hybrids, it’s not necessary to add fertilizer at any point.
Choose an organic, granular, slow-release fertilizer that is higher in phosphorus and lower in nitrogen (which can encourage leaf production at the expense of flowers). Check out Jessica’s article that explains fertilizer numbers. Many organic fertilizer companies seem to have their own bulb formula, so that removes the guesswork out of what to purchase. You can find examples here and here.
Add your granular bulb fertilizer to the garden when shoots first start to appear in the spring. Use the package to determine how much you should use. Sprinkle the recommended amount around the plant (but not on it).
Everything you’ve heard about the importance of allowing the bulb foliage to die back naturally (and not cutting it away once the flowers have bloomed) is true. You can interplant your bulbs among perennials that will hide the dying foliage. However you should deadhead tulip flowers once they have faded to prevent them from forming seeds. (This drains the energy that should be going back into the bulb for the following season.)
After blooming, and as the foliage of all spring-flowering bulbs starts to yellow and die back, photosynthesis revitalizes the bulb by using the sun’s energy to make the nutrients required for it to regrow. Wait about six weeks before clearing away the leaves.
Make a note in your calendar to also add a bit of slow-release fertilizer to those planted areas again in the fall. (I plant my bulbs in October or November.)
How and when to fertilize bulbs in poor soil
I’ve always had good luck planting bulbs in my gardens that have consistently healthy soil. However I realized there are situations where flower bulbs will benefit from adding a bit of fertilizer at planting time in the fall.
My front yard garden, for example, doesn’t have the best soil. I’ve been trying to amend it over time. It’s a bit hard-packed and not as loose and friable in spots like in my other gardens. The bulbs I’ve planted in that garden did grow the first year, but their blooms and leaves were a bit lackluster.
For gardens with poor soil, amending the soil with compost, and adding a granular fertilizer at the time of planting will give those bulbs an extra boost of nutrients. This should help with flower production. Top-dress the soil around the newly planted bulbs rather than adding the fertilizer to the planting hole, as direct contact can burn the bulbs.
Add a layer of shredded leaves to the planting site as a winter mulch. As they start to break down, they will help to improve the soil, too.
Fertilizing bulbs to keep squirrels away
If you have a problem with squirrels, I have an extra fall bulb-planting tip to share. Luckily squirrels don’t bother with foul-smelling bulbs, like fritillaria, hyacinths, alliums, and daffodils. But tulips to them are meal-worthy. And once they see the freshly disturbed soil of a newly planted tulip garden, they want to know what’s underneath. Those bulbs are sitting ducks.
Before adding a winter mulch of shredded leaves, I will sprinkle hen manure on the planting site, a squirrel-deterring tip I got several years ago from garden designer Candy Venning of Venni Gardens. She also recommended bone meal, another squirrel repellent.
When to fertilize bulbs that bloom in the summer and fall
Summer-flowering bulbs, such as dahlias and caladium, and fall-flowering balls, like colchicums, can also benefit from fertilizer. Once you choose where you’re going to plant your bulbs, be sure to amend the soil with lots of organic matter. After planting, sprinkle a slow-release bulb fertilizer around the planting area.
And just as you do with spring-flowering bulbs, allow the foliage to die back completely before trimming it away.
Interesting flower bulb choices and planting advice
Learn about tulip planting depth in this video:
- Unusual flower bulbs for your garden and how to plant them
- Deer-resistant bulbs for spring color in the garden
- Bulb-planting design tips and inspiration from the Keukenhof gardens
- Alliums for the garden: The best long-blooming allium varieties
- Types of lilies: 8 beautiful choices for the garden